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Preliminary and HSC Golden Bell Frog NSW

HEINEMANN

Biology • Kate Mudie • Judith Brotherton

Principal reviewer: Jan McBryde

Lake Burrendong NSW after drought

Heinemann A division of Reed International Books Australia Pty Ltd 22 Salmon Street, Port Melbourne, Victoria 3207 World Wide Web hi.com.au Email [email protected] Offices in Sydney, Brisbane, Adelaide and Perth. Associated companies, branches and representatives throughout the world. © Kate Mudie and Judith Brotherton 2004 First published 2000 Second edition 2004 2007 2006 2005 2004 10 9 8 7 6 5 4 3 2 1 The purchasing educational institution and its staff have the right to make copies of the whole or part of this book, beyond their rights under the Australian Copyright Act 1968 (the Act), provided that: 1. the number of copies does not exceed the number reasonably required by the educational institution to satisfy their teaching purposes; 2. copies are made only by reprographic means (photocopying), not by electronic/digital means, nor stored nor transmitted; 3. copies are not sold or lent; 4. every copy made clearly shows the footnote (e.g. ‘©Reed International Books Australia Pty Ltd 2000. This sheet may be photocopied for non-commercial classroom use’). Any copying of this book by an educational institution or its staff outside of this blackline master licence may fall within the educational statutory licence under the Act. The Act allows a maximum of one chapter or 10% of this book, whichever is the greater, to be copied by any educational institution for its educational purposes provided that that educational institution (or the body that administers it) has given a remuneration notice to Copyright Agency Limited (CAL) under the Act. For details of the CAL licence for educational institutions contact CAL, Level 19, 157 Liverpool Street, Sydney, NSW, 2000, tel (02) 9394 7600, fax (02) 9394 7601, email [email protected] Copying by others Except as otherwise permitted by this blackline master licence or under the Act, for example, any fair dealing for the purposes of study, research, criticism or review, no part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means without prior written permission. All enquiries should be made to the publisher at the address above. Publisher: Malcolm Parsons Editor: David Meagher Designer: Effie Evgenikos Cover designer: Cynthia Nge Illustrations: Guy Holt, Andrew Plant, Trudi Canavan Photograph researcher: Lara Artis, Janet Pheasant Typeset in 10/12 Berling by Palmer Higgs Film supplied by Splitting Image Colour Studios, Australia Printed in Hong Kong by H&Y Printing Limited National Library of Australia cataloguing-in-publication data: Mudie, Kate. Heinemann biology. 2nd ed. Includes index. For senior biology students in NSW. ISBN 1 74081 371 5. 1. Biology—Textbooks. I. Brotherton, Judith. II. Andrews, Carol, 1958- . III. Sanders, Yvonne. IV. Title : Biology. 570 (Pack ISBN: 1 74081 227 1; CD ISBN: 1 74081 372 3) Disclaimer The selection of Internet addresses (URLs) given in this book/resource were valid at the time of publication and chosen as being appropriate for use as a secondary education research tool. However, due to the dynamic nature of the Internet, some addresses may have changed, may have ceased to exist since publication, or may inadvertently link to sites with content that could be considered offensive or inappropriate. While the authors and publisher regret any inconvenience this may cause readers, no responsibility for any such changes or unforeseeable errors can be accepted by either the authors or the publisher.

Contents Contents Introduction Acknowledgments

1 2

Chapter 1 A local ecosystem 1.1 Terrestrial and aquatic environments 1.2 Local ecosystems: interactions and responses Chapter summary Exam-style questions

Chapter 2 Patterns in nature

3

2.1 2.2 2.3 2.4 2.5

Organisms: cells and structure Cell membranes: form and function Obtaining nutrients Exchanging gases Growth and repair Chapter summary Exam-style questions

Chapter 3 Life on Earth

4

3.1 3.2 3.3 3.4

The origins of life Fossils and the evolution of life Procaryotes: the first living things Taxonomy: classifying organisms Chapter summary Exam-style questions

Chapter 4 Evolution of Australian biota 4.1 4.2 4.3 4.4

Gondwana: ancient supercontinent Changes in Australian flora and fauna The continuation of species The future of Australia’s biota Chapter summary Exam-style questions

v vi

1 2 18 39 41

43 44 57 66 82 94 102 104

107 108 114 124 132 143 145

147 148 157 170 198 206 208

iii

5 6

Chapter 5 Maintaining a balance 5.1 Activity and temperature 5.2 Water for transport 5.3 Regulation of substances Chapter summary Exam-style questions

Chapter 6 Blueprint of life 6.1 6.2 6.3 6.4 6.5

7

The evidence for evolution Mendel and the inheritance of characteristics Chromosome structure—the key to inheritance The mechanism of inheritance Reproductive technologies and genetic engineering Chapter summary Exam-style questions

Chapter 7 The search for better health 7.1 7.2 7.3 7.4 7.5 7.6 7.7

What is a healthy organism? The importance of cleanliness The search for microbes as causes of disease Protecting the body: defence barriers The immune response Epidemiological studies Strategies to prevent and control disease Chapter summary Exam-style questions

Glossary Index

iv

211 212 227 243 258 260

263 264 278 288 297 309 323 326

329 330 334 343 356 361 368 380 387 389 391 400

Introduction

Functioning organisms

Heinemann Biology is a full-colour senior biology text which closely supports the New South Wales Board of Studies syllabus for Biology Stage 6 (2002) and other senior biology courses. This new edition has been developed by the authors and publishers after extensive consultation with teachers. Students and teachers will find that the new textbook provides a comprehensive and enjoyable study of biology, with the most up-todate biology available presented in Australian contexts. Features of the text include: • seven chapters covering the seven core modules of the New South Wales syllabus • chapters 1 to 4 cover the preliminary course, and chapters 5 to 7 cover the HSC core modules • four option modules are available on eBiology Student CD • an opening context statement and a list of the syllabus outcomes at the beginning of each chapter • chapters divided into clear-cut sections • a modern and stimulating design. Three levels of exercise complement the text: • Each section includes questions to consolidate the key concepts learned and test knowledge and understanding. • Further questions develop the application of knowledge and address the broader syllabus requirements for skills development. • Exam-style questions provide experience with multiple choice and short answer questions, and extended responses in biology. Two styles of boxes supplement the text: • Unshaded boxes surrounded by a blue border provide examples of biology in an applied situation or relevant context. These include the issues surrounding the nature and practice of biology, applications of biology, as well as the historical development of ideas.

• Shaded purple boxes contain material which goes beyond the core content of the syllabus. This material may be conceptual, contextual or provide additional information. Teachers can direct their students to this information as appropriate. Other features include: • key terms highlighted in bold for easy identification in the text • Biofacts to add interesting snippets of information to the text • key points in the margin which state the main ideas of the section and will assist students to locate facts or summarise information • chapter summaries to aid understanding and revision of the chapter outcomes • a list of practical activities for each chapter, covering the syllabus requirements—these activities can be found in the Heinemann Biology Activity Manual • a glossary of terms and comprehensive index. The Heinemann Biology Activity Manual contains a broad range of activities that, together with this book, ensure full coverage of the syllabus, including practical and field work requirements. All activities are crossreferenced to the student text. Many contain information technology opportunities. Teacher notes, safety advice and guidelines on introducing information technology into the classroom are also included. Support material: hi.com.au/biol Heinemann Biology has dedicated website support including downloadable software for spreadsheets and graphing packages, multi-media presentations, datalogging activities and advice, useful information and internet links.

v

Acknowledgments The authors and publisher would like to thank Barbara Evans, Pauline Ladiges, John McKenzie and Philip Batterham for their kind permission to use text and illustrations from Heinemann Biology One and Two; the editor, David Meagher; Yvonne Sanders and Carol Andrews for their contributions and the members of the teacher review panel, Jan McBryde, Marie Grant, Phil Lachman, Jenny Williams and Monika Khun for their invaluable input during the development of this book. The authors would like to acknowledge the assistance of Professor Robert Burton (Director, Anti-Cancer Council of Victoria), Dr Eugenia Pedagogos (Royal Melbourne Hospital), the Australian Kidney Foundation, George James Bouhalis and Marisa Butera. The authors and publisher would like to thank the following for their permission to reproduce the copyright material in this book. The Age, pp. 365, 378; ANT/Jan Tyler, p. 184; ANT/M. J. Tyler, p. 192; ANT/Dave Watts, p. 24 (top right); Kathie Atkinson, pp. 4 (centre right), 21, 192 (left); Australian Picture Library, p. 122; Australian Picture Library/John Carnemolla, pp. 153, 265; Australian Picture Library/Sean Davey, p. 36 (bottom); Australian Picture Library/Greenpeace, p. 303 (bottom); Australian Picture Library/Minden Pictures, p. 73 (bottom); Bill Bachman, pp. 18, 30 (right), 177 (left); Big Island Photographics, p. 195; Biophoto Associates, pp. 44, 55 (bottom right); Dr Geoff Brown, p. 35; Graeme Chapman, p. 158 (right); Gene Cox, pp. 73 (middle), 94, 95 (bottom left); Cancer Council of NSW, p. 381; Bruce Fuhrer, pp. 18, 23 (bottom right), 30 (left), 45 (top), 78, 117 (left), 124, 132, 136 (bottom left, bottom right), 137 (top right), 138, 151 (bottom), 159 (left), 187, 264, 269; Pauline Ladiges, pp. 130, 152 (right); Lochman Transparencies/Eva Boogaard, p. 189 (top); Lochman Transparencies/ Jeremy Colman, pp. 1, 7; Lochman Transparencies/Jiri Lochman, pp. 23 (left), 34 (top), 147, 194 (bottom), 180, 181, 189, 191, 331 (right), 334; Lochman Transparencies/Marie Lochman, pp. 165 (top), 177 (top right), 331 (left); Lochman Transparencies/Peter & Margy Nicholas, pp. 2, 4 (bottom left); Lochman Transparencies/Dennis Sarson, pp. 36 (top), 37; Mary Evans Picture Library, pp. 166, 168 (right); David Meagher, pp. 3 (top and centre left), 4 (centre left, top and bottom right, bottom middle), 19 (both), 23 (top),

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24 (left), 29, 32 (top left and right), 67, 83, 137 (top and bottom left, bottom right), 273, 278, 279; Museum of Victoria, p. 109; Nature Focus/Australian Museum, p. 199; Nature Focus/C. Andrew Henley, p. 20; Oxford Scientific Films/Doug Allan, p. 190 (above); Oxford Scientific Films/Peter Parks, p. 266; Panos Pictures/J. Hartley, p. 372; PhotoDisc, pp. 377, 378 (top); The Photo Library/Nick Green, p. 167; The Photo Library/Gary Lewis, p. 165; The Photo Library/Michael McCoy, pp. 66, 76; The Photo Library/Science Photo Library, pp. 168 (left), 380, 382, 383; The Picture Source, pp. 34 (bottom), 154; The Picture Source/Richard Thom, p. 221; The Picture Source/Sherman Thomson, p. 136 (top left); G. R. (Dick) Roberts, p. 32 (bottom); Royal Melbourne Hospital/Arthur Wigley, pp. 368, 373; Science and Society Picture Library, pp. 297, 298; Barry Silkstone, p. 3 (top centre); Ken Stepnell, pp. 3 (top and bottom right), 157, 158 (left), 159 (right), 170, 185; Sydney Fish Market, p. 204; Andrew Tatnell, p. 194 (top); University of Melbourne/Richard Walters, pp. 48, 352 (bottom); University of Melbourne/George J. Wilder, pp. 87, 238, 240 (top); Dr. M. Vesk, pp. 53 (bottom), 54 (bottom); Professor P. Vickers Rich/Monash University, p. 201; Visuals Unlimited, pp. 177, 178, 319, 349; Visuals Unlimited/Michael Abbey, pp. 356, 359; Visuals Unlimited/Nancy P. Alexander, p. 372; Visuals Unlimited/Jack M. Bostrack, pp. 45 (bottom), 90; Visuals Unlimited/R. Calentine, pp. 224, 352 (middle bottom); Visuals Unlimited/George Chapman, p. 55 (left); Visuals Unlimited/John D. Cunningham, pp. 73 (top), 352 (top left); Visuals Unlimited/Don W. Fawcett, pp. 53 (top), 56 (left); Visuals Unlimited/Ken Greer, p. 352 (middle left); Visuals Unlimited/T. C. Malhotra, p. 337; Visuals Unlimited/Monsanto, p. 320; Visuals Unlimited/David M. Phillips, pp. 55 (top right), 63 (right), 179. Every effort has been made to trace and acknowledge copyright. The authors and publisher would welcome information from anyone who believes they own copyright to material in this book.

Chapter 1

A LOCAL ECOSYSTEM

An ecosystem is any environment containing living organisms that interact with each other and with the non-living parts of the environment. Ecosystems are largely self-sustaining, because materials and energy are exchanged between the organisms and their environment. Energy from sunlight enters the system through photosynthesis in plants, then flows through other living organisms via food webs. Materials such as carbon, nitrogen, oxygen and water cycle through an ecosystem, and can also pass from one ecosystem to another. The size of a population of organisms does not remain constant in an ecosystem. Populations can increase or decline dramatically. The contributing factors for this variation include disease, predation, competition, availability of resources, and increasing human activity and interference. Humans have often disturbed natural ecosystems to meet their own needs. The clearing of vast areas of forests and woodlands for agriculture is one example. The interactions between organisms and their environment are often complex and not immediately obvious. The study of ecology enables us to understand these interactions. Studying a local ecosystem can give an insight into how other ecosystems function. What is your local ecosystem? Carefully analysing the biotic and abiotic factors operating in your local area will allow you to identify and understand important biological concepts. You are encouraged to analyse and report on aspects of the local environment that have been affected by people, and suggest solutions to the problems that exist.

This chapter increases students’ understanding of the nature, practice and applications of biology.

1.1

Terrestrial and aquatic environments OBJECTIVES When you have completed this section you should be able to: ● distinguish between abiotic and biotic factors in the environment ● make comparisons between the abiotic factors of terrestrial and aquatic environments ● identify factors that affect the distribution and abundance of species ● suggest reasons for using different sampling techniques to estimate population size, for example, quadrats and the capture–recapture technique. ● describe the roles of photosynthesis and respiration in ecosystems ● state the equation for cellular respiration and understand that aerobic cellular respiration occurs through a chain of chemical reactions ● identify uses of energy by living organisms

The environment of an organism is its surroundings—everything around it, both living and non-living, that affects it. An ecosystem is any environment containing living organisms interacting with each other and with the nonliving parts of that environment.

The habitat of an organism is the place where it lives.

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The environment of an organism is its surroundings—everything around it, both living and non-living, that affects it. An ecosystem is any environment containing living organisms interacting with each other and with the non-living parts of that environment. An ecosystem can be any size, from a drop of rainwater to the whole Earth. It can be a pond, a forest, a desert, or a small area you are studying in a piece of bushland. The term ‘ecosystem’ tells us that it is being studied as a system. This system involves the exchange of materials and energy between organisms and their environment. Ecosystems are largely self-sustaining. Environments have abiotic and biotic factors. Abiotic means non-living; biotic means living. Abiotic factors include physical and chemical factors such as the temperature, rainfall, type of soil, and the salinity of the water. Biotic factors include all the living organisms, how many types there are, their numbers, distribution and interactions. The habitat of an organism is the place where it lives. The organisms which are found living together in a particular place form a community. The study of the relationships living organisms have with each other and with their environment is called ecology.

Australian environments Terrestrial environments are environments on land. Land covers about 35% of the Earth’s surface. Differences in the climate, the topography of the land, the availability of water, and human actions have produced many different terrestrial environments. They include rainforests, open forests, mountain tops, deserts, grasslands, heathlands, farms and cities. Some terrestrial environments in Australia are shown in Figure 1.1.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

FIGURE 1.1 Some Australian terrestrial ecosystems: (a) rainforest, (b) open forest, (c) desert, (d) mountain top, (e) grassland, (f) city, (g) farm.

A local ecosystem 3

Organisms that live in water live in an aquatic environment. Aquatic environments may be saltwater or freshwater. Saltwater environments include the open seas, estuaries and saltwater lakes. Oceans cover about 65% of the Earth’s surface. Tides, waves, currents and winds continuously move the water in the surface layers. Freshwater environments include still water such as lakes, ponds and swamps, and moving water such as springs, creeks and rivers. Some aquatic environments in Australia are shown in Figure 1.2.

(a)

(b)

(c)

(d)

(g)

(e) (f) (h) FIGURE 1.2 Some Australian aquatic ecosystems: (a) open sea, (b) estuary, (c) rocky shore, (d) saltwater lake, (e) coral reef, (f) freshwater lake, (g) swamp, (h) river.

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Abiotic characteristics Terrestrial and aquatic environments have very different abiotic characteristics. These differences mean that, in order to survive, animals and plants living in an aquatic environment will be very different from the animals and plants living in a terrestrial environment. These differences are summarised in Table 1.1. TABLE 1.1 A comparison of the abiotic characteristics of aquatic and terrestrial environments.

Characteristic

In aquatic environments

In terrestrial environments

Viscosity is a measure of how hard it is to move through a gas or a liquid (fluid).

Water has a high viscosity. This makes it more difficult for organisms to move through it.

Air has a low viscosity. This makes it easier for organisms to move through it.

Buoyancy is the amount of support experienced by an object immersed in a liquid or gas. It is equal to the weight of the liquid or gas displaced (Figure 1.3).

The buoyancy of water offers support to both animals and plants. It may help them to maintain their shape, and enables some organisms to function at different depths.

Animals and plants do not experience much buoyancy from air. They need to be able to support themselves.

Temperature variation The main source of heat is from the Sun’s radiation. The radiation intensity depends on latitude. It is greater at the equator than at the poles. Animals and plants can survive only within a certain temperature range.

Water heats up more slowly than air. Temperatures in the surface ocean layers vary from 30˚C at the equator to freezing point in arctic regions. However, the temperature in a particular region varies only a little from year to year. Deep waters everywhere are cold (Figure 1.4). Small bodies of water may show considerable daily and seasonal variation.

Surface temperatures on land vary far more than in water. The highest recorded is 60˚C, and the lowest is less than –80˚C. Daily and seasonal variations may be very great. Temperatures beneath the ground do not vary so much. The ability to avoid or tolerate heat gain and loss is important in land organisms.

Pressure variation The Earth’s gravitational field (the pull of gravity) gives rise to pressure differences between the upper and lower layers in both air and water. At any one level pressure is constant.

Pressure in water increases rapidly with depth. At a depth of 10 metres the pressure is twice that on the surface. Very few organisms live at great depths. Changing depth rapidly may be difficult for many organisms.

Atmospheric pressure decreases with height above sea level and also fluctuates over time. It may affect breathing by animals and flight.

Availability of gases Oxygen (O2) and carbon dioxide (CO2) are important gases for living organisms.

Gas availability in water is low and depends on the temperature. Diffusion is slower. More gases can be dissolved at lower temperatures. Oxygen concentration also decreases with depth. Oxygen availability affects the number and distribution of aquatic organisms, and also their body structure. Carbon dioxide dissolves in water to form carbonate and bicarbonate ions.

Gases are freely available in air and diffusion is rapid. Air contains about 20% oxygen and 0.03% carbon dioxide. The remainder is mostly nitrogen. Gas availability is not usually a limiting factor for land organisms, except at high altitudes.

A local ecosystem 5

Characteristic

In aquatic environments

In terrestrial environments

Availability of water

Water availability is rarely a problem in aquatic environments but the osmotic effects of fresh and salt water are important to organisms.

Water availability varies. The amount of rainfall and when it falls particularly affect plants. Obtaining water and preventing its loss may be a problem for all land organisms, especially in arid environments.

Availability of ions

Saltwater (marine) environments contain 3.5% dissolved salts— mostly sodium and chloride ions. Freshwater environments have a low ion concentration. Organisms need to be able to cope with any osmotic differences between their cells and the external environment.

Ions are available in the soil. The type and amount depend on the composition of the soil. Soil type and pH influence the type and amount of plant growth.

Light penetration Light received is from the Sun’s radiation. The light intensity is greater at the equator than at the poles.

Light falling on water may be reflected, scattered or absorbed. Light penetration in water decreases rapidly with depth (see Figure 1.5). Light availability affects the distribution of organisms in water.

Light can pass freely through air. Plenty of light is available to land organisms. Dense plant growth or topography may affect light penetration to some areas. The amount of light received is important for plant growth.

Availability and type of substrates There are many different types of rocks, soils, sands and other material formed from rock. They vary in their mineral and nutrient status.

Bottom-dwellers are affected by the type and amount of substrate available. Free-swimming and surface level aquatic organisms are less affected, although the amount of sediment (turbidity) in water is important.

The amount and type of soil is important for plant growth and for the provision of habitats for ground-dwellers and animals that live underground. The steepness and rockiness of the land is important.

Strength of natural forces

Tides, currents and waves may vary in strength according to the season and the weather. Some organisms cannot survive in moving water, while others cannot survive in still water.

Winds and rain vary in strength and duration according to the season and the climate. Many organisms cannot survive exposure to these factors in open environments.

Availability of shelter

Not all aquatic organisms require shelter. The substrate, rocks, vegetation and coral reefs may provide for those that do.

Most animals require shelter. Some plants will grow only in sheltered environments.

Availability of space

May be a limiting factor in some aquatic environments, especially for animals requiring territory.

May be a limiting factor on land for both plants and animals, particularly those requiring territory, shelter or nesting sites.

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upthrust from water

FIGURE 1.3 Buoyancy in water. The fish’s body is supported by buoyancy. If the upthrust is greater than the fish’s weight, the fish rises. If it is less, the fish sinks.

weight of fish solar radiation depth in metres 0 warm surface water 1000

zone of change

2000

3000 cooler, deeper water 4000

5000

layering in the ocean, east coast of Australia

(a)

(b)

FIGURE 1.4 (a) In large bodies of water, the surface layers of water warm up in spring and summer. In calm conditions two layers form: a warmer surface layer and a colder layer below. In autumn and winter, when air temperatures are colder and there is more turbulence at the surface, this layered effect disappears. (b) In very cold regions, a layer of ice forms on the surface of water in winter. This insulates the water below, allowing aquatic organisms to survive. depth in metres 3 5

LIGHT red orange yellow

10 green 20 blue 25 30

indigo violet

FIGURE 1.5 Light penetration in water. When light enters water, the different wavelengths are absorbed at different depths. Red, orange and yellow are absorbed first. The green, blue and violet wavelengths are absorbed with increasing depth. This is why the underwater world is dominated by blues and greens. Almost no light penetrates below a depth of about 30 metres.

A local ecosystem 7

Distribution and abundance The distribution of a species is all the places in which it is found.

Distribution The distribution of a species describes where it is found. No species is spread evenly through an entire natural ecosystem. Organisms occupy the areas where the biotic and abiotic factors of the environment suit them. They live where their chances of survival are high, where their requirements for survival are met, and where they are able to avoid predators. Figure 1.6 is a profile sketch showing where three different species were found by an ecologist who walked in a straight line for 140 m across a slight dip between two ridges at Myall Lakes, NSW. The places where the plants were found are indicated by shading. As you can see, closely related species (the two banksias) can occur in different zones. The boundaries between different species are not always as distinct as this; these three species seem to have quite different requirements for survival. Figure 1.7 is a plan sketch of the same area, giving us more information about the extent of the three species than the profile does. Banksia serratifolia Banksia asplenifolia

height above ground in cm

Boronia parvifolia

110

transect

50

0 20

40

60

80

100

120

140

metres FIGURE 1.6 Profile sketch of the distribution of three species of plants in a sample area at Myall Lakes, NSW.

FIGURE 1.7 Plan sketch of the area covered in Figure 1.6. (For clarity, the length has been compressed in this sketch.)

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Heinemann Biology

Banksia asplenifolia

Boronia parvifolia

transect 2 metres wide

Banksia serratifolia

0

metres

140

Abundance The abundance of a species means how many members of the species live throughout the ecosystem. Abundance is not the same throughout the area, and changes over time. A species will increase in abundance if the birth or germination rate exceeds the death rate; that is, if its resources are plentiful and there is not much predation or disease. Increases in the abundance of animals are caused by births and immigrations; decreases are due to deaths and emigrations. Plant abundances increase through the germination of seeds or spores, and decrease by plants dying or being consumed. Change in abundance is often represented graphically (Figure 1.8). 100

ferns

60

gras ses 40

mo sse

Ground cover (%)

80

s

20

l 0

e i ch

1

ns

c 2

3

nu o co

ee t tr

4 Time (years)

s

5

6

7

FIGURE 1.8 This graph shows the changes in abundance of various plants that became established on the island of Krakatoa after a volcanic eruption. Abundance is measured here by the amount of ground covered by each type of plant.

Factors affecting distribution and abundance Now try to think of as many factors as possible to explain the distribution and abundance of species. • Why do they occupy certain areas and not others? • Why aren’t there more (or fewer) individuals of each species? All organisms have their own requirements for successful survival and maximum growth and development. The word ‘resources’ is often used in reference to factors affecting distribution and abundance of organisms. Resources are anything in an environment that organisms use. Resources are usually limited; organisms that need the same resource will be in competition. Plants need light and water for photosynthesis; their rate of growth depends on temperature and soil or water quality. The distribution and abundance of plants directly affect the distribution and abundance of animals. Animals depend on plants for food; they may also need shelter and nesting sites. Organisms are only found in areas that can supply their needs. The following is a list of some factors, both abioic and biotic, which may affect the ability of an organism to survive in an ecosytem.

Abiotic factors • amount of light • amount and strength of wind and rainfall A local ecosystem 9

• • • • • • •

temperature: daily and seasonal variations effect of topography, altitude and depth strength of tides, currents and waves water: amount, salinity, pH and availability type and availability of substrate availability of space and shelter oxygen availability

See Table 1.1 on pages 5 and 6 for details of these factors.

BIOFACT When white clover and strawberry clover are planted together in the same field, the strawberry clover with its longer leaf stalks grows taller, overtopping the shorter white clover. Strawberry clover competes better for space and shades out the other species, eventually excluding it from the field.

Biotic factors • seasonal availability and abundance of food for animals—suitable plants for herbivores and suitable prey for carnivores • number of competitors—these may be from the same species or other species with similar requirements; the birth rate and death rate of a species may be important here • number of mates available—animals need to find mates for the species to survive and reproduce in a given ecosystem • number of predators • number and variety of disease-causing organisms In your field studies you will have to measure some of these factors. Those which are significant for the organisms you are studying will depend on the ecosystem you are investigating and the types of organisms.

Distribution and abundance: case studies Your practical work will include measuring the distribution and abundance of a named species in the field. The following examples show how a knowledge of the distribution and abundance of organisms can help us understand their requirements, contribute to our knowledge of the complex interactions of ecosystems, and even perhaps help us to prevent their extinction.

Case study 1: the humpback whale The humpback whale (Megaptera novaeangliae) is a large aquatic mammal up to 15 metres long, with a humped back that carries a small fin. The tail and long flippers have white patterns or markings that are unique to each whale. The head, jaws and flippers also have noticeable bumps or knobs on them. Humpback whales are filter feeders. Instead of teeth they have baleen plates on their upper jaw that sieve or filter small organisms from the water. An adult female produces a single offspring every 2 to 3 years. The estimated life span of a humpback whale is more than 70 years.

Distribution FIGURE 1.9 A humpback whale ‘breaching’. Note the bumps on the jaws and flipper.

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Humpback whales have a world-wide distribution. There are two distinct large groups—the northern hemisphere and southern hemisphere humpback whales—and each group is made up of different geographical populations. All the populations are migratory, moving from cold

Summer feeding areas Winter breeding areas Migration routes

FIGURE 1.10 Humpback whale migration and distribution patterns in the Australasian region.

subpolar feeding grounds in summer to warm tropical breeding grounds in winter (Figure 1.10). In Australian waters there are two distinct populations: those that migrate along the coast of Western Australia and those that migrate along the eastern coast.

Abundance The humpback whale is an endangered species. It was hunted extensively throughout its range until the mid 20th century. The western Australian humpback whales were reduced from about 17 000 to fewer than 1000 by 1962, when whaling in Australian waters was banned. Their numbers have now recovered to about 5000. The eastern Australian population fell from an estimated 10 000 whales to between 200 and 500 by 1962. The population is now increasing, and is currently about 2000 to 3000 whales. Estimates of abundance are conducted by shore, aerial and waterbased observations (whale-watching), including photographic identification of individual whales. This method is possible because the migration routes of the whales is known. Capture–recapture using tags is possible where the migration movement of a group of whales is unknown. The recovery rate of tagged whales has, however, been very low.

Special factors affecting distribution and abundance We still know very little about the behaviour and migration of humpback whales, and the factors that control their population numbers in the wild. Arctic and Antarctic waters are rich in krill (small shrimp-like crustaceans) that are the main food of the humpback whale. In summer the whales feed and grow before moving to warmer waters where their young are born and where they mate. Some of these migrations cover several thousand kilometres. Their numbers declined so drastically when hunted by humans because of the ease with which they were caught. Their feeding, mating and calving grounds are usually found close to shore, they are found in groups, and they are slow swimmers. The humpback whale’s low reproductive rate makes the recovery of populations slow.

1

Explain why there is an international ban on whaling of many species of whales, including the humpback whale.

2

Suggest some of the factors, other than human activity, that could control the abundance of humpback whales in the world.

3

Whale watching has become a popular tourist attraction in many parts of the world. What concerns might scientists have about how this new human activity could affect the numbers of humpback whales? A local ecosystem 11

Case study 2: the bush rat

(a)

Bush rats (Rattus fuscipes) are inconspicuous, nocturnal native rodents. Each individual lives in a burrow and has its own home range or area. Bush rats are placental mammals, not marsupials. They can breed all year round although most young are born in late spring and summer. Usually about five young are born in each litter. Their life span is, on average, one year.

Distribution Bush rats live mainly in coastal areas. Only in south-eastern Australia are they found more than 100 kilometres inland. They live in many different habitats, from dry, sandy hills to tropical rainforests, but they are more common in open forests, particularly where there is dense undergrowth, and near watercourses. There are four distinct subspecies, and their ranges do not overlap (Figure 1.11).

Abundance (b)

4

1

2

3

1 Rattus fuscipes fuscipes 2 Rattus fuscipes greyi Both these rats are small with long, soft grey-brown coats 3 Rattus fuscipes assimilis a large rat with a dark-grey shaggy coat 4 Rattus fuscipes coracius a large rat with a reddish-brown, short, smooth coat FIGURE 1.11 (a) The bush rat, Rattus fuscipes. (b) The distribution of the bush rat.

Although bush rats are widespread within their habitats, their population densities tend to be quite low. Numbers estimated using trapping techniques (see p. 14) were between 11 and 22 rats per hectare, depending on the type of forest habitat. Population numbers were found to be lowest in winter and highest in summer, after the main breeding season. Bush rats living on islands off the coast tend to have higher population densities. Estimates from trapping these rats range from 60 to 150 animals per hectare. A likely reason for this is the lack of predators, such as foxes and cats, on the islands.

Special factors affecting distribution and abundance Bush rats can live successfully in a wide range of habitats, limited only by their need for water. They can survive only four days without water. They eat a broad range of foods. Their diet may include insects, fungi, plant materials such as seeds, mosses, roots, berries, grasses and herbs, and occasionally small vertebrates, e.g. mice. They have a high reproductive capacity (females can breed at 3 months old and gestation lasts 22–25 days) and they can reproduce all year round if environmental conditions are favourable. 1

What factors control the bush rats’ numbers in the wild?

2

Do you think bush rat numbers could ever reach plague proportions as those of some rats and mice do?

3

What influence do you think climate plays in the distribution of bush rats?

Population estimates A group of similar organisms living in a given area at the same time is known as a population.

12 Heinemann Biology

A group of similar organisms living in a given area at the same time is known as a population. In your field studies you will measure the distribution and abundance of different populations in your chosen ecosystem.

This could be done directly but usually involves making population estimates. The reason that population estimates are made is because of the difficulty in describing in detail any large area. It would be impractical and time-consuming to count every living thing, even if this were possible. It would also cause considerable damage to the environment! Scientists have developed methods called sampling techniques to make estimates of the distribution and abundance of species.

Measuring distribution When studying organisms in the field, it is usual to describe their distributions on a sketch of the area, as in Figures 1.6 and 1.7 (page 8). The sketch may be an aerial view (plan) or a side view (profile) that you draw yourself. If the area is too large to be drawn entirely, take a narrow section of it, called a transect. This is a strip that crosses the entire area from one side to the other. You work only within the transect, recording what organisms are found as you cross the field. On your sketch you indicate the types or names of organisms found in each place.

activity Methods of estimating population numbers

A transect is a narrow strip that crosses the area being studied, from one side to another. Transects are used mainly to study the distribution of species in the area being studied.

FIGURE 1.12 A transect is a narrow strip through the area you are studying.

Measuring abundance You could measure the abundance of a species simply by counting all the individuals that occur in the area. If a certain tree is scattered through the area you are studying and the area is not too large, it may be quite realistic to walk around the area recording each tree as you find it. But this would be very time-consuming and difficult to do if there were a lot of species to measure. Plants and small invertebrates may be too numerous to count reliably. Many animals are not always visible and may move rapidly. The technique used to estimate abundance needs to be carefully chosen, and often depends on the type of organism you are studying.

A quadrat is a small area that represents the larger area being studied. Quadrats are used mainly to study the abundance of species in the area being studied.

Plant abundance A useful method in a small area is to estimate the percentage cover. To record the abundance of grass in a field, or algae in a pool, simply estimate how much of the area is covered by the plant and record it as a percentage. Often, however, this is impossible to do. When organisms are very numerous or scattered over a huge area, it is quicker and easier to study small, randomly chosen areas of vegetation, called quadrats,

FIGURE 1.13 Using a ready-made frame to mark out a 1 x 1 metre quadrat.

A local ecosystem 13

If your quadrat is one metre square you can easily calculate the number of organisms per square metre, and from this estimate the total in the area you are studying.

BIOFACT Biologists cannot always catch the animals they are studying, so they use many other techniques for finding out what animals are present in an area. They include: • scat analysis: analysing animal droppings (scats) to find out what animal made them and (if it is a predator) what animals it had eaten • tracks: checking tracks in sandy or muddy areas to see what animals made them • bones: identifying animals from old bones (skulls are best, especially if they still have teeth) • call recognition: analysing animal calls to identify the species (especially birds, bats and frogs) • photo traps: still or video cameras can be set up to record every animal that passes by • hair-tube analysis: special traps are set up to catch samples of fur from animals that brush past; the fur can be analysed to find out the species it came from.

and count the numbers or estimate the percentage covers in them. By comparing the size of the quadrats to the total area, we can estimate the total abundance. Quadrats can be any size, depending on the type of vegetation, but they are usually between 1 and 50 metres square. If you are studying a 1 ✕ 1 metre quadrat, you can use a ready-made square frame, so that every quadrat will be exactly the right size (Figure 1.13).

Animal abundance If the species we are studying is constantly on the move, it may be easier to use the technique which is known as capture–recapture. With this technique you capture and tag a sample (say, 5) of the species then release them. After they have had time to mix, you recapture a sample (say, 10) and count the number of tagged animals. If only one of them is tagged, it is reasonable to expect that the original 5 tagged animals represent 10% of the total population. So the total population is probably about 50 (Figure 1.14). abundance =

number captured

×

number recaptured

number marked in recaptured Capturing animals requires various trapping techniques. These include using specially designed traps and nets which catch animals alive and unhurt, digging small pits into which small animals will fall, and using spotlights at night and then catching the located animal with a net.

tag a sample of 5 ‘X’ and release them

x xx x x

x

x

x recapture a sample

x FIGURE 1.14 The capture–recapture method. Tagged animals are shown as ‘X’, and others are shown as dots.

x

1 tag in 10 = 5 tags in 50 Total pop. = 50

Photosynthesis and respiration Photosynthesis is the process by which plant cells capture energy from sunlight and use it to combine carbon dioxide and water to make sugars and oxygen.

14 Heinemann Biology

Photosynthesis is the process by which plant cells capture energy from sunlight and use it to combine carbon dioxide and water to make sugars and oxygen (see Chapter 2, p. 68). All living things ultimately depend on this process. The compounds plants make during photosynthesis provide nutrients and energy to organisms that consume plants. Organisms that

consume the plant-eaters gain nutrients and energy from them, so both energy and materials are passed from organism to organism. Respiration is the process by which cells obtain energy. In this process organic molecules, particularly sugars, are broken down to produce carbon dioxide and water, and energy is released. When we look at these two processes, they appear to be almost the reverse of each other. However, this is not true: the sequence in one is not the reverse in the other. The processes themselves are related because energy from the Sun is incorporated into the products of photosynthesis, which are used by plants. When animals consume plants, they obtain nutrients that are used in respiration in their body cells so that they too can obtain energy. This energy drives all the metabolic processes in an organism such as growth and repair, and ultimately also drives ecosystems. In photosynthesis, plants capture light energy and transform it into chemical energy. This chemical energy, which is stored in complex organic compounds, is transferred from plants to animals via food chains. In the process of respiration, which releases energy for organisms to use, some of the chemical energy is transformed to heat energy and lost. In an ecosystem there is no re-use of energy: it is either used by a living thing or lost as heat. Because of this, a continual input of energy is needed to keep living systems functioning.

Aerobic cellular respiration All living cells need energy all the time to stay alive. Energy-rich food materials, either made by a plant during photosynthesis or consumed by an animal, are used by cells in the process of aerobic respiration. Respiration involves a series of chemical reactions. It is a controlled process, occurring as a sequence of about 50 different reactions, each one catalysed by a different enzyme. Energy is released slowly in small amounts. The chemical energy held in the bonds of complex organic molecules, such as sugar, is released when the bonds are broken. The energy is transferred to the energy carrier molecule ATP. ATP is produced at several points along the way. The process begins in the cytoplasm, but most ATP comes from the steps that occur in a cellular organelle called a mitochondrion. Glucose is a carbohydrate containing carbon, hydrogen and oxygen. When glucose is broken down, carbon dioxide and water are formed. We can represent respiration by the following generalised equation: glucose + oxygen → carbon dioxide + water + energy ATP is the energy store of the cell. When energy is available, ADP collects it. When energy is needed, ATP supplies it. In fact, respiration can be thought of as the process by which ATP molecules are made in a cell. For each molecule of glucose that is broken down to carbon dioxide and water, about 38 molecules of ATP are produced. ADP + P + glucose + oxygen

many reactions

Respiration is the process by which cells obtain energy.

Energy is needed to sustain ecosystems. The ultimate source of all energy for life on Earth is the Sun. Plants use chlorophyll to capture some of the Sun’s energy in photosynthesis. This energy then flows through ecosystems and keeps them functioning.

Aerobic means requiring the presence of oxygen. An enzyme is a substance that alters the rate at which a reaction occurs, but is not used up in the reaction.

BIOFACT The ATP (adenosine triphosphate) molecule consists of a base–sugar group (adenosine) linked to three phosphate groups: A–P–P–P If either of the two end phosphate groups is removed, a large amount of energy is released from the bond. When ATP detaches one phosphate group, ADP (adenosine diphosphate) forms and energy is released. The reaction is reversible: if enough energy is available, ADP and phosphate can combine to form ATP.

ATP energy out

energy in

carbon dioxide + water + ATP

38ADP + 38P + C6H12O6 + 6O2 → 6CO2 + 6H2O + 38ATP

Stages of respiration The process of respiration can be thought of as occurring in two stages. The first stage occurs in the cytoplasm of the cell and results in the

ADP +P ATP is the form in which energy is carried in cells and made available when needed.

A local ecosystem 15

splitting of the 6-carbon sugar molecule into two 3-carbon molecules. This process involves at least 12 steps. The 3-carbon molecules formed are called pyruvate, and two molecules of ATP are gained. The second stage occurs in the mitochondria (see Chapter 2 p. 54). It involves the use of oxygen and results in the complete breakdown of pyruvate into carbon dioxide and water. A total of 36 molecules of ATP can form in this series of reactions. The energy is released gradually; it is ‘packaged’ as ATP and released at several points in the pathway. enzymes 2 x C3

CO2 + H2O

36 ATP molecules released at intervals

Overall, 40% of the energy in glucose is converted to ATP. The rest is lost as heat.

C6

enzymes

How energy from respiration is used

2 ✕ C3 2 ATP

Some of the energy from respiration which is released as heat is useful because cells and enzymes function best at warm temperatures in endothermic animals (see Chapter 5, p. 217). The energy that is released as ATP is used by organisms in a number of cellular processes:

2 (ADP + P)

BIOFACT Aerobic respiration is also called ‘oxidation’ because it uses oxygen. In oxidation, oxygen is added or hydrogen removed from a substance. If you burned sugar in the laboratory, oxygen from the air would be used up. The reaction would form carbon dioxide and water, and energy would be released as light and heat.

• synthesis of complex molecules such as proteins, lipids, carbohydrates and nucleic acids • growth involving the division, elongation and differentiation of cells • repair and maintenance of damaged or old cells • active transport of materials across cell membranes • functioning of special cells that need extra energy, such as nerves, muscles, liver and kidney in mammals • transport of materials within organisms, such as in the phloem of plants (see p. 88) and circulatory systems of animals (see p. 90).

Questions 1

Define the term ‘ecosystem’.

2

a Describe the differences between an organism’s environment, habitat and community. b Define ‘ecology’.

3

The following features are part of a granite hilltop ecosystem. Identify which are biotic factors, and which are abiotic. rain

snow-grass

16 Heinemann Biology

snail

air

sunlight

earthworm dragonfly eucalypt wind temperature frog phosphorus rock wallaby butterfly soil moss shelter altitude predator bacteria 4

a Distinguish between the ‘distribution’ and the ‘abundance’ of a species. b List some factors that affect the distribution and abundance of a named aquatic organism and a named terrestrial organism.

5

6

a Define what is meant by the ‘resources’ that an organism needs. b Make a list of the general resources that must be available to i a plant ii an animal if they are to survive in their environments. a List the reasons why populations of organisms are usually estimated rather than counted. b Explain how transects and quadrats can be used to estimate the distributions and abundances of organisms. Use diagrams in your answers.

c Animals cannot always be seen or captured during field studies. Describe other methods scientists can use to determine the presence and abundance of animals. 7

a Define ‘photosynthesis’. b Explain the significance of photosynthesis for organisms in ecosystems.

8

a Describe the process of aerobic cellular respiration and write down the balanced chemical equation for this process. b List the ways living organisms use the energy made available through cellular respiration.

F u r ther questions 1

alpine grassland rainforest open forest woodland desert mangrove swamp rocky shore heathland saltwater lake a Name a national park in New South Wales where this ecosystem occurs. b Describe the abiotic factors of the ecosystem. c List some of the kinds of plants and animals that you would expect to find there. Study the following graph, which shows changes in the abundance of insects on eucalypts. The abundance of the insects is normally stable, at a level well below the highest number that the eucalypt forest can support. The population is kept in check by two predators: one is a tiny parasite that kills the young insects; the other is a bird that eats the adults. At point X on the graph, on the coldest winter’s night for many years, all the parasites were killed.

Population density

2

a What would have happened to the number of young insects at this point? b What effect would this have had on the supply of eucalypt leaves? c Suggest two reasons for the sudden ‘crash’ in the number of insects.

Choose one example from the following list of Australian ecosystems.

3

An ecologist studying a population of platypuses in the Shoalhaven River in New South Wales used the capture–recapture method, and calculated that there were 3 platypuses in the river. On her next visit to the river she estimated the population to be 11, using the same technique. Suggest why the second results were much higher than the first.

4

Aerobic cellular respiration involves a number of complex chemical reactions. Use a branching flow chart to summarise the process. Include: • the two stages of the process • where each stage occurs • the compounds that result from each chemical pathway • the amount of energy (ATP) that is made available.

5

Explain the relationship between the processes of photosynthesis and cellular respiration.

control level X Time

A local ecosystem 17

1.2

Local ecosystems: interactions and responses OBJECTIVES When you have completed this section you should be able to: ● explain the short-term and long-term consequences on ecosystems of competition for resources between species ● describe the role of decomposers in ecosystems ● use examples to describe the relationships between producers, consumers and decomposers in food chains and food webs ● use food chains, food webs and biomass pyramids to explain trophic interactions between organisms in ecosystems ● outline the factors that affect the numbers of predators and prey in an ecosystem ● recognise examples of parasitism, commensalism, mutualism and allelopathy in ecosystems and describe the roles of the different organisms in these associations ● define the term ‘adaptation’ and be able to identify and describe adaptations of plants and animals to environmental factors ● recall and analyse the impact of the activities of humans on ecosystems ● describe the relationship between different kinds of pollution and contamination of the environment ● discuss strategies aimed at balancing the needs and activities of humans with the need to conserve, maintain and protect the environment.

Interactions between organisms activity A local ecosystem field study and report

18

Heinemann Biology

Organisms that live in the same ecosystem may either increase or decrease each other’s chances of survival. These interactions can be between members of the same species or members of different species.

Population trends When the number of organisms in a population increases dramatically in a short time, we say there has been a ‘population explosion’.

BIOFACT A crab has recently been found which lives in coral and attacks the crown-ofthorns starfish.

Populations of organisms do not remain at a constant level within an ecosystem. Many factors may affect their numbers. When the same species is found in an ecosystem year after year in approximately the same numbers, scientists say the population is stable or in balance. In other words, the resources required by that species are sufficient to maintain steady population numbers within that ecosystem. Sometimes the numbers in a population increase dramatically; we refer to this as a population explosion. Population numbers may also decline. Disease, predation, competition from other species, and human activities can all contribute to the decline and possible extinction of an organism in an ecosystem. The population of most Australian mammals has fallen dramatically over the past 200 years, and many populations have reached levels where their survival is endangered.

Exploding starfish! Population explosions of the crown-of-thorns starfish (Acanthaster planci) on the Great Barrier Reef have damaged coral reefs. Adult starfish eat anemones and the soft-bodied polyps of corals, leaving behind the skeleton of the coral. The starfish has poisonous spines. This feature reduces the chances of it being caught and eaten. Why do starfish populations explode? No-one knows exactly, but infestations appear to be due to

natural causes. Heavy rains following dry weather result in higher than normal levels of nitrates and phosphates being washed into coastal waters. This might be worsened by the use of fertilisers on farms, and by the disturbance of soils. Higher nutrient levels enable more phytoplankton to grow, and this provides more food for the crown-of-thorns larvae. As a result, more starfish survive to the adult stage.

FIGURE 1.15 The crown-of-thorns starfish, Acanthaster planci. The right-hand photograph shows the underside of the animal, with the suckered tube feet and central mouth.

1 Explain what appears to be the main factor causing this population explosion.

A local ecosystem 19

Even in stable populations, numbers are never constant. They show cyclical or periodic changes, usually related to the availability of food and water, predation levels and the species’ own reproductive rates. An individual species may show regular fluctuations in numbers, because of the changing availability of food throughout the year. Abiotic factors such as drought or unseasonably low temperatures may reduce plant growth levels. Sufficient water and warm temperatures may produce a surge in plant growth, making more food available for animals. Animals may move in and out of ecosystems according to the availability of food. The reproductive cycle of organisms also causes numbers to rise and fall, usually in an annual cycle. An increase in the number of plants in an area is usually followed by a rise in the numbers of one or more animal species. Can you think why this might be? In your field studies on distribution and abundance of organisms in your local ecosystem, it is important to look for trends in population numbers and to try to explain them.

The decline of a wombat The northern hairy-nosed wombat, Lasiorhinus kreftii, is a heavily built marsupial weighing about 35 kg (Figure 1.16a). It has a distinctive nose covered with short brown hair. It has soft fur, mainly brown but mottled with grey, fawn and black. It has black patches around its eyes, and its ears are slightly long and pointed, with white fur on the edges. Its short, strong legs and claws are used to dig burrows or to help it collect plants, including the native grasses that form much of its diet. The northern hairy-nosed wombat is active at night. Although it tends to live by itself, it often shares

(a)

burrows with other wombats. The female gives birth to a single young during the wet season (November –April). The young wombat stays in the mother’s pouch for up to 9 months, and finally becomes independent at about 12 months. Fossil records show that the northern hairy-nosed wombat once inhabited a wide area. But since the 1800s it has been found in only three areas: near Deniliquin, in south-central New South Wales; Moonie River near St George, south Queensland; and Epping Forest, near Clermont in central Queensland. Today, the species is endangered. It is

(b)

FIGURE 1.16 (a) The northern hairy-nosed wombat, Lasiorhinus kreftii, and (b) its past (pink areas) and present distribution (arrow).

20 Heinemann Biology

known to occur only in the Epping Forest area, which is now within Epping Forest National Park.

Causes What caused this species to become endangered? The main reason is the loss of its natural habitat caused by farming. Competition with rabbits, sheep and cattle for food, and the effects of long droughts, have led to the decline of the wombat. The small populations that remain are susceptible to disease, fire and inbreeding, and also to predators such as dingoes and foxes.

Solutions In 1971 the Epping Forest National Park was established to protect the habitat of the northern hairynosed wombat. The park is fenced to keep out sheep and cattle. In 1982 cattle were removed from the

park, and by 1989 wombat numbers had increased from 35 to about 70. Programs to control introduced grasses aim to improve the supply of native grasses, which provide more nutrition for the wombats. Access to the park is restricted to park management staff and researchers, who are examining the population and feeding ecology of the wombats. In particular, they are monitoring the size and distribution of the population, the wombats’ diet, body condition and activity pattern. They determine the extent of genetic variation within the population, and the amount of inbreeding that is occurring. The effects of predators and competitors are also being monitored. Today there are 62 wombats in the park, including only 15 females capable of breeding. A captive breeding program is now being undertaken to try to rescue the northern hairy-nosed wombat from the edge of extinction.

1

Give three reasons for the decline in numbers of the wombat population.

2

Identify strategies being used to halt the decline in wombat numbers.

FIGURE 1.17 The laughing kookaburra (Dacelo gigas) is a common predator in forests and woodlands. It sits on a low branch until it spots its prey, which might be a small snake, lizard, rodent or worm, then swoops quickly to the ground and seizes the prey in its powerful beak.

Predation This is a feeding relationship in which one animal, the predator, obtains its food by killing another animal, its prey (Figure 1.17). This relationship increases the predator’s chance of survival and reproduction at the expense of the prey’s. Predator–prey relationships may be one reason to account for fluctuating population numbers in an ecosystem. Rises and falls in predator numbers usually follow rises and falls in prey numbers. The numbers of predators and prey in an ecosystem depend on a number of factors: • The size of any given ecosystem will determine how many organisms it can support. • There are usually fewer predators than prey. • The availability of the prey’s food will largely determine the number of prey at any given time. This may depend on factors such as the time of year and the weather.

Predation is a feeding relationship in which one animal, the predator, obtains its food by killing another animal, its prey.

A local ecosystem 21

• The reproductive cycles of both prey and predator affect their numbers. Large numbers of young prey may be followed by an increase in predator numbers. The prey are eaten, their numbers decline, and numbers of predators then also decrease. • There is often competition between predators for the same prey. For example, cormorants and gannets compete with large fish for the smaller fish they all prey on. • Diseases can affect both predators or prey. If prey are affected, then the food supply for predators will be less and they will also decline in number. If predators are affected, the numbers of prey will increase. • Seasonal migrations of predators or prey will affect populations. When prey are abundant, some predators might move into an ecosystem to take advantage of the abundance.

zone of inhibition

FIGURE 1.18 Allelopathy. A chemical produced by the blackbutt, Eucalyptus pilularis, prevents seedlings from germinating close to the parent plant.

Sucker and hooks attach the parasite to the inner wall of the host’s gut.

Segments full of eggs can detach and leave the host’s body in the faeces. FIGURE 1.19 The tapeworm is an example of a parasite. The tapeworm gains nutrients and a safe habitat within its host, while its host is usually seriously ill and receives no benefits at all.

22 Heinemann Biology

Allelopathy Allelopathy is the production by a plant of specific chemicals (allelochemicals) that can be detrimental or beneficial to another plant. These chemicals influence the growth and development of neighbouring plants by repelling predators and parasites, or poisoning competitors. The visible effects on other plants nearby may include prevention of seed germination, deformed roots (including the death of root tips and lack of root hairs), slow growth and poor reproduction. Camphor produced in leaves of the camphor laurel tree accumulates in the soil, preventing germination or growth of seedlings around each established group. In a similar way, blackbutt eucalypts (Eucalyptus pilularis) inhibit the germination of their own seeds close to the parent plant. Few trees live happily with grass. The competition is particularly noticeable on cleared or burnt ground. The woollybutt or alpine ash, Eucalyptus delegatensis, a forest tree of the mountains of southeastern Australia, has its growth inhibited by chemicals produced by tussock grass. If the woollybutt seeds germinate in the forest underneath the parent trees, they remain healthy but grow only very slowly, often remaining under a metre in height for over 40 years. When overhanging shade is removed—by the death and fall of a large tree, for example—they respond immediately with vigorous growth. If the forest is burnt or cleared, however, germinating woollybutt seedlings grow well until grass cover develops. If there are a sufficient number of seedlings close together, their leaves provide enough shade to suppress the growth of the grass and they will continue to grow and survive. But if they are widely separated, the grass continues to thrive and the trees have their growth inhibited, so that many die. The result is that a natural patchwork develops in the area, with some patches dominated by tussock grass and others dominated by trees.

Parasitism A parasite obtains its food from a host. Although the host is harmed in some way, it does not necessarily die. Most free-living organisms have parasites. Many bacteria, viruses and fungi which cause diseases are parasites. Other examples of parasitic relationships involve ticks, fleas and tapeworms (Figure 1.19). The host also provides a safe place to live for the parasite. The parasite benefits and may increase its own chance of survival and reproduction at the expense of its host.

Symbiosis Symbiosis is a type of interaction between organisms where two different species live together in a close association. The association benefits at least one of them, and the other is not disadvantaged. The two common types of symbiosis are commensalism and mutualism.

Symbiosis is a type of interaction between organisms where two different species live together in a close association.

Commensalism A relationship that benefits one species and does not harm the other is known as commensalism. The organisms are not dependent on this type of relationship: they could survive without each other (Figure 1.20). Mutualism If two organisms are more closely associated so that both benefit, the relationship is called mutualism. For example, lichens are an association between an alga and a fungus (Figure 1.22). The cells of the alga are found amongst the hyphae of the fungus. The alga provides food and oxygen by photosynthesis and the fungus provides a moist environment for the alga. Together they can survive in environments where at least one, if not both, could not survive on their own. A mutualistic relationship exists between emus and the nitre bush. Emus live almost exclusively on the fruit of this bush when it is available in late summer. This benefits the nitre bush because digestion of the seed coat increases its rate of germination—only 6% of seeds falling from the bush germinate, compared to 62% of seeds passed out by emus. In some cases the relationship is so strong that survival of one member is impossible without the other. For example, termites chew and swallow wood but cannot digest the cellulose. Protozoans living in their gut do this for them. The termite is provided with food, and the protozoans obtain food and a safe place to live (Figure 1.21).

FIGURE 1.21 The gut of the termite contains microscopic protozoans (inset) that digest the cellulose that the termite eats.

FIGURE 1.20 An example of commensalism. The ferns growing on the tree trunk are epiphytes. They are supported by the tree but do not obtain food from it, and they do not affect the health of the tree.

FIGURE 1.22 An example of mutualism. A lichen consists of an alga and a fungus growing together as a single organism. Both the alga and the fungus benefit from this arrangement.

A local ecosystem 23

(a)

(b)

FIGURE 1.23 Two examples of animals that form groups: (a) beaked mussels (Brachidontes rostratus), and (b) magpie geese (Anseranas semipalmata).

Living in groups predator

detritivores

Organisms of the same type are often found in groups (Figure 1.23). Some plants may form large groups, or even dominant communities, in ecosystems. This increases the chances of pollination, and may reduce the risk of damage to individual plants. For animals, living in groups may improve an individual’s chances of finding a mate, avoiding predators and getting enough to eat.

The role of decomposers

decomposers

FIGURE 1.24 Detritivore and decomposer food chains in the pond start with dead organic matter.

Trophic interactions are feeding relationships between organisms.

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Decomposers have an important role to play in ecosystems: they absorb nutrients from dead organisms or waste materials and return organic matter to the soil. Detritus is dead organic matter derived from plants, animals or other organisms. This material is eaten by detritivores such as snails, worms, termites, millipedes and mites, and by decomposers—bacteria and fungi. Detritivores play a critical role in breaking down organic matter into smaller particles that are finally consumed by decomposers. Bacteria and fungi secrete digestive enzymes that break down detritus into soluble organic molecules such as sugars, and eventually into inorganic molecules such as carbon dioxide and nitrogen. Many food chains and webs involving decomposers exist in forests, woodlands and other terrestrial ecosystems where dead plant matter accumulates, and also in freshwater and saltwater aquatic ecosystems.

Trophic interactions Trophic interactions are feeding relationships between organisms. ‘Trophic’ comes from the Greek word trophikos, meaning ‘relating to food’.

detritivore

detritivore predator

detritus (dead matter)

decomposers FIGURE 1.25 Detritivore and decomposer food chains are very important in terrestrial ecosystems such as forests, where plant litter accumulates on the ground.

Food chains A food chain represents the flow of energy from one living thing to the next. green plant → plant-eater → animal-eater Food chains in the ocean are usually longer than terrestrial ones. For example: phytoplankton → zooplankton → small fish → bigger fish → even bigger fish → shark Food chains begin with producers, which are usually plants or algae. Plant-eaters are herbivores. Animal-eaters are carnivores. Animals that eat both plants and animals are omnivores. Herbivores, carnivores and omnivores are all consumers. The first consumer in a food chain (a herbivore) is called a primary consumer; it is eaten by the secondary consumer, which in turn may be eaten by the tertiary consumer, and so on. Scavengers are consumers that eat dead animals. Decomposers are organisms such as fungi and bacteria that cause decay. Decomposers have an important role to play: they make the materials produced by decomposition available to plants. It is then recycled into the food chains.

A local ecosystem 25

Food webs In most ecosystems there is more than one primary consumer, and animals often eat more than one thing. To show the complex feeding interactions in an ecosystem, we use a food web. The role of an organism in a series of relationships is its niche. For example, in Figure 1.26 the fish occupies a carnivore niche.

FIGURE 1.26 A food web in a pond ecosystem. ‘Plankton’ means all the floating microorganisms in the water. These may be either plants (phytoplankton) or animals (zooplankton). ‘Detritus’ means decomposing matter, including dead plants and animals.

26 Heinemann Biology

koala

honeyeater

possum

leaves

aphids

stick insect fruits and flowers

mistletoe bird

flying fox

mistletoe

kookaburra

moth

wagtail

bees

borer

sap dried leaves

earthworm

cat

FIGURE 1.27 A food web in a forest ecosystem.

A local ecosystem 27

solar energy

sea birds

fish

zooplankton

fish

meat and fat humans

seals and whales

algae (phytoplankton)

viscera and meat

huskies walrus molluscs (oysters, clams) FIGURE 1.28 A food web in an Arctic ecosystem. The highest-order consumer is the human.

Biomass is the amount of living material in an organism or group of organisms at any one time.

20%

sunlight 39% 40%

39% evaporates water from plant 1% used in photosynthesis 20% is reflected from plant

40% warms soil, air and vegetation FIGURE 1.29 Absorption of energy from sunlight by the leaf of a photosynthetic plant. Only 1 per cent of the available energy is used for photosynthesis. The rest is reflected from the leaf or passes through it.

28 Heinemann Biology

Biomass and energy pyramids Producer organisms trap energy from sunlight and use it to make simple organic compounds in the process of photosynthesis. These compounds are then used to build the more complex compounds that make up plant tissues. Less than 1 per cent of the Sun’s energy that reaches the Earth’s surface is used for photosynthesis (Figure 1.29). Biomass is the amount of living material in an organism or group of organisms at any one time. The amount or mass of living plant material produced by photosynthesis in an ecosystem is the plant biomass. Because they are producers, plants have the greatest biomass of all organisms in terrestrial ecosystems. It has been estimated that only about 10 per cent of the available plant biomass is consumed by animals. Of this, most is lost (Figure 1.30), so the herbivore biomass is much less than the plant biomass. Similarly, carnivores have less biomass as a group than herbivores. The biomass therefore decreases along the food chain. Herbivores have a greater biomass than carnivores, and the top consumer usually has the lowest biomass (Figure 1.31). At each step in a food chain, energy is also lost. The amount of energy at each trophic level can be shown by an energy pyramid. Energy pyramids look very similar to biomass pyramids because the energy is transferred in food that makes up the biomass. Energy pyramids indicate that the further along a food chain an organism is, the less energy is available to it (Figure 1.32).

Both energy and matter are lost at different feeding or trophic levels—energy is mostly lost as heat, and matter is lost as undigested food and wastes. As much as 90 per cent of energy and matter is lost at each level along a food chain.

BIOFACT Humans make up about 0.000 000 000 005% of the mass of the Earth—about 3 x 1011 kg out of 5.976 x 1024 kg.

40% FIGURE 1.30 A kangaroo gets energy from the plants that it eats, but only about 20% of the food it eats is converted into animal tissue. About 40% is used by the kangaroo in respiration to provide energy for its activities, and this energy is eventually lost as heat. The remaining 40% is lost in faeces and urine.

20%

Level 4 40%

carnivore

heat Level 3 animal-eaters

carnivore

Level 2

plant-eaters herbivore

plants FIGURE 1.31 A simple biomass pyramid. A biomass pyramid shows the amount of matter (biomass) in the organisms at each trophic level.

Level 1 producer

FIGURE 1.32 A pyramid showing the decline in biomass and energy along a food chain, from producer to herbivore to top carnivore. In general about 10 per cent of the energy at one level appears in the next.

Adaptations An adaptation is a feature of an organism that makes it well suited to its environment and lifestyle. An adaptation may be structural—a physical characteristic relating to the structure of an organism’s body; physiological—related to the way the organism functions; or behavioural—how an organism responds to its environment. Adaptations help an organism to survive and reproduce in an ecosystem. When looking at an organism, it is helpful to know as much as possible about where it lives and what conditions in its environment are like. What is the availability of water, what are the daily and seasonal temperatures like? What sort of predators does it have? If it’s an animal, what does it eat and how does it move about? Knowing this information

An adaptation is a feature of an organism that makes it well suited to its environment and lifestyle.

A local ecosystem 29

Adaptations are inherited characteristics. They are the result of natural selection.

can help to relate any features the organism shows to conditions in its environment. Without it, possible explanations to explain any features are guesswork. It is hard to infer that a characteristic is an adaptation without knowledge of its environment. Adaptations are inherited characteristics. They are the result of natural selection. When looking for adaptations in organisms, care needs to be taken that the characteristics are in fact inheritable, and not the result of a particular individual’s life history (see Chapter 4, p. 157).

BIOFACT

Adaptations: case studies

The southernmost stand of mangroves in the world occurs at Millers Landing in Corner Inlet in Victoria. In most other parts of the southern coast, mangroves have been cleared to improve boat access, or killed by pollution. The mangrove in Figure 1.33b is at Millers Landing.

When looking at organisms in your local environment you will try to identify features which can be interpreted as adaptations. The two examples below may help you in your understanding of how organisms are suited to their environments.

Case study 1: mangroves Mangroves form woody plant communities in warm, shallow tidal water over more than half of the Australian coastline. Twenty-nine species occur along the tropical north coast of Australia, but the number of species decreases further south. Around Sydney only two species are found: the grey mangrove, Avicennia marina, and the river mangrove, Aegiceras corniculatum. In South Australia and southern Victoria only the grey mangrove is found. There are none in Tasmania. Mangroves are found on sheltered muddy shores and along estuaries. Depending on the conditions and the species, they range from 2 m high shrubs to 30 m high trees.

Support and movement Mangroves are upright woody plants. They are anchored by complex root systems in the shifting environment of tidal mud flats. They have vertical anchor roots to which are attached spreading cable roots. Together these form a dense mass which helps stabilise the mud. Some mangroves support themselves on ‘stilt’ roots that lift the plant out of the salty water (Figure 1.33a).

(a) FIGURE 1.33 Two adaptations of mangroves: (a) stilt roots and (b) pneumatophores.

30 Heinemann Biology

(b)

Gaseous exchange There is a lack of oxygen in the water-logged soil, which is exposed at low tide but covered with water at high tide. Mangroves of the genus Avicennia have aerial roots or pneumatophores which push upwards through the mud and salt water. Their tips have pores or lenticels through which gaseous exchange can occur (Figure 1.33b).

Control of water balance Mangroves are halophytes (salt-tolerant plants). The water available to them is salt water with a high ion concentration. They control their salt level in three ways: • accumulation: their cells maintain higher than normal concentrations of cell solutes • secretion: some mangroves have salt glands on their leaves which actively secrete salt • exclusion: some mangroves use energy to excrete salt.

Control of internal temperature Mangrove leaves have a thick cuticle and are hard and leathery. These features help to control water loss and prevent wilting in hot weather. In dense mangrove swamps the thick canopy of leaves helps maintain a lower temperature in the lower layers.

Obtaining light Mangroves are the dominant plant in their community. The shape of the plant and arrangement of leaves ensure abundant light is available for photosynthesis. The leaves are dark green because they are rich in chlorophyll. The leaves high on the plant are angled and those lower down are horizontal to best capture sunlight.

Reproduction Mangroves have flowers and, following pollination and fertilisation, fruits containing one seed develop. These seeds begin germination before they drop from the parent plant (Figure 1.34). The seeds are buoyant and dispersed by the tide. Their initial development, particularly of the root system, ensures the new plant can rapidly anchor itself and grow rapidly once it is deposited in the mud. 1

List three environmental factors affecting mangroves.

2

For each of these factors, state the adaptations shown by mangroves.

3

Briefly explain each adaptation that enables mangroves to survive in their habitat.

A local ecosystem 31

FIGURE 1.34 In some mangroves the seeds germinate while still attached to the plant. When fully formed these propagules drop from the plant and spear into the mud, where they begin to grow. (a) Ceriops, (b) Bruguiera.

(a)

BIOFACT The crew of the Endeavour were the first Europeans to encounter kangaroos. In June 1770, Lieutenant James Cook wrote in his ship’s log: ‘I saw myself this morning ... one of these Animals ... it was of a light Mouse colour and the full size of a grey hound and shaped in every respect like one, with a long tail which it carried like a grey hound ... it jumped like a Hare or a deer.’

(b)

Case study 2: kangaroos Members of the genus Macropus are all physically very similar. Larger specimens (over 20 kg) are called kangaroos, and smaller species are known as wallabies and wallaroos. Species inhabiting steep, hilly areas rather than flat plains are usually called wallaroos. Few species are solitary—most congregate in groups or mobs. Kangaroos are widespread across Australia. They are all grazing herbivores that feed on grasses and herbs. The red kangaroo inhabits inland plains. The western and eastern grey kangaroos are found in grasslands, eucalypt woodlands and open forests from Tasmania to central Queensland and across to Western Australia. The antelopine wallaroo is found in the tropical north of Australia from Queensland across to Western Australia. The euro, or common wallaroo, is found in central and southern Australia.

Support and movement Kangaroos have an internal bony skeleton. Their well-muscled hind legs are far larger than their forelegs (Figure 1.35). Only the hind legs are used when the animal is travelling at high speed. This method of bounding along is more efficient in terms of energy use when compared with animals which run on all four legs. When moving slowly, kangaroos ‘hop’. Their weight is pressed down on their forelimbs and tail and their large hind legs are swung forwards.

FIGURE 1.35 The long tail of kangaroos and their relatives is an adaptation that helps the animals to balance when hopping.

32 Heinemann Biology

The long tail of the kangaroo is a useful structure. It is used as a balancing counterweight when bounding, as an extra limb when hopping, and it helps the kangaroo to remain upright when standing still.

Gaseous exchange Kangaroos have lungs as internal respiratory surfaces.

Control of water balance Kangaroos in arid areas, such as the red kangaroo and euro, can survive for long periods without drinking water, provided there is sufficient green plant material available. Kangaroos reduce water loss by sweating only during exercise. When they stop moving, sweating stops.

Control of body temperature Kangaroos that live in hot, dry regions seek the shade of rock crevices and caves during the hottest part of the day. Kangaroos in other areas will bask in the sunshine, but when conditions are hot they seek the shade of trees and bushes. When the weather is hot, euros may lick their forelimbs where the blood vessels run close to the surface and heat is lost from the body. It is thought that the evaporation of the saliva has a cooling effect.

Obtaining light Kangaroos have binocular vision. They are mainly nocturnal animals but may also be active in early morning and early evening.

Reproduction Kangaroos are marsupial mammals. They have internal fertilisation and a very short gestation period in the uterus. At birth the young climb into the mother’s forward-opening pouch. They attach to a teat and continue development while suckling (Figure 1.36). When they leave the pouch there is a weaning period before parental care ends. In the red kangaroo the young are born after 33 days in the uterus and weigh less than a gram. They remain suckling in the pouch for 235 days. They leave the pouch weighing 4–5 kg and have a weaning period of up to 4 months, during which they suckle and eat grass. Kangaroo population numbers are controlled through reproduction. Under good environmental conditions numbers can increase rapidly, because female kangaroos can be almost continuously pregnant when adult. They mate again directly after giving birth. If the mother is still suckling her newborn young, the fertilised egg does not develop until the young leaves the pouch. This is known as delayed implantation. At any one time a female kangaroo may have a joey (young kangaroo) being weaned, a young one being suckled in the pouch and an embryo in the uterus awaiting development. Kangaroos have the amazing ability to produce two kinds of milk at the same time. The milk produced by the teat for the developing young in the pouch contains much less fat than the milk produced by the teat being used by the joey outside the pouch. When environmental conditions are not good, such as in times of drought, young joeys do not survive and any fertilised egg does not implant. Females do not begin reproducing again until conditions improve.

A local ecosystem 33

FIGURE 1.36 A newborn kangaroo suckling in the mother’s pouch.

Competition is the struggle between organisms for the same resource.

(a)

(b) FIGURE 1.37 (a) The fast-moving black nerite, Nerita melanostragus, and (b) the slow-moving variegated limpet, Cellana tramoserica, compete for the same food on seashore rocks.

34 Heinemann Biology

1

List three environmental factors affecting kangaroos.

2

For each of these factors state the adaptations shown by kangaroos.

3

Briefly explain how each adaptation enables the kangaroos to survive in their habitat.

Competitive interactions Competition in ecosystems is the struggle between organisms for the same resource. A particular ecosystem can support only a certain number of each type of species. Competition may be between members of the same species, or between members of different species. In the short term, competition reduces the chance of survival and restricts the abundance of all the competitors. In the long term one of the competitors will usually be more successful and drive out or significantly reduce the numbers of other competitors. Each year many more offspring are produced by most organisms than can survive. While some young will be consumed as food by other organisms, many die in the competition for limited resources. A eucalyptus tree may produce thousands of gum nuts each year but only a few of the germinating seedlings will survive in the competition for light, nutrients and space. Different species of intertidal molluscs often occur together, eating the algae that grow on the rocks. Two such molluscs are the black nerite, Nerita melanostragus, and the variegated limpet, Cellana tramoserica (Figure 1.37). If algae are in short supply, the faster-moving nerites remove most of the algae and the limpet may starve. Species that are introduced into natural ecosystems by humans often successfully compete with the native inhabitants, because they may not have the same predators or diseases. For example, feral goats use rock shelters and food needed by rock wallabies in the Flinders Ranges of South Australia, and have caused a decline in the number of rock wallabies. More usually in ecosystems, organisms within the same habitat do not compete directly for the same resource. For example, different species of lizards living in the same desert ecosystem may eat different sizes of insects. Similarly, in urban gardens, the common garden spider, Araneus, catches and eats larger insects than the St Andrew’s Cross spider, Argiope, does.

The impact of humans on ecosystems Human activities have a great impact on ecosystems. We are members of many ecosystems, but unlike most other organisms we deliberately change our environment to suit our own needs. In doing so we bring about rapid alterations and widespread change. Our effect is often destructive to the original environment and its other inhabitants.

Land clearing In Australia more than 50% of eucalypt forests and 75% of rainforests have been cleared or modified since the arrival of Europeans. Land clearing destroys habitats and leads to soil erosion and land degradation. In many areas there has been increased salinity of both soil and water.

BIOFACT Habitat fragmentation is the division of a once-large area into smaller, unconnected fragments through clearing or other disturbance. Each fragment by itself is too small to maintain the original ecosystem. The self-sustaining cycle of materials and the flow of energy break down, and species are lost from the fragments, leading to local extinctions. If the fragmentation occurs over a wide area, species can become extinct.

Salting of a river system

Riv er

The Murray–Darling River system in eastern Australia (Figure 1.38) is one of the longest river systems in the world. About three-quarters of all the water used in Australia for domestic, industrial and agricultural needs comes from it. The Murray–Darling region supports much of Australia’s food production. Unfortunately, the rivers and the country that they drain are suffering enormous problems of salinity and land degradation. The lower Murray River has always been somewhat salty, but now the water that flows into South Australia carries 1.3 million tonnes of salt per year—about 2.5 tonnes of salt per minute! The water can be so salty that it exceeds the World Health Organization’s maximum level for drinking. Much of the salt in the Murray River comes from an increase in the inflow of water caused by rising

watertables in the surrounding country. As the water rises through the soil it picks up dissolved salts, mainly sodium chloride. The salty ground water is sometimes so close to the soil surface that it kills crops, and the land becomes an unproductive salt-flat (Figure 1.39). The salty water also flows into rivers, and this explains the extra saltiness in the Murray. Watertables have risen as a result of clearing native vegetation and irrigating crops. Without a full cover of native vegetation that can use up the water, groundwater levels have risen. Adding water to the land through irrigation has caused the watertable to rise further. A 1999 report into salinity in the region found that the problem is still increasing, and that if nothing is done the river water will soon be unsuitable for both drinking and irrigation.

Brisbane

er Riv

i mo

e Ri v

iver nR r hla c Murr La u bidge Riv er

eR

r

iv e

Sydney

m

Adelaide

a ri

rray Mu

r ive gR n i l ar

M a cqu

D

r

a yre lgo acInt Rive Cu M Gwydi r Na

e R.

Melbourne

FIGURE 1.38 The Murray–Darling river system connects three Australian states—New South Wales, Victoria and South Australia.

FIGURE 1.39 A devastated landscape, the result of salinity caused by over-irrigation.

A local ecosystem 35

BIOFACT Since European settlement, more than 1900 plant species have been introduced into Australia from elsewhere. About half of these are now listed as weeds.

Introduced species In Australia many introduced species have become pests, for example the red fox, cane toad (Figure 1.40), European carp, lantana and privet.

Pollution Air and water pollution and the dumping of wastes spoil and poison the environment for all organisms including humans.

BIOFACT The Landcare movement has developed in Australia over the past 20 years as a partnership between landholders, land managers, industry and government. There are more than 1000 Landcare groups in New South Wales. These groups are concerned with managing all natural resources in their areas in a more sustainable way.

Extinction of other species By destroying habitats and hunting without constraints we have caused the extinction of many species of animals and plants. The list of species at present threatened with extinction is a long one. Conserving the environment is a global issue. In Australia, national, state and local governments, community groups and individuals are working to find the balance between human activities, the need to protect the quality of the environment and the need to conserve our unique plants and animals.

FIGURE 1.40 The cane toad (Bufo marinus), a species from tropical South America, was introduced into Queensland in 1935 to control cane beetles in sugar cane crops. But the cane toad was no better than the native frogs at controlling the beetles, and became a pest itself. It breeds rapidly, has no major predators, and has few competitors or parasites. Its poison glands kill many would-be predators. It is rapidly extending its range into the Northern Territory and New South Wales. As it is a tropical animal, environmental factors such as cold and drought are currently the only controlling factors limiting its spread.

Environmental pests and weeds can be controlled by physical, chemical or biological means. • Physical control includes shooting animals and cutting down or digging out plants. • Chemical control includes use of poison baits and insecticides for animals and herbicides for plants. • Biological control involves the introduction of a predator or parasite.

BIOFACT The use of motor vehicles and the combustion of fuels such as wood and gas account for most of the air pollution in Australian cities.

36 Heinemann Biology

FIGURE 1.41 In large cities such as Sydney, photochemical smog caused by gas emissions from vehicles and factories can be an extremely serious problem.

Pollution is the spoiling or poisoning of the environment through human activity.

BIOFACT

FIGURE 1.42 Most wastes from urban and industrial areas are dumped in tips without treatment. In the past, polluting chemicals such as pesticides and paints were also dumped in this way, polluting the soil and water.

Endangered Australian animals include: ● Baw Baw frog ● bent-wing bat ● bilby ● brush-tailed rock wallaby ● corroboree frog ● eastern bristlebird ● flatback turtle ● ghost bat ● giant freshwater crayfish ● giant Gippsland earthworm ● ground parrot ● helmeted honeyeater ● humpback whale ● Leadbeater’s possum ● mountain pygmy-possum ● northern hairy-nosed wombat ● numbat ● orange-bellied parrot ● speartooth shark.

Questions 1

a What is a food chain? b Write down an example of a food chain in an ecosystem.

2

a Define the terms ‘producer’ and ‘consumer’. b Outline the role of each of the following types of consumers in ecosystems, giving one Australian example of each: herbivore carnivore omnivore scavenger decomposer

3

a Define ‘biomass’. b What information is expressed in a biomass pyramid that a food chain cannot express? c Not all of the energy at one level of a food chain is passed on to the next level. What happens to it?

4

Describe each of the following kinds of relationships that exist between organisms in ecosystems, and give an Australian example of each:

i ii iii iv

predator–prey parasite–host commensalism mutualism.

5

Outline some factors that can affect the numbers of predators and prey in ecosystems.

6

a Explain what is meant by the term ‘allelopathy’. b Describe two examples of allelopathy, including at least one Australian example.

7

a Define the term ‘adaptation’. b Describe an adaptation to an environmental factor displayed by i an Australian animal ii an Australian plant.

A local ecosystem 37

F u r ther questions 1 A particular epiphytic plant has a bulbous stem that is honeycombed with chambers. In these chambers lives a species of ant. The epiphyte obtains mineral nutrients from the ant nest in its stem, so it can survive in areas that cannot support other epiphytes. Of what benefit is the relationship to the ants? What type of association exists between the plant and the ants? Explain your answer. 2

3

Complete the table (below) of adaptations shown by various plants and animals that make them wellsuited to their particular environments. In each case, decide whether the adaptation is structural (S), behavioural (B) or physiological (P). Explain how the feature is beneficial to the organism. Biomagnification is the increase in concentration of substances (such as DDT) at each level of a food pyramid.

b Why do all food chains begin with producer organisms? 5

Consider the food web in Figure 1.28. a Identify the producers, primary consumers, decomposers, and top-order consumer. b Draw a food chain for this food web that shows the seal in the position of i secondary consumer ii tertiary consumer. c Use an example from this food web to explain why food webs are more stable than food chains.

6

Write out a food chain in which you are a link. What niche do you occupy?

7

Using library and Internet resources, investigate the Earth’s growing human population. Discuss the implications, problems and solutions, from an ecological viewpoint.

8

a Outline the impact of human activity on the extent of forest cover in Australia since European settlement in 1788. b Why have Australian forests been cleared?

9

The cane toad is an example of an introduced species that was imported to Australia as a means of biological control. Unfortunately it became a pest itself. a What is meant by ‘biological control? b Outline the reasons that contributed to the cane toad becoming a pest. c Describe the effects that this pest species has had in Australia.

DDT concentration herring gulls

98.8 ppm

fish

4.4 ppm 0.44 ppm

shrimps

0.014 ppm mud in

in mud Lake Michigan

DDT

a How much has the concentration of DDT increased by the time it is taken up into the gulls’ tissues? b Investigate how DDT affects birds, and summarise your findings in less than 100 words. c Suggest the possible consequences of DDT for other organisms as a result of its effects on toporder consumers. 4

a What is the original source of energy for any food chain or food web in an ecosystem?

O rganism

Feature

Grey mangrove

pneumatophore

Coast saltbush

halophytic

Red kangaroo

licks forelimbs in hot weather

Three-lined skink

basks in sun during cool weather

Common wombat

retreats to burrow on a hot day

Blackbutt

releases a chemical into soil (see p. 22)

38 Heinemann Biology

10

Choose an example of an introduced plant that has become a pest in New South Wales. a What effects has the pest species had in the state? b Describe the successful measures, if any, that have been used to remedy the problem.

S, B or P?

Benefits for organism

Chapter summar y Practical activities 1.1



Methods of estimating population numbers

1.2



A local ecosystem field study



A local ecosystem report

1.1 ●













Ecosystems are the systems formed by organisms interacting with each other and with their environment. These systems may be terrestrial (on land) or aquatic (in water), or a combination of both. Abiotic factors are the non-living components of an ecosystem, such as availability of gases, rainfall and temperature. The abiotic factors of terrestrial environments are very different from those of aquatic environments. Biotic factors are the living organisms in an ecosystem, such as vegetation, mates, predators and prey. The distribution, diversity and abundance of organisms in an ecosystem are determined by both abiotic and biotic factors. Methods used to investigate ecosystems include observing, measuring, collecting and making estimates of populations using particular sampling techniques, such as transects and quadrats. Photosynthesis is the process by which plants capture the radiant energy from sunlight to produce simple organic compounds (sugars). Respiration is the process by which all living organisms obtain energy to use to perform all their activities. In respiration, energy is released from reactions involving sugars and oxygen, and water and carbon dioxide are produced. This general statement for aerobic respiration in cells is a summary of a sequence of many biochemical reactions. Energy is used by organisms in all their activities, including growth, general body metabolism and reproduction.

1.2 ●





Population estimates can change over season and time. Factors affecting numbers in predator and prey populations can include the size of the ecosystem, the availability of the prey’s food, reproductive cycles, competition and disease. Interactions between organisms such as commensalism and mutualism benefit at least one organism. Other interactions such as predation and parasitism may be harmful to one of the organisms. In allelopathy, chemicals from one plant can affect other organisms in its environment. Decomposers are important in ecosystems. They break down dead plants and animals and their wastes to simple compounds that are recycled and used by other organisms.

A local ecosystem 39











40 Heinemann Biology

The flow of energy from one living thing to another can be shown in a food chain. The trophic or feeding interactions that occur in an ecosystem can be represented in a food web. An adaptation is a feature of an organism that makes it suited to its environment and helps it to survive and reproduce. Adaptations can be physiological, structural or behavioural. Competitive interactions are when organisms compete for the same resource. These relationships may be between members of the same species or between different species. The result of these interactions affects the numbers of the organisms in an ecosystem. In the short term, competition reduces the chance of survival and restricts the abundance of all competitors. In the long term, one competitor is usually more successful and reduces the number of other competitors. Human activities, such as pollution, land clearing and the introduction of new species, can have a severe impact on natural ecosystems.

EXAM - STYLE QUESTIONS Multiple choice 1

2

3

Which statement best describes an organism’s environment? A It is the ecosystem in which it lives. B It is the sum of all of the living and non-living things that affect it. C It is the set of physical factors that influence its survival. D It is the place where it lives. In environmental terms, what are deserts, woodlands, grasslands and forests? A They are examples of ecosystems found in parts of Australia with different climates. B They are examples of the interactions of living organisms. C They are stages in the succession of ecosystems. D They have the same inputs, outputs and processes. Which description best explains the environmental factors of temperature, availability of gases and amount of light? A They are biotic factors that can influence the distribution and abundance of species. B They are abiotic factors that can influence the distribution and abundance of species. C They are biotic and abiotic factors in the environment. D They are physical factors that affect the distribution of plants, but not animals.

4

Which of the following is true for the distribution of a particular species? A It does not usually vary much. B It can be changed by human activities but not by natural processes. C It is determined by the availability of its requirements. D It tells us the numbers to be found in a particular area.

5

seagrass pop. 1



fish pop. 2



6

Why do we use transects and quadrats when studying population size? A To determine the number of organisms in an ecosystem. B To ensure random sampling of an ecosystem. C To estimate the distribution of a population. D To estimate the abundance of a population.

7

Why is aerobic cellular respiration an important process in ecosystems? A It transforms light energy into energy-rich organic compounds. B It releases energy stored in organic compounds. C It creates the energy that cells need. D It involves a series of important chemical reactions in cells.

8

In an area of an ecosystem, a biologist estimated the following biomasses (V to Z represent populations of particular species): V = 500 kg, W = 600 kg, X = 5000 kg, Y = 50 kg, Z = 5 kg Which of the following represents a possible food chain in this ecosystem? A V➝W➝X➝ Y B Z➝ Y➝ V➝ X C Z➝ Y➝ X➝W D X➝ V➝Y➝ Z

9

A tiny flatworm, Coavoluta roscoffensis, lives buried in the sand in the tidal zone of the seashore. It has a green alga living in its tissues. When the tide is out during the day, the worm comes to the surface. In the light, the green alga can photosynthesise. The flatworm lives on the starch produced as a result of photosynthesis by the alga. Which of the following best describes the relationship between the flatworm and the alga? A mutualism B commensalism C parasitism D predator–prey.

10

What role do decomposers play in ecosystems? A Decomposers are bacteria and fungi in soil and water. B Decomposers contribute to the recycling of materials. C Decomposers live on dead organic matter. D Decomposers are the end of the food chain.

pelicans pop. 3

The diagram shows three populations that make up a food chain. If a new predator moves in that also feeds on fish, what might happen to the numbers in the three populations? A Numbers in all three populations decline. B Numbers in population 2 decline but 1 and 3 increase. C Numbers in population 1 increase but 2 and 3 decline. D Numbers in population 2 decline but 1 and 3 stay the same.

Studying A localecosystems ecosystem 49 41

Short answers 1

You are on a field trip to Cope Creek on the Bogong High Plains of Victoria to investigate the revegetation of areas where cattle are now excluded. Your task is to estimate the population of the silver snow daisy in a fenced plot. Outline a sampling technique that would allow you to estimate the plant population without counting every plant of that species in the plot.

2

Three species of small arthropods are found living in a river estuary. The diagram and graph below show the distribution of these species and the water salinity in the estuary. a Describe how the salinity of the water affects the distribution of each species. b Suggest another environmental factor which may affect the distribution of each species.

5

LAYER X LAYER Y

LAYER Z

species 2 species 1 sea

26

20 10 0 Distance from mouth of estuary (km) Distribution of three arthropod species in estuary

Salinity level

sea water

3

fresh water 26

high water low water

7–18˚C

4–7˚C

Study the following diagram. human activities CO2

X

Y

20 10 Distance from mouth of estuary (km) Salinity of water in estuary

0

A

Study the following two diagrams. Which diagram best represents the feeding relationships in a field? Give three reasons for your choice.

bird grasshopper grass

4

6

salinity gradients

18–24˚C

a Describe the relationship between depth and temperature. b i Describe the relationship you would expect between depth and light penetration. ii How is light penetration likely to affect the distribution of plants in the lake? iii In which layer (X, Y or Z) would you expect the greatest abundance of animal life? Explain your answer. c Identify two other physical factors that might affect the organisms living in the lake.

species 3

river

Study the following cross-sectional diagram of a lake in summer.

bird grasshopper grass

Locust plagues are common in some inland areas of Australia following seasons of good rainfall. a Describe the short-term effect on an ecosystem of the arrival of the locust plague. b What is the long-term effect on the ecosystem?

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a Name the two processes X and Y represented in the diagram. b Name substance A. c List three uses of energy by the kangaroo. 7

Study the food web shown in Figure 1.28. a Identify the producer organisms in this food web. b Describe three predator–prey relationships that are represented. c Give an example of commensalism in this food web. Fully explain the relationship between the organisms you have listed. d Draw a food pyramid that is representative of all of the organisms in the food web.

Chapter 2

PATTERNS IN NATURE

When we examine general features across a range of living organisms, patterns in structure and function can be identified. The fundamental processes in living things are concerned with inputs (absorbing necessary chemicals) and outputs (releasing wastes). Cells are the building blocks from which all living organisms are made. Patterns in their structure and function can be seen at the microscopic and biochemical level. Structural similarities reflect similar biochemical processes. The fundamental difference between plant cells and animal cells is related to the process of photosynthesis that occurs only in plant cells. To take in materials and remove wastes efficiently, living things have evolved often complex systems with large surface areas. The collection and distribution of nutrients and wastes is performed by specialised transport systems. In living things the processes of sexual reproduction show similar patterns reflecting similar functions. It is likely that these patterns may reflect common evolutionary origins for multicellular organisms, both plants and animals.

This chapter increases students’ understanding of the history, applications and uses of biology.

2.1

Organisms: cells Cell formation and structure OBJECTIVES When you have completed this section you should be able to: ● state the cell theory ● recount the historical development of the cell theory, including the contributions of Robert Hooke and Robert Brown ● describe evidence that supports the cell theory ● discuss the significance of advances in technology, especially the light and electron microscopes, to developments in cell theory ● identify that there are different types of cells in multicellular organisms, organised into tissues, organs and organ systems ● identify cell organelles which are visible using light and electron microscopes ● discuss the relationship between the structure and function of cell organelles.

The cell theor y activities ● ●

The light microscope Observing cells

The cell theory states that: 1 Cells are the smallest units of life. 2 All living things are made up of cells. 3 All cells come from pre-existing cells.

Over the past few hundred years, biologists have found that all lifeforms share a fundamental similarity. They are all made up of units we call cells. Cells are the building blocks of which all living organisms are made. The basic structure of all cells is similar and they have many chemical substances in common. They also have similar basic functions. Using cells, plants and animals can build specialised structures (tissues and organs) as part of their bodies, for a variety of purposes. This gives us the great diversity of living things that we see today. In 1839, the German biologists Matthias Schleiden and Theodor Schwann stated their cell theory, as follows: 1 Cells are the smallest units of life. 2 All living things are made up of cells. In 1858 another German, Rudolf Virchow, added a further point to the theory: 3 All cells come from pre-existing cells. These three statements form the modern-day cell theory.

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Spontaneous generation vs cell theory Prior to the seventeenth century, biologists developed a theory—the ‘theory of spontaneous generation’—that some animals such as worms and frogs could spontaneously emerge from mud or water, and that organisms such as maggots developed from rotting meat. However, the Italian scientist Francesco Redi showed that maggots developed only on meat that flies had laid their eggs on. In the nineteenth century, the theory of spontaneous generation was finally shown to be fundamentally incorrect when Louis Pasteur’s experiments proved that microorganisms arose only from other microorganisms. As a result, the ‘cell theory’ developed which in turn formed the scientific basis for the ‘germ theory of infection’ and revolutionised the development of antiseptic procedures in medicine.

Historical development Most cells are so small that a microscope is needed to see them. The development of the cell theory went hand in hand with technological advances in the manufacture of lenses and magnifying devices in the 19th century. The microscopes produced in the early part of that century enabled much greater detail to be seen (Figure 2.1). Magnifying devices have been used since ancient times. Chinese porcelain vases, dated to around 1000 BC, show people wearing glasses, and the Greeks and Romans used glass flasks filled with water to magnify small objects. However, the first recorded study of cells was not until the 17th century, when Robert Hooke used a microscope he developed to view a thin piece of cork (Figure 2.2). Robert Brown’s work in 1831 identified the nucleus as a large body to be found inside cells. Table 2.1 shows the major events in the development of the cell theory.

FIGURE 2.1 This microscope was one of the best available in the early 19th century. It enabled cells and their contents to be seen in detail.

cork cells

light source

micro-organisms in a drop of water

(a)

(b)

FIGURE 2.2 (a) Hooke’s microscope, and what he saw. (b) Leeuwenhoek’s microscope, and what he saw.

FIGURE 2.3 Modern compound light microscope, which can magnify up to 1000×.

Patterns in nature 45

TABLE 2.1 Major events in the development of the cell theory.

Ti m e

Event

13th century

Roger Bacon (1214–1294) used a convex lens as a magnifying glass.

1485

Leonardo da Vinci (1452–1519) used glass lenses to study small objects.

about 1600

Two spectacle-makers, Hans and Zacharias Janssen, constructed the first compound microscope using two lenses inside a tube.

1665

Robert Hooke (1635–1703) further developed the compound microscope, including the use of the iris diaphragm. Hooke kept a detailed record of his observations. He first described cells (or rather, plant cells) in his description of a slice of cork he observed under the microscope. He described it as ‘. . . all perforated and porous, much like a honeycombe . . . the pores or cells . . . consisted of a great many little boxes . . .’

1672

Marcello Malpighi (1628–1699) stated that all plants are built of chambers (‘utricles’).

1676

Anton van Leeuwenhoek (1632–1723) described unicellular organisms from his observations of a drop of stagnant rainwater as ‘animalcules’. Leeuwenhoek also soaked some peppercorns in water for several days. When he examined a drop of this water under his microscope, he did not find the little needles he expected that made pepper ‘hot’, just more unicellular organisms.

1683

Leeuwenhoek discovered bacteria from his observations of saliva.

1824

René Dutrochet (1776–1847) stated that all animals and plants are made up of cells.

1831

Robert Brown (1773–1858) noted that the cells of orchids he was observing under the microscope contained a structure inside the cell. He called this the ‘nucleus’.

1839

Matthias Schleiden (1804–1881) and Theodor Schwann (1810–1882) formulated the cell theory that all living matter is composed of small units called cells. Johannes Purkinje (1787–1869) used the word ‘protoplasm’ for the contents of a cell.

1858

Rudolf Virchow (1821–1902) stated that ‘where a cell exists there must have been a pre-existing cell, just as the animal arises only from an animal and the plant only from a plant’.

1880

Walther Flemming (1843–1905) described cell division (mitosis) from observations on living and stained cells.

20th century

There was continued development of light microscopes and their uses; for example, dark-field microscopy, use of polarised light, phase-contrast microscopy.

1933

Ernst Ruska built the first electron microscope. Transmission electron microscopes and scanning electron microscopes were developed in the following decades.

1960s–present

X-ray microscopes use X-rays produced by synchrotrons to examine matter at the atomic level, particularly the structure and interactions of proteins.

BIOFACT Robert Brown is not only remembered for his discovery of the nucleus in cells. He was already a famous naturalist and a friend of Charles Darwin, and had travelled all round the world discovering and describing new plants and animals. He arrived in Australia with Matthew Flinders aboard HMS Investigator in 1798, and spent several years investigating the strange new plants he found here, many of which he described and named. The Australian plant family Brunoniaceae and its single species, Brunonia australis, are named after him.

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Technological advances Technological advances in the design of microscopes and techniques in preparing materials to be viewed have added to our knowledge of cells. The structure of cells was visible to some degree with the early light microscopes and was further revealed when techniques were developed for cutting very thin sections of tissue from multicellular organisms and staining them for observation. In staining techniques, differently coloured dyes are taken up by different parts of the cell. For example, the densely packed threads visible at times in the cell nucleus absorb stain readily and became known as chromosomes, which means ‘coloured bodies’. In 1933 Ernst Ruska built the first electron microscope, enabling more detailed observations of all structures to be made. Technological advances continue in microscopy. Laser scanning and the use of three-dimensional imaging software have improved our knowledge of cell and tissue structures. X-ray microscopes allow the shape and structure of biological molecules such as proteins to be determined.

Inside microscopes A compound light microscope passes a focused beam of light through the specimen. This light passes through the objective lens, then up the barrel to the ocular lens (the eyepiece) and into the eye, where a magnified image is formed. An electron microscope uses a beam of electrons to enable us to see cells and their contents at extremely high magnification, and with great clarity. The image from an electron microscope can be produced on a TV screen or a photographic plate. There are two main types of electron microscope: transmission and scanning. Transmission electron microscopes require specimens which are so thin that electrons can pass through them to form an image. They are used to show such things as the internal structure of cells. Scanning electron microscopes produce images of the surface features of objects. Specimens are usually coated with a very thin layer of metal atoms to enhance the image. Living cells can be observed using a laser scanning microscope. A computer-controlled scanning mirror moves a beam of laser light across a section or slice of the specimen. The light signal is detected and converted into a digital image that can be stored or displayed on a computer screen. Using scans at (a)

LIGHT MICROSCOPE eye image

ocular lens magnifies (e.g. 10x)

objective lens magnifies (e.g. 100x) slide with specimen

different positions, a three-dimensional image of the specimen can be constructed. Confocal laser scanning uses gas lasers and allows thick specimens to be examined. Fluorescence from the specimen is focused by the objective lens through a pinhole aperture to reduce blurring of the image. Multiphoton laser scanning uses solid state lasers, which do not require a pinhole aperture. The longer wavelengths of light that are produced are more penetrating and cause less damage to cellular matter.

FIGURE 2.4 Human chromosomes seen under a scanning electron microscope. (b)

ELECTRON MICROSCOPE screen image

projector lens (electromagnets)

objective lens (electromagnets) specimen (in vacuum) condenser lens (electromagnets)

condenser focuses light onto specimen light source

source of electrons

FIGURE 2.5 The operation of (a) a compound light microscope, and (b) a transmission electron microscope.

Patterns in nature 47

Magnification and resolution Light microscopes magnify objects about 1000 times and electron microscopes magnify objects up to 1 million times. Microscopes enable us to distinguish two objects close together as separate objects; in other words, it allows us to see detail. The degree to which a microscope can do this is called its ‘resolving power’. The resolving power of the human eye is 100 µm. If two points are 0.1 mm apart (100 µm) your eyes see them as two points; if they are any closer, they appear as one point. The resolution of the light microscope can be up to 0.2 µm, or about 500 times better than the human eye. The resolution of the best electron microscope today is about 0.0002 µm, which is about 5 million times better than our eyes. The electron microscope gives us greater magnification of cells, with very clear details of their contents (Figures 2.6, 2.7).

FIGURE 2.6 (left) Cells from a rat aorta viewed with a light microscope (1000×). FIGURE 2.7 (right) Cells from the same rat aorta viewed with an electron microscope (10 000x)

TABLE 2.2 Comparison of light microscopes and electron microscopes.

Feature

Light microscope

Electron microscope

Magnification

up to 1500×

1000000×

Resolution

up to 0.2 mm

up to 0.0002 mm

Advantages

samples prepared quickly; coloured stains can be used; living cells can be viewed

high magnification and resolution allow objects as small as molecules to be viewed

Disadvantages

limited visible detail

only non-living sections can be viewed because electrons must be kept in a vacuum to prevent scattering expensive and long preparation of materials for viewing

100 µm 80 µm 60 µm 40 µm 20 µm 0 plant cell

FIGURE 2.8 The sizes of some cells.

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human cheek cell

human red blood cell

bacterium

Synchrotrons Sometimes referred to as a ‘supermicroscope’, a synchrotron is a large facility that produces beams of very bright light used by scientists to examine matter at an atomic level. In a synchrotron electrons are emitted by an electron gun and then accelerated in a linear accelerator (linac). They pass to a circular accelerator or booster synchrotron and then into a large, ringshaped vacuum tunnel with a circumference of 1000 to 1500 metres. Magnets on the tunnel walls bend the electrons as they travel through the tunnel at almost the speed of light. This causes the beam line 8

beam line 7

beam line 1 booster synchroton

beam line 6 storage ring

beam line 2 linac

beam line 5 electron gun

beam line 3 beam line 4

experimental station

FIGURE 2.9 Plan view of a synchrotron. There is a station at the end of each beam line where scientists can use the radiation beam to examine samples at the molecular level.

electrons to emit radiation, which is formed into beams. The beams are then directed to experimental stations where scientists can use the radiation for research. Special mirrors, lenses and filters are used to focus and direct the radiation onto sample materials. While the radiation produced spans the whole of the electromagnetic spectrum, synchrotrons can be designed to produce beams of specific wavelengths. Light of shorter wavelengths, such as ultraviolet (UV) light and X-rays, are particularly useful because they can be used to view atoms and molecules that are measured in nanometres. For biologists, the use of X-ray microscopes using synchrotron radiation is leading to a better understanding of cellular structures and biological systems at the molecular level. Both living and non-living samples can be examined, the arrangements of complex molecular structures such as proteins can be identified and biochemical processes can be tracked. There are about fifty synchrotrons worldwide. A synchrotron is currently being built in Melbourne and will be ready for use in 2007. For more information visit www.synchrotron.vic.gov.au. For more information about synchrotron radiation and its applications, visit the website of the Australian Nuclear Science and Technology Organisation (ANSTO) at www.ansto.gov.au and view the work of the Australian Synchrotron Research Program.

The structure of cells Cell contents The cell under the light microscope The light microscope revealed that cells are not empty, and that protoplasm (the contents of a cell) is not uniform in structure. Although cells show great variation, they have certain structures in common. All cells have a clearly defined shape or boundary. This is maintained by a flexible cell membrane which encloses the internal contents of the cell. Sometimes this membrane is called the plasma membrane. The protoplasm inside most cells reveals definite structures, called organelles (‘little organs’). Organelles have a particular job to do for the cell. They are either enclosed in or made up of membranes. Organelles and the cell membrane are continually changing, breaking down and reforming in a dynamic pattern of activity. One organelle that can be seen with the light microscope is the nucleus. Inside the nucleus is a darker staining area called the nucleolus; the small threads which show up clearly in some nuclei, by staining, are

BIOFACT Because they are so small, cells are measured in micrometres and nanometres. ● One micrometre (µm) is one millionth of a metre, or 10–6 m. ● One nanometre (nm) is one thousand millionth of a metre, or 10–9 m. Plant and animal cells range in size from 5 to over 100 µm wide (see Figure 2.8).

Patterns in nature 49

All cells have a clearly defined shape or boundary. This is maintained by a flexible cell membrane which encloses the internal contents of the cell. The cell wall, found in plant cells, consists of a network of cellulose microfibrils (long strands of cellulose molecules) in a cement of pectin and other substances.

position of cell membrane

nucleus (contains chromosomes)

cytoplasm

chloroplast

cell wall large vacuole

nuclear membrane

Plant cell cell membrane

cytoplasm Animal cell

nucleus (contains chromosomes) nuclear membrane

Note: no cell wall no large vacuoles no chloroplasts

FIGURE 2.10 Generalised cells seen through a light microscope (1000×).

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the chromosomes. The contents of the cell between the nucleus and cell membrane is called the cytoplasm. Cytoplasm looks like a clear fluid with particles in it. Some of these particles or granules are easily visible with the light microscope, but others cannot be distinguished clearly. Some of the granules visible in the cytoplasm may be food reserves or food vacuoles. The food they contain, such as starch or oils, may be made visible by selective staining techniques. With the best light microscopes, small structures called mitochondria (singular = mitochondrion) can be seen, but not in detail. The smaller details of internal cell structures are distinguishable only with an electron microscope. When cells were first examined under the light microscope, it became apparent that there are two basic and commonly seen cell types. One is surrounded by a cell membrane only, and the other has a much thicker cell wall around it. Cell walls are non-living structures which give shape and rigidity to the cell they surround. The cell membrane still surrounds the internal cell contents but is often not visible because it is pressed up against the cell wall. The cell wall, found in plant cells, consists of a network of cellulose microfibrils (long strands of cellulose molecules) in a cement of pectin and other substances. Lignin (wood) may be one of these. The wall protects and supports the cell (see Figure 2.14, p. 53). Many cells with cell walls may also contain large round or oval membrane-bound organelles called plastids which are easily seen in the cytoplasm. They are found only in cells taken from plants. Plastids often contain pigment. Green-coloured plastids or chloroplasts contain the pigment chlorophyll, giving plants their green colour. Chromoplasts produce the colours of flowers and fruits, and leucoplasts store nutrients such as starch. Vacuoles are sacs surrounded by a single membrane. Vacuoles may be temporary structures, such as food vacuoles, which contain substances brought into the cell from outside. Plant cells usually also contain a large central vacuole. It is filled with a fluid called the cell sap, consisting mainly of water and dissolved substances such as sugars, salts and sometimes coloured pigments. In some plant cells the vacuole may occupy 80–90% of the total cell volume. TABLE 2.3 Main cell structures visible with a light microscope.

Name

Function

Nucleus

contains the chromosomes; the information in the chromosomes is used to control the development and functioning of the whole cell; without a nucleus most cells will die

Cell membrane

forms the boundary between the cytoplasm and the outside environment; controls the entry and exit of substances to and from the cell

Cytoplasm

contains many organelles; is where most cell activities are carried out

Cell wall

gives protection, support and shape; all plant cells have a cell wall

Chloroplasts

contain the green pigment chlorophyll, and are the site of food manufacture (photosynthesis) in plants

Vacuoles

store water and other substances; large and important in plant cells

TABLE 2.4 A comparison between plant and animal cells.

Plant cells

Animal cells

cell walls made of cellulose

no cell walls

the cell has a definite outline and regular shape due to the cell wall

the cell has a more flexible and variable shape

although there is a wide variation in size, plant cells tend to be larger on average than animal cells

tend to be smaller than plant cells

chloroplasts present in many plant cells

chloroplasts never present

usually contain a large central vacuole

vacuoles not usual; if present, vacuoles are smaller and often temporary

no centrioles

centrioles present

Nucleus rules—OK! The role of the nucleus was clearly demonstrated in a set of experiments carried out in the 1930s using Acetabularia, a large single-celled marine alga consisting of a branching foot, stalk and cap. The nucleus is located in the foot. Two closely related species with different shaped caps are A. mediterranea and A. crenulata. When a cap is removed from either species, a cap of the same shape grows back (Figure 2.11a). (a)

In the next part of the experiment (Figure 2.11b), caps were removed from individuals of each species, then the stems were cut and transplanted onto roots of the other species. The first caps that regrew were intermediate in form between the two species. When these were removed, all following regenerated caps had the form of the root species. The nucleus was obviously controlling the functions of the cell.

A. mediterranea

A. crenulata cap

stalk nucleus

branching foot

(b)

FIGURE 2.11 Experiments with Acetabularia showing the role of the nucleus.

How does this experiment show that the nucleus controls the functions of the cell? (Explain in one or two sentences only.)

The cell under the electron microscope Figure 2.12 shows cell structures visible with an electron microscope. You will see that it shows some organelles that are not visible with the light microscope; for example, the Golgi body and ribosomes. Patterns in nature 51

PLANT CELLS

ANIMAL CELLS ribosomes nucleus cell membrane cytoplasm

Golgi body mitochondria cell wall

vacuole

nucleus chloroplast

cytoplasm

FIGURE 2.12 The parts of plant and animal cells. Cell membrane: a complex structure which contains the cytoplasm and controls movement of substances into and out of the cell.

Golgi body: a stack of flat membrane sacs where final synthesis and packaging of protein in membrane-bound vesicles occurs before secretion.

Cytoplasm: the fluid content of the cell. It is more than 90% water and contains ions, salts, enzymes, food molecules and organelles other than the nucleus.

Cell wall: a non-living cellulose structure outside the cell membrane in plant cells. The cell wall provides support, prevents expansion of the cells and allows water and dissolved substances to pass freely through it.

Nucleus: a large organelle that is surrounded by a double layer of membrane. It stains differently from cytoplasm and so often looks darker in prepared slides. The nucleus contains genetic material and controls cellular activities.

Vacuoles: membrane-bound structures found in most cells. They may contain food, enzymes or fluid. Plant cells typically have large fluid-filled vacuoles that provide support.

Mitochondrion: an organelle composed of many folded layers of membrane. Michochondria are involved in the energy transformations that take place in cells.

Chloroplasts: green organelles, found in those plant cells in which photosynthesis takes place. They are composed of many folded layers of membrane.

Ribosomes: tiny organelles that are sites of production of proteins.

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Organelles and their function Organelles are the functional units of cells. All organelles are surrounded by a membrane and have a particular job to do for the cell. Because they are surrounded by a membrane, they are easily identifiable. The nucleus is spherical and relatively large in terms of other cell structures. It is surrounded by a double membrane, called the nuclear envelope, which has pores in it to allow fairly large molecules in and out. The nucleus contains most of the genetic material of the cell. The nucleus controls the activities of the cell by controlling which proteins are made in the cell. The genetic material (DNA and associated proteins) in a cell is seen most clearly as chromosomes when a cell is about to divide. The chromosomes contain the genes, the inherited information that determines whether proteins are made or not made (see p. 299). In a non-dividing cell, the nucleolus is a conspicuous structure. It is the region where the genes for ribosomal RNA are found and is the site of ribosome formation.

(a)

nucleolus

pores (b)

double nuclear membrane

FIGURE 2.13 (a) Electronmicrograph and (b) diagram of a nucleus, showing the nuclear pores.

Intercellular connections greatly enlarged

cellulose fibres

(a)

Tissues are organised groups of similar cells, which combine together to form organs—structures with particular functions. The connections between the cells in a tissue or organ are critical to its function. Plant cells are separated from one another by cell walls, and adjacent cells are connected by plasmodesmata, which are cytoplasmic strands that pass along channels through the cell walls (Figure 2.14b). Cells in animal tissues are held together by junctions between the cell membranes. These junctions fall into three main groups (Figure 2.14c). Dense fibrous connections (desmosomes) hold cells of a tissue together, giving it mechanical strength. Tight junctions between cell membranes limit the movement of fluid outside cells. For example, in the lining of the gut, tight junctions prevent the gut contents leaking out into the body. Gap junctions allow nutrients and ions to pass from the cytoplasm of one cell to another.

plasmodesmata tight junction microvillus desmosome

cytoskeletal fibres gap junction

(b)

(c)

FIGURE 2.14 (a) Section of the cell wall of a plant. (b) A leaf cell, showing a plasmodesma (4000×). (c) Diagram of an animal cell, showing a desmosome, tight junction and gap junction.

Patterns in nature 53

Organelles are the functional units of cells. All organelles are surrounded by a membrane and have a particular job to do for the cell.

FIGURE 2.15 (a) Diagram of a mitochondrion. (b) A mitochondrion from a mouse’s intestine (×28 000).

Mitochondria are usually about 0.5 µm wide and up to 7 µm long. They are surrounded by a double membrane; the outer membrane is smooth, but the inner membrane is greatly folded. On these folds, called cristae (singular crista), the chemical reactions of respiration occur, producing energy for the cell. The folding greatly increases the surface area over which the reactions can occur. Mitochondria also contain ribosomes, DNA and RNA (see p. 55). circular DNA inner membrane

outer membrane (a)

FIGURE 2.16 An electronmicrograph showing a lysosome (arrow) in the process of breaking down unwanted material.

microtubule

FIGURE 2.17 Centrioles consist microtubules.

of

bundles

of

FIGURE 2.18 Rough endoplasmic reticulum from the intestine of a mouse (×24 000).

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matrix

crista (fold) ribosome (b)

Lysosomes are small, spherical, membrane-bound organelles about 0.4 µm in diameter. They are very common in animal cells but are rare in plant cells. Lysosomes are very acidic and contain digestive enzymes. They fuse with vacuoles containing food or other substances that have been taken into the cell, or with old and damaged organelles within the cell, and digest them or break them down. If lysosome enzymes are released into the cell, they will break it down and destroy it. Microtubules are tiny hollow tubes only about 25 nm in diameter. They help control the shape of the cell and assist with movement. Cytoplasmic extensions, such as cilia and flagella, consist of microtubules. Centrioles are found in pairs in animal cells, and are made up of microtubules. They organise the formation of the spindle, also made of microtubules, during mitosis. (This process is described on pp. 94–96.) The endoplasmic reticulum (which means ‘a network inside the cell’) is a system of membranous sacs and tubules that is connected to the nuclear envelope. It provides: (1) an internal surface for many of the chemical reactions in the cell, and (2) a series of channels through which material can be circulated. ‘Rough’ endoplasmic reticulum has tiny grains (ribosomes) attached. Proteins are made in the ribosomes and transported from the endoplasmic reticulum to the Golgi body in transport vesicles. ‘Smooth’ endoplasmic reticulum has no ribosomes attached. It is the site of lipid manufacture and also helps to inactivate some drugs, such as alcohol and barbiturates. Ribosomes are tiny spherical bodies made of RNA and protein and are approximately 20 nm in size. They may be attached to the endoplasmic reticulum or lie freely in the cytoplasm. Each ribosome is made up of two sub-units, one large and one small. Ribosomes are made in the nucleolus area of the nucleus. They are the site of protein synthesis (manufacture).

The Golgi body (also called the Golgi apparatus or Golgi complex) consists of stacks of flattened membrane sacs. It chemically modifies, stores and distributes substances made by the endoplasmic reticulum. These are received in transport vesicles from the endoplasmic reticulum and repackaged ready for secretion either into or out of the cell. Chloroplasts are green plastids found only in green plant cells. They are 4–6 µm in diameter, surrounded by a double membrane. Their internal area, the stroma, contains a complex system of membranes or lamellae. Photosynthetic lamellae are also called thylakoids. These thylakoids occur in stacks called grana. The membranes of the thylakoids contain the chlorophyll pigments and enzymes needed for photosynthesis. They also contain ribosomes and DNA.

(a)

starch grain stack of grana (thylakoids) stroma ring of DNA

double membrane

lamella

(b)

ribosomes (a)

FIGURE 2.19 (a) A Golgi body. (b) Electronmicrograph of a Golgi body in a cell from the intestinal wall of a mouse (×9000).

(b)

FIGURE 2.20 (a) Diagram of a chloroplast. (b) Chloroplast of a plant cell (×4500).

Chloroplasts are green plastids found only in green plant cells.

Cells within cells within cells Both mitochondria and chloroplasts have their own DNA, and they produce their own membranes and ribosomes and some of their proteins. Their DNA and ribosomes are more similar to the DNA and ribosomes in bacterial cells than in plant and animal cells. Cell growth occurs as cytoplasm increases and is not linked to cell division. These observations have led to an intriguing theory, now widely accepted, about the evolution of cells. This theory proposes that, about 1.5 billion years ago, an aerobic bacterium was engulfed by a simple predatory cell with a membrane-bound nucleus, and the bacterium continued to live symbiotically within its host (Figure 2.21). This ‘guest’ became the forerunner of mitochondria, and the predatory cell became the first of a new type of cell, which eventually gave rise to protists, fungi, plants and animals. Some time later, another engulfment took place; this time it was of a photosynthetic bacterium, which became the forerunner of chloroplasts. This second event led to the evolution of green plants. Recent studies have shown that there is a double membrane envelope around the chloroplasts of

brown algae, suggesting that a third engulfment (probably of an ancestral red seaweed) may have taken place. A nucleus-like structure has been found between the two chloroplast membranes. bacterium

green plants

animals

bacterium simple predatory cell mitochondrion chloroplast nucleus

brown seaweeds

FIGURE 2.21 The endosymbiotic theory of evolution of encaryotic cells suggests that primitive cells engulfed different forms of bacteria, resulting in more complex cells. (See pp. 124–125.)

Patterns in nature 55

Questions 1

Describe the cell theory that is accepted by scientists today.

a Name each of the organelles X, Y and Z. b Describe the function of each organelle.

2

Outline the historical development of the cell theory. Include a discussion of the contributions or evidence of Robert Hooke and Anton van Leeuwenhoek.

Comment on the relationship between the structural features shown and the function you have stated.

3

Prepare a point-form summary comparing the light and electron microscopes. Include explanation of how the respective microscopes operate, their magnifying powers, resolution and applications.

4

Prepare a table that summarises the cell structures visible using these two different kinds of microscope.

5

a Identify the structures indicated on the diagram of the cell shown. b Describe the function of each of the organelles you have identified in (a).

3

Study the diagram of the following cell. Is it an animal cell or a plant cell? Give three reasons for your answer.

X

Z

X

Match the term listed in the left column with its correct description or role in the right column. organelle dark-staining area within nucleus protoplasm membrane-bound sac within cytoplasm, may contain dissolved nutrients or wastes for storage cell wall small spherical body; site of protein production nucleolus sub-cellular structure with a particular function vacuole rigid cellulose structure surrounding plant cells that provides support for cells chloroplast contents of cell ribosome stack of flattened membrane sacs that stores and distributes substances within cell and secretes substances out of cell Golgi body green plastid found in plant cells; site of photosynthesis

W

Y

6

7

Study the following electronmicrographs.

Y

Z

F u r ther questions 1

2

Light and electron microscopes together provide a clear picture of the structure of living cells. Summarise the limitations of the light microscope, compared with the electron microscope. Suggest why there is no description of cells in the writing of biologists 400 years ago.

cytoplasm food particle

vacuole 56

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chloroplast cell wall

nucleus

(x400)

2.2

Cell membranes: form and function OBJECTIVES When you have completed this section you should be able to: ● identify the major groups of substances that occur in cells and describe their role in cell activities ● recall that molecules move into and out of cells ● describe the current model of membrane structure ● explain how the structure of cell membranes accounts for the movement of some substances into and out of cells ● define the processes of diffusion and osmosis ● explain how the surface area to volume ratio of cells affects the rate of movement of substances into and out of cells.

The substances in cells All substances are either organic or inorganic. Living things are composed of organic substances, but they also contain inorganic substances. Organic molecules always contain carbon atoms. The major groups of organic molecules found in cells are carbohydrates, lipids, proteins and nucleic acids. Inorganic molecules do not usually contain carbon atoms and are found in living and non-living things. Some simple carbon compounds, including carbon dioxide (CO2), are often considered to be inorganic even though they contain carbon.

Inorganic substances Cells contain the inorganic substances water, oxygen and salts. Examples of salts are chlorides, phosphates and sulfates of various metals such as sodium, potassium and calcium. These salts are usually present in the form of ions (Table 2.5). For example, sodium chloride or common salt (NaCl) dissolves in the water in the human body, forming sodium ions (Na+) and chloride ions (Cl–).

activities ● ● ●

Chemical substances found in tissues Diffusion, osmosis and cell membranes Surface area to volume ratio

Organic molecules always contain carbon atoms. Inorganic molecules do not usually contain carbon atoms and are found in living and non-living things.

Patterns in nature 57

TABLE 2.5 How the human body uses some inorganic substances.

Name

Symbol

Use in the body

Calcium ion

Ca2+

builds strong bones, teeth; helps blood clotting and proper nerve and muscle functions

Iron ion

Fe2+

carries oxygen (attached to haemoglobin in red blood cells)

Oxygen gas

O2

used to release energy in respiration

Phosphate ion

PO43–

part of ATP, the substance that stores energy; also found in nucleic acids and bones

Sodium ion

Na+

functioning of nerves

Water (H2O) is the most abundant inorganic compound in the body. About 70% of the body’s molecules are water molecules. Water is needed by all cells. Most of the substances involved in cell chemistry are dissolved in water. For example, most nutrients and wastes dissolve in water in the blood, which transports them around the body to the various sites where they are either used or eliminated. Water is required for many cell processes and is also often produced as a result of them. Carbohydrates are compounds consisting of carbon, hydrogen and oxygen atoms (C, H and O).

Organic substances Carbohydrates

FIGURE 2.22 Some animals store fat in specialised body parts. The fat-tailed dunnart (Sminthopsis crassicaudata) survives over winter by using fat stored in its swollen tail.

Monosaccharides are the simplest carbohydrates, consisting of single units of sugar.

Disaccharides are also simple carbohydrates, consisting of double units of sugar.

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Carbohydrates are compounds consisting of carbon, hydrogen and oxygen atoms (C, H and O). The general formula for carbohydrates is (CH2O)n where n can be any number. Thus the ratio of hydrogen to oxygen is 2:1, as in water (H2O). Carbohydrates are important sources of energy in cells; they are broken down to glucose when energy is required. When humans eat more carbohydrate than they need, it is stored as fat. The body never wastes energy; any excess is stored under the skin or around the organs. To lose weight, less energy must be taken in than is needed, so that the fat stores are used up. Some carbohydrates are important in cell structure; for example, plant cell walls are made of a complex carbohydrate called cellulose.

Monosaccharides and disaccharides There are three groups of carbohydrates: monosaccharides, disaccharides and polysaccharides. Monosaccharides are the simplest carbohydrates, consisting of single units of sugar. They include glucose, the ‘instant energy’ in many sports drinks; fructose, a sugar in fruits; and ribose, a sugar in nucleic acids. All the monosaccharides are soluble in water. Monosaccharides are the basic building blocks of more complex carbohydrates. Disaccharides are also simple carbohydrates, consisting of double units of sugar. They include sucrose (table sugar), which consists of a glucose molecule and a fructose molecule joined together. Lactose (milk sugar) contains one molecule of glucose linked with one molecule of galactose; maltose contains two linked molecules of glucose. When two organic molecules link up, a water molecule is produced. This reaction is called condensation.

CH2OH

CH2OH

H

C

O

H

H

C

O

H

CH2OH O

C

H OH

H

C

C

H OH

H

C

C

C

C

OH

C

C

O

H

OH

HO

HO

H

OH

glucose molecule (C6H12O6)

CH2OH O C H

CH2OH

C HO

H

C

C

OH

H

C

O

C

H HO

OH C

OH

H

HO

C

C

OH

H

C CH2OH

sucrose molecule (C12H22O11)

H

H

H

ribose molecule (C5H10O5)

H

H

H

C

C

O

H

OH

H

C

C

C

OH H

H

C

O

OH

OH C H

FIGURE 2.23 Some monosaccharides and disaccharides.

CH2OH

lactose molecule (C12H22O11)

Polysaccharides Polysaccharides are complex carbohydrates consisting of multiple sugar units condensed to form huge molecules. Starch is a polysaccharide that is the main food store in plants such as potatoes and beans. One starch molecule contains 2000–3000 condensed glucose molecules. Cellulose, the main component of plant cell walls, contains more than 2000 condensed glucose molecules. Polysaccharides are insoluble in water.

+

+ glucose

fructose

CH2OH H

C

C

H OH

H

C

C

H

OH

H C

H

C

C

H OH

O

FIGURE 2.24 The process of condensation.

sucrose

CH2OH O

water

CH2OH O H

C

C

H

OH

H C

H

C

C

H OH

O

CH2OH O H

C

C

H

OH

H C

H

C

O

H

C

H OH

H

C

C

C

H

OH

H

OH

C

C

H

H

C

O

O

O

starch molecule (C6H10O5)n CH2OH H

C

C

H OH

O

O

H

C

C

C

H

H

OH

C H

H

OH

CH2OH

C

C

H

C

OH H

H

C

H OH

H

C

C

OH H

C

C

H

H

C

H

OH

C CH2OH

O

H C O

O

cellulose molecule (C6H10O5)n

O

CH2OH

O

FIGURE 2.25 Two polysaccharides: starch and cellulose.

Patterns in nature 59

glycerol three molecules of fatty acids FIGURE 2.26 A triglyceride: a typical lipid.

Lipids Lipids contain carbon, hydrogen and oxygen (C, H and O), but the ratio of H to O is never 2:1. There is very little oxygen in lipid molecules. A typical lipid in your body has the structure shown in Figure 2.26. Lipids are usually insoluble in water. This large group of organic molecules includes fats, oils, waxes and steroids. Fats contain more than twice the energy of carbohydrates.

TABLE 2.6 Some lipids and their properties and uses.

Lipid

P roperties

Uses

Fats

usually solids at 20°C; highly saturated molecules; common in animals

used as stores of energy, e.g. fat tissue

Oils

usually liquids at 20°C; unsaturated molecules; common in plants

used as stores of energy, e.g. coconut oil

Waxes

more common in plants than animals

used as waterproof coatings on leaves and fruits, e.g. the leaf cuticle

Steroids

have many important roles in plants and animals

form part of membranes, e.g. cholesterol; coordinate functions, e.g. cortisone and sex hormones

Proteins Proteins are large molecules made up of smaller molecules called amino acids joined together. The bonds linking the amino acids are called peptide bonds. A chain of linked amino acids is called a polypeptide.

BIOFACT Spider webs, silk, wool and feathers all consist of proteins. Feathers consist mostly of the same protein, keratin, that makes up human nails and hair.

Proteins are the most abundant organic molecules in cells, and they are needed by the body for growth and repair. Proteins contain atoms of carbon, hydrogen, oxygen and nitrogen (C, H, O and N). They are large molecules made up of smaller molecules called amino acids joined together by peptide bonds to form a polypeptide. A polypeptide may contain between 50 and 1000 amino acids linked in various sequences. Polypeptide chains fold and twist into various shapes (Figure 2.27). There are 20 different amino acids in cells, and they can combine in many different ways. To give you an idea of the number of possible combinations, imagine the number of words you could make using a 20letter alphabet.

The 20 amino acids are : alanine arginine asparagine aspartic acid cysteine glutamic acid glutamine

glycine histidine isoleucine leucine lysine methionine phenylalanine

proline serine threonine tryptophan tyrosine valine

The name ‘protein’ means ‘most important’. Proteins are certainly very important to cells: the structure and function of cells depend on the proteins from which they are made.

The structure of proteins A protein consists of one or more polypeptide chains. Different amino acids in the polypeptides have different side groups attached, and these side groups behave in particular ways. Some side groups are attracted to the water around them, some are attracted to other side groups in the chain, and others are repelled by the water or by neighbouring side groups. This occurs spontaneously. 60

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FIGURE 2.27 (a) A polypeptide chain may contain any assortment of amino acids joined together. Each symbol represents a different amino acid. (b) A protein molecule. The shape of the molecule results from folding one or more chains, brought about by forces between the amino acids.

(b)

(a)

All the competing forces interact to produce a particular shape of protein molecule which is stable and specific to that molecule. The three-dimensional shape of the protein molecule causes particular side groups to be exposed, and these determine the special chemical behaviour of the protein. For example, it determines the shape and nature of the active site of an enzyme, the electric charge and the pH (acidity) on the outside of the molecule (see p. 214), and whether it is attracted to oils or water. When a protein is heated, the atoms within may move apart, and the three-dimensional structure may be altered. This results in an alteration of the protein’s behaviour, a process called denaturing. You can observe this when egg white, which is made of protein, is cooked and ‘sets’.

Nucleic acids Nucleic acids are organic molecules which contain linked sugar molecules, nitrogen bases and phosphate groups. These molecules play an important part in determining heredity. Figure 2.28 represents the structure of a nucleic acid. The base–sugar–phosphate unit is called a nucleotide; it may be repeated hundreds of times or more in one nucleic acid molecule. There are two types of nucleic acid. Deoxyribonucleic acid (DNA) is mostly found in the chromosomes. Ribonucleic acid (RNA) is found throughout the cell and is needed for the manufacture of proteins. Both contain a pentose (5-carbon) sugar. RNA contains the sugar ribose. DNA contains the sugar deoxyribose (ribose with some oxygen missing).

The primary protein structure is the unique sequence of amino acids forming a polypeptide chain. The secondary structure is the initial folding or coiling of the polypeptide chain. The tertiary structure is the linking between side chains to form the final specific shape of the protein. The quaternary structure is the final shape of a protein if more than one polypeptide chain makes up the protein.

Nucleic acids are organic molecules which contain linked sugar molecules, nitrogen bases and phosphate groups.

phosphate pentose (5 carbon) sugar

base: 4 types one nucleotide

FIGURE 2.28 The structure of nucleic acid.

Patterns in nature 61

The movement of molecules Membrane structure: the fluid mosaic model Molecules move in and out of cells. Living cells are continually exchanging materials with their external environment.

Every cell is surrounded by the cell (or plasma) membrane. The cell membrane regulates the flow of substances in and out of the cell. It is differentially or selectively permeable; that is, only certain substances can cross it. Cell membranes appear to be dynamic structures that can form, reform and change. The fluid mosaic model (Figure 2.29) has been proposed to best account for these features. It is known that the cell membrane is composed of about 40% lipid and 60% protein. According to the fluid mosaic model, the cell membrane is a thin sheet composed of two layers like a sandwich (a ‘bilayer’) of special lipids called phospholipids. The sheet is very fluid and the phospholipids can move about quite easily within it. Other lipids such as cholesterol are also found within the membrane. Phospholipids have a ‘hydrophilic’ (water-loving) phosphate portion that can dissolve in water. Phospholipids also have a ‘hydrophobic’ (water-hating) lipid portion that repels water, and is soluble only in fats. The lipid portions are attracted to each other and line up in the centre of the sandwich (Figure 2.29). Proteins are scattered throughout the lipid bilayer. These are of two types: integral proteins which span the membrane, and peripheral proteins which are attached to the integral proteins on the inside or the outside of the membrane. The proteins are also capable of movement within the membrane. This structure accounts for the way that membranes prevent or allow materials to move across them by both passive and active means, as described below. outer film of lipid (hydrophobic)

carbohydrate

< 10 nanometres

integral protein

FIGURE 2.29 The fluid mosaic model of membrane structure.

lipid bilayer protein molecules

cytoplasm

hydrophilic layer

Diffusion Diffusion is the passive movement of molecules from a region where the concentration of those molecules is high to a region where the concentration is low.

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Diffusion is very important to living cells. It is one way they take in materials from the environment; it is also one way they rid themselves of unwanted materials produced in the cell. A diffusion gradient exists whenever two areas have different concentrations of a substance; the substance moves until the two concentrations are equal. This requires

no energy. Movement requiring no energy is known as passive transport. Water, oxygen, carbon dioxide and other small ions and molecules can diffuse freely through cell membranes.

molecules moving about become evenly distributed

Facilitated diffusion Facilitated diffusion is a type of passive transport in cells whereby substances are moved down a concentration gradient by ‘carrier proteins’ in the cell membrane (Figure 2.30). The membrane is a barrier to many water-soluble substances, such as glucose, urea and many ions. However, some fat-soluble substances such as oxygen, carbon dioxide and alcohol can cross the membrane. The proteins and cholesterol that are able to move within the membrane also affect its permeability. This is why it is called a selectively permeable membrane — it lets some molecules through but not others. The membrane thus controls separation from, and links with, the external environment.

Osmosis Osmosis is a special case of passive transport involving the diffusion of water molecules across a selectively permeable membrane. Water will move by diffusion from an area where there is more of it to an area where there is less of it. This results in the movement of water from a dilute (less concentrated) solution to a stronger (more concentrated) solution. It is the way that water enters or leaves cells.

Osmosis and diffusion compared Both osmosis and diffusion involve the passive spreading of molecules from regions where they are more concentrated to regions where they are less concentrated. Osmosis is the movement of a solvent, usually water, through a semipermeable membrane. The membrane allows the solvent through, but not larger molecules. The solvent moves across the membrane from where it is in high concentration to where it is in low concentration, until the two concentrations are equal. Diffusion is the movement or spreading out of any type of molecules, whether liquid or gas, from where they are more concentrated to where they are less concentrated. Diffusion occurs whether or not a membrane exists.

Active transport across membranes It can be difficult for some molecules to pass through a membrane; they might be blocked because of a diffusion gradient or because of their own properties. For example, some molecules cannot cross the membrane if they are too large, if they cannot dissolve in the membrane, or if they carry electric charges or fatty sections that bind them to the membrane. In active transport specific carrier proteins in the membrane bind with these molecules and carry them through the membrane. This process requires the expenditure of energy. Receptor molecules on the outer surface of the membrane bind to their special substrate molecules, activating the protein carrier molecules or membrane channels, thus allowing the previously excluded molecules to enter the cells. For example, the hormone insulin binds to insulin receptors on the membranes, which then become permeable to glucose molecules.

(a)

greater concentration outside cell (b)

concentration equal on both sides of cell membrane

FIGURE 2.30 The process of diffusion. (a) Molecules move from regions of higher concentration to regions of lower concentration. (b) If the concentration is higher outside the cell, molecules will diffuse through the cell membrane into the intercellular fluid.

Osmosis is the movement of water molecules across a selectively permeable membrane from a dilute solution (where the number of water molecules is high) to a more concentrated solution (where the number of water molecules is lower).

BIOFACT The disease called adult onset diabetes can be a consequence of obesity. The body cells become insulin-resistant through the loss of some of the membrane insulin receptors and impairment of the membrane transport system.

Two solutions of equal concentration are said to be isotonic; a more concentrated solution is hypertonic and a more dilute solution is hypotonic.

Patterns in nature 63

Endocytosis

There is a higher concentration of water molecules outside the cell than inside, so water diffuses into the cell.

Endocytosis is a type of active transport in which large molecules are transported across a membrane. The molecules are enclosed in a vacuole or vesicle formed by the membrane, and then discharged on the other side. There are three types of endocytosis. In pinocytosis the material transported is a liquid. In phagocytosis the material is solid. In receptor-mediated endocytosis the molecules bind to specific receptor sites on the membrane called coated pits.

Surface area to volume ratio

The extra water makes the cell swell up. The cell membrane now lies close to the cell wall, and the cell is turgid. FIGURE 2.31 Osmosis in a plant cell.

Consider a new cell beginning to grow. It needs nutrients, and takes them in through its surface membrane. The area of this surface affects the rate at which nutrients can enter the cell, as well as the rate at which wastes can leave. As the cell grows, its needs are probably greater, but the rate of exchange of materials with its surroundings falls. We can use a model to see why this is so. Figure 2.32a shows a cube with faces 1 cm × 1 cm. total surface area = 6 × 1 × 1 = 6 cm2 volume = 1 × 1 × 1 = 1 cm3 ∴ surface area to volume ratio = 6 : 1 Figure 2.32b shows a cube with faces 2 cm × 2 cm. total surface area = 6 × 2 × 2 cm = 24 cm2 volume = 2 × 2 × 2 cm = 8 cm3 ∴ surface area to volume ratio = 24 : 8 = 3 : 1

(a)

(b) 2 cm

1 cm 1 cm

1 cm 2 cm 2 cm

FIGURE 2.32 The volume of a cube increases at a faster rate than the surface area, so a larger cube has a smaller surface area to volume ratio.

FIGURE 2.33 Many planarians (flatworms) are very thin, so their surface area to volume ratio is very large. Can you suggest both advantages and disadvantages of this?

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So as the cube increases in size, the relative amount of surface decreases. This is why the capacity to take in enough nutrients and lose wastes decreases as cells grow. One reason for the small size of cells could be that the rate of exchange of substances through the membrane is more efficient than in a large cell. When a cell reaches a certain size, it divides into two smaller cells, restoring a more favourable surface area to volume ratio. In multicellular organisms, tissues and organs involved in the exchange of materials with the external environment often have large surface areas in relation to their volumes, to enable the exchange to be conducted efficiently (Figure 2.33).

Questions 1

a Explain the difference between organic and inorganic substances. b Complete the following table summarising the major groups of organic compounds found in living things. Organic compound

Elements

Function

3

A key role of cell membranes is to regulate the passage of materials into and out of cells. The feature of cells that allows for this regulatory role is the selectively permeable cell membrane. What is meant by ‘selectively permeable’?

4

a Distinguish between the processes of diffusion, osmosis and active transport. Use diagrams to illustrate your answer. b List the substances that move across cell membranes by each of these different processes.

5

What is ‘facilitated diffusion’?

6

a What is meant by ‘surface area to volume ratio’? b Cells have a large surface area to volume ratio. Explain why this is important for cells.

carbohydrates proteins lipids nucleic acids 2

Describe the fluid mosaic model of the cell membrane.

F u r ther questions 1

a Use the results to determine the initial concentrations of the glucose solutions in each of the test-tubes, in relation to the 2% glucose solution into which the test-tubes were immersed. b Explain fully what has occurred in each test-tube.

a Divide the following list into organic and inorganic substances. water iron protein starch lipid oxygen monosaccharide calcium nucleic acid cellulose Explain the basis upon which you have grouped these substances.

3

When a kidney patient undergoes dialysis, their blood is passed through the dialysis machine to remove metabolic wastes. Materials are exchanged between the blood and dialysing fluid inside the dialysis unit. It is important that the concentration of the dialysing fluid is the same as the concentration of the blood. Explain why this is so.

4

How do large multicellular organisms overcome the problem of a small surface area to volume ratio so that the rate of exchange of nutrients and wastes can occur efficiently?

b Carbon dioxide contains carbon but it is classified as an inorganic compound. Explain why. 2

Students conducted an experiment to investigate the movement of glucose across a selectively permeable membrane. They set up three tubes, each with its end secured with a selectively permeable membrane. Each tube contained different concentrations of glucose solution. They then placed the tubes into a beaker of 2% glucose solution and left the experiment for 24 hours. The results were as follows: A

B

C

Initial

A

B

C

Final 2% sugar solution Patterns in nature 65

2.23

Obtaining nutrients Cell formation O OBBJJEEC CTTIIVVEESS When this section youyou should be be Whenyou youhave havecompleted completed this section should able ableto: to: ●● distinguish between abiotic factors in describe the organisation ofand cellsbiotic into tissues, organs and the environment systems ● identify some examples to demonstrate the structural and functional relationships between cells, tissues, organs and organ systems ● explain the difference between autotrophs and heterotrophs ● explain the significance of photosynthesis for ecosystems ● list the materials needed for the process of photosynthesis ● write the word equation for the process of photosynthesis ● understand that the general equation for photosynthesis represents a summary of complex biochemical reactions ● explain the relationship between the arrangement of plant structures involved in the uptake of water and minerals and the surface area available for this uptake ● explain the relationship between the internal and external structure of leaves and their role ● explain the importance of increasing the surface area of foods for efficient chemical digestion, especially the role of teeth ● explain the relationship between the structure of vertebrate digestive systems and diet, including a comparison of herbivores and carnivores ● outline the function of specialised structures in the digestive systems of herbivores and carnivores.

Cells and systems activities ● ●

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Photosynthesis Comparison of mammalian digestive systems

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In multicellular organisms, different cells can become specialised to perform different functions. Different types of cells have different structures and activities, but they work together so that the organism functions as a coordinated whole. Groups of cells that are similar in structure and function are called tissues. Groups of tissues make up organs. Organs make up systems within the body, such as the digestive system in humans (Figure 2.35).

For an example of the types of cells and their functions in the small intestine of the digestive system, see page 77 and Figure 2.55. For an example of the types of cells and their functions in the leaf of a plant, see page 74 and Figure 2.49. the digestive system from the human organism

muscle cells

FIGURE 2.34 The leaves of the moss Calyptrochaeta apiculata contain two types of cells with different functions. The hexagonal lamina cells carry out photosynthesis and the other normal functions of leaf cells. The longer, narrower marginal cells give the leaf strength and protect it from mechanical damage.

tissue from the stomach the stomach, an organ

Multicellular organisms are composed of many cells. There is an enormous range of multicellular organisms, including all plants, animals and fungi, and most algae. A human is a multicellular organism consisting of cells packed together.

FIGURE 2.35 How cells, tissues and organs make up the human digestive system.

Autotrophic and heterotrophic cells There are many different types of cells—there is no such thing as a ‘typical’ cell. But we can talk in general terms about common characteristics of plant cells and compare them with the common characteristics of animal cells. One essential difference is that plant cells are autotrophic while animal cells are heterotrophic. Autotrophic means ‘self-feeding’. Plant cells which contain chloroplasts are able to produce their own nutrient requirements by photosynthesis. Heterotrophic means ‘feeding on something different’. Animal cells do not contain chloroplasts. They must obtain the substances they need from their external environment.

Patterns in nature 67

The autotrophic nature of green plants is fundamental to the existence of all organisms: all heterotrophic organisms depend ultimately on the ability of autotrophs to synthesise organic materials. Revise the differences between plant and animal cells (p. 51).

Photosynthesis Materials for photosynthesis, and its role in ecosystems All living things ultimately depend on the process of photosynthesis.

BIOFACT Chemosynthetic or chemoautotrophic organisms obtain their energy from inorganic sources. Sulfur bacteria such as Thiobacillus obtain their energy from the oxidation of hydrogen sulfide. (See also p. 127.)

Word equations can be used to describe reactions. This is a useful way to summarise the process of photosynthesis: light energy chlorophyll carbon + water sugar + oxygen dioxide

Photosynthesis can occur only if plant cells can obtain carbon dioxide, water and light from their external environment. The products of photosynthesis are sugars and oxygen. All living things ultimately depend on the process of photosynthesis, which provides the continuous input of energy necessary to sustain ecosystems. Photosynthesis enables green plants to obtain energy directly; animals obtain this energy indirectly from the food they eat.

The photosynthesis equation A generalised equation for photosynthesis can be written as follows: 6CO2 + 6H2O → C6H12O6 + 6O2 In fact, this equation is more correctly written as: 6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O Notice that six H2O molecules have been added to each side of the equation. This is done to show that all the oxygen released comes from the water, not the carbon dioxide. There are 12 atoms of oxygen in the six molecules of oxygen gas on the right-hand side now, and 12 atoms of oxygen in the water on the left-hand side. The process of photosynthesis can be thought of as occurring in two sets of reactions or stages, although it is actually continuous: the products of the first stage become the raw materials of the second stage. Only the first set of reactions use light; these are called the light reactions of photosynthesis. The second set use carbon dioxide and are light-independent; they are also called the carbon-fixation stage of photosynthesis.

Light reactions of photosynthesis The term photolysis is often used for the process by which light splits water (photo = light, lysis = split).

Radiant energy from sunlight is absorbed by the chlorophyll pigments in the chloroplasts of green plant cells and is converted to chemical energy. Some of this energy is used to split water molecules into hydrogen and oxygen, as expressed in the following equation: light

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Radioactive tracing The fact that water (not carbon dioxide) is the source of the oxygen released in photosynthesis was revealed by the use of radioactive isotopes. This method is called radioactive tracing because it tells us precisely where atoms come from and where they go. The first tracer experiments were performed in 1920, and tracers have since been used by many

scientists to follow chemicals in living systems in plants and animals. For example, when photosynthesising plants are watered with H2O containing radioactive atoms of oxygen, all the radioactive atoms are eventually lost from the cells as oxygen gas (O2). Figure 2.36 shows an example of a radioactive tracer experiment. FIGURE 2.36 A radioactive tracer experiment.

dilute acid potassium hydroxide solution absorbs CO2 to prevent escape of radioactive CO2

light aluminium foil

heavy weight

radioactive CO2

film blackened only by regions of leaf which received light reaction produces 14CO 2 photographic film

plant exposed to radioactive CO2 in the light

leaf detached after 4 hours light, pressed on photographic film in total darkness

film developed

This stage occurs on the internal membranes of the chloroplasts (thylakoids) in plant cells (see p. 55).

Light-independent reactions of photosynthesis Hydrogen released from the first reactions combines with carbon dioxide to form sugars. This process, which is a building-up reaction, requires energy. The energy needed is supplied from some of the energy absorbed from light in the first set of reactions in photosynthesis or from energy stores (ATP) in the plant. energy hydrogen + carbon dioxide → sugars These reactions occur in the stroma or fluid matrix of chloroplasts. At least two different pathways are known. In many plants the C3 or Calvin cycle occurs where the first stable compound produced contains three carbon atoms. In some plants, such as sugar cane and corn, the C4 pathway results in the first stable compound produced having four carbon atoms.

BIOFACT The light-independent reactions are sometimes referred to as dark reactions. However, most biologists now avoid this term because the whole process of photosynthesis takes place in sunlight, and because these reactions must have the products of the light reactions to operate.

Patterns in nature 69

BIOFACT Most sugars contain 6 or 12 carbon atoms in each molecule. Common household sugar is sucrose, a disaccharide (see p. 58). In Australia this is obtained mostly from sugar cane, which is grown in Queensland and northern New South Wales. In some countries, such as the USA, the main source of household sugar is sugar beet. Sucrose is a compound of two other sugars: fructose and glucose.

The production of sugars The sugars made in cells may be used in cell respiration as they are formed. During the day there is usually enough light for photosynthesis to produce sugars faster than they are used in respiration. As a result, sugars are converted to starch and stored in the cells. This is why we test a green leaf for starch when we are looking for evidence of photosynthesis (Figure 2.37). At night, some starch is converted back into sugars, which are transported in the phloem to any cells that need it for respiration. Sugars that are not used in respiration may be built up into proteins, helping the plant to grow, or may be stored as starch or lipid (Figure 2.38).

green part of leaf white part of leaf methylated spirit warming

variegated leaf

boiled leaf boiling water

burner extinguished

iodine (yellow-brown) iodine turns blue-black if starch is present

FIGURE 2.37 Testing a leaf for starch.

photosynthesis

sugars

FIGURE 2.38 Organic molecules made by plants from the products of photosynthesis.

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carbohydrates e.g. sugars for respiration, starch for storage, cellulose for cell walls

lipids e.g. lipoproteins in cell membranes or oils for storage

nitrogen compounds (from the soil)

proteins e.g. enzymes

The role of enzymes in photosynthesis

Factors affecting the rate of photosynthesis The rate of photosynthesis, like all chemical reactions, depends on the availability of the required substances. The concentration of carbon dioxide, the availability of water, and the intensity of light are all important factors in this process (Figure 2.39). If the supply of any one of these is low, it is described as a limiting factor because it limits the rate at which the whole process of photosynthesis can occur. The rate of photosynthesis also increases with temperature, mainly because the activity of the enzymes involved increases. However, very high temperatures can slow down the reaction.

Rate of photosynthesis

Enzymes are substances that speed up the rate of reactions, without being used up in the reaction. Each of the many reactions in photosynthesis is controlled by a particular enzyme. The enzyme molecule provides a site (called the active site) at which the reactants can come together. On the active site, the reaction occurs faster than it would otherwise do. When the reaction is complete, the products leave the active site and the enzyme is free to be used again (see Chapter 5, p. 213).

20 18000 units of light

15 6000 units of light

10

2000 units of light

5

667 units of light

0

0.20 0.30 0.01 0.10 Concentration of CO2

FIGURE 2.39 The effect of light intensity and carbon dioxide concentration on the rate of photosynthesis. Note how the amount of light limits the rate of photosynthesis, even though the rate increases with increasing carbon dioxide concentration.

Obtaining nutrients—plants Plants use specialised structures to obtain the materials required for photosynthesis from their environment.

Obtaining water and minerals In an aquatic environment, water and minerals can be absorbed across the whole surface of the plant. In most terrestrial plants, water and minerals are obtained through root systems, which also anchor the plants in the soil. Plant roots need a large surface area over which absorption can occur. They achieve this by having a branching structure, and root hairs just behind the root tips.

Different types of roots Figure 2.40 shows the two major types of root systems found in flowering plants. Tap root systems have a main root from which side (lateral) roots emerge. Tap roots may penetrate deeply into the soil. Some tap roots may act as storage organs and swell up as food reserves are deposited. Carrots, beetroots and parsnips are examples of this. Fibrous roots form a network of roots close to the soil surface. They may spread out widely to anchor the plant, helping to bind the soil and prevent erosion. Some types of plants, for example banksias, form cluster roots. These are groups of many tiny roots. These increase the surface area for the uptake of mineral ions in nutrient-poor soils. Patterns in nature 71

Mycorrhiza Over 90% of flowering plants have their root systems associated with fungi. The fungi provide the plant with additional mineral nutrients and the plant provides the fungus with carbon products from photosynthesis. Fungal threads (hyphae) may form a sheath surrounding the roots of the plant. Aerial roots In some plants, roots may grow above the ground. These may help to support the plant and are often called prop roots or buttress roots. Some plants that grow in wet or waterlogged conditions produce aerial roots that help in gas exchange (see p. 31). Rhizoids Mosses, liverworts and hornworts do not have true roots. They are anchored to their substratum by hair-like structures called rhizoids.

tap root

External root structure

fibrous root FIGURE 2.40 Two types of root systems.

The root system of a plant is usually entirely below ground. Because roots do not contain chloroplasts, they do not photosynthesise. They also do not have leaves or buds. The roots of most plants grow down into the soil. Their growing point is protected by a root cap. As roots grow through the soil, they form a branching network which helps to anchor the plant. Just behind the growing point is the region of root hairs, which provide a large surface area where most of the absorption of water and mineral ions takes place.

tip of root

soil crumbs

;;;; ;;;;; ;;;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;; ; ; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ;;;;;;;;;;;;;;;;;;;;;;;; ; root ; ;;;; ;;;; ;;; ;;; ; hair ;; ;;; air spaces

water film (a)

FIGURE 2.41 (a) The external structure of a root. (b) Root hairs on a radish seedling. (b)

Internal root structure Epidermis The protective outer layer or epidermis of roots usually lacks a cuticle. In many young roots the epidermis is covered with a slimy coating or sheath called mucigel. Cortex In the roots of many plants the large cells of the cortex act as a storage area for excess materials. There are air spaces between the cells for the circulation of gases. 72

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Vascular tissue In roots the vascular tissue forms a cylinder in the centre. It consists of xylem and phloem (see pp. 85–86), and is sometimes called the stele. central

cortex

vascular tissue

cortex root hair

root hair cell wall

phloem

cytoplasm

xylem epidermis

zone of elongation cell division occurs here root cap FIGURE 2.42 Internal root structure.

vacuole soil particle

FIGURE 2.44 Cross-section of a Ranunculus root.

phloem FIGURE 2.43 Cross-section of epidermal cell, showing root hair.

cortex cells

Obtaining sunlight and carbon dioxide All plants need light to photosynthesise. Some aquatic plants drift in the surface layers of oceans or large bodies of fresh water. Aquatic plants which remain anchored by root systems must be in shallow water in order to obtain sufficient light. On land, most plants are anchored in the soil and must be exposed to enough light in order to survive. Plants require carbon dioxide for photosynthesis. They obtain it either from the water if they are submerged aquatic plants, or from the air. The specialised plant structure for obtaining light and carbon dioxide in most plants is the leaf. It is in the leaf where most photosynthesis for the plant occurs.

xylem FIGURE 2.45 Central vascular tissue of a Ranunculus root.

The structure of a leaf The external structure of a typical leaf is visible to the naked eye. It consists of a lamina (the blade of the leaf), a petiole which joins the leaf to the stem, and a network of veins (Figure 2.47). The internal structure of a leaf is visible through a microscope, which reveals specialised tissues (Figure 2.49). These structures are all related to the main function of the leaf as the organ of photosynthesis for the plant.

External leaf structure Arrangement Leaves are usually arranged along the stems of plants in a way that exposes them to the maximum amount of sunlight possible. They are usually angled so that the sunlight strikes the upper surface of the leaves.

FIGURE 2.46 Water-lilies have long stems that enable them to be anchored in the mud in shallow water, while having the floating leaves fully exposed to sunlight.

Shape Most leaves are broad, thin and flat (Figure 2.48). This gives them a large surface area for the capture of sunlight and the exchange with the environment of gases needed in photosynthesis and respiration. Patterns in nature 73

Internal leaf structure

blade

midrib

Cuticle Leaves secrete a waxy substance which forms an outer layer or cuticle. This helps to maintain the shape of the leaf and provides protection. In terrestrial plants it plays an important role in reducing water loss by evaporation. It may be thin or absent in aquatic plants. Epidermis The epidermis forms a single protective layer of cells on the upper and lower surfaces of the leaf. It is transparent, so sunlight can readily penetrate through to the photosynthetic cells within. Stomates Stomates are pores in the leaf that can open and close (see p. 89). When open they permit the exchange of gases between the leaf and the external environment. At the same time, however, water is lost by evaporation.

vein petiole FIGURE 2.47 The external structure of a typical leaf.

a

b

c

d

e

f

Mesophyll The word mesophyll means ‘middle leaf’. The mesophyll cells in the centre of the leaf consist of two types (Figure 2.49). Palisade mesophyll cells are found most commonly in one or two rows below the upper epidermis. They are regularly arranged, elongated cells packed with green chloroplasts. It is in these cells that most of the plant’s photosynthesis occurs. Spongy mesophyll cells are usually situated between the palisade cells and the lower epidermis. They contain fewer chloroplasts than palisade cells and are irregularly arranged with large spaces between them. This arrangement enables gases and water vapour to move easily between the cells and stomates. Veins Veins are tubes of vascular tissues consisting of xylem and phloem cells, which transport materials to and from the leaf. (The xylem transports water and mineral ions from the roots to the leaves; the phloem transports the products of photosynthesis from the leaves to the rest of the plant; see p. 86). Veins form a branching network throughout the leaf. This helps give rigidity to the leaf, maintain its shape and structure, and ensures that every leaf cell is close to a vein.

midrib GREATLY ENLARGED

g

h

i

palisade mesophyll cell

epidermis xylem vessel

cuticle

vein water-carring xylem vessel

FIGURE 2.48 Some common leaf shapes; (a) lanceolate, (b) obovate, (c) oblong, (d) rhomboidal, (e) ovate, (f) sagittate, (g) linear, (h) deltoid, (i) elliptical.

vein air space

FIGURE 2.49 The internal structure of a leaf.

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stomate phloem cell spongy mesophyll

guard cell

food-carrying phloem cell

Mammalian digestion The digestive system Animal cells are heterotrophic. Unlike plant cells, they cannot make their own food, so they must obtain it from their external environment. In heterotrophic organisms such as mammals, the digestive system provides the means by which external nutrients required by cells are taken into the organism and broken down or digested. Large, insoluble food molecules are converted into small, soluble ones that can be absorbed and made available to the body cells. In mammals, the process of digestion involves both the mechanical and chemical breakdown of food. This is followed by its absorption into the body. Complex molecules are broken down into simple molecules by the action of specialised proteins called enzymes. Most digestive enzymes split food molecules by hydrolysis, in which a molecule is split at a particular point by adding a water molecule. There are three kinds of digestive enzymes: ● amylases, which act on carbohydrates ● proteases, which act on proteins ● lipases, which act on lipids. These enzymes are produced by specific cells along the length of the gut, as well as in the salivary glands and pancreas. Large food molecules might need to be broken down by several enzymes acting in sequence. An overview of the structure and function of mammalian digestive systems is illustrated in Figure 2.51, using the human digestive system as an example.

Herbivores and carnivores compared

(a) cutting cropping

(b) shearing

(c)

grinding

The mouth and teeth In the mouth, food is usually broken up by the teeth and mixed with saliva, which lubricates the food so it is easier to swallow. Chemical digestion of carbohydrates may also begin. The action of teeth greatly increases the surface area of foods to expose them to the digestive chemicals that will help to break them down. Most mammals have four kinds of teeth: incisors and canines (which are single teeth for cutting), and premolars and molars (which are double teeth for grinding). Think about the way you bite and chew an apple. The single teeth cut it and the double teeth grind it. In mammals, the type of teeth depend largely on the sorts of food eaten. Carnivores (animals that eat other animals) have large canines. Their cheek teeth are specialised for a slicing action to cut and tear flesh and bones. The jaws of a carnivore are short and have a powerful bite. They can only move up and down. Carnivores typically gulp their food. Carnivorous mammals include dogs, cats, tiger quolls and Tasmanian devils. Herbivores (animals that eat plants) have no canines, but have large flat double teeth to grind plant food with jaws that can move from side to side. Herbivorous mammals include cows, kangaroos and koalas. Omnivores (animals that eat both plants and animals) have some teeth for tearing and cutting meat, and others for grinding plant food. Omnivorous mammals include humans, pigs and some rodents.

(d) chewing

indicates movement of teeth

FIGURE 2.50 In mammals, tooth structure is adapted for the mechanical breakdown of different types of foods. (a) Incisors are typically used for cutting and tearing. (b) Carnivores have large, powerful cheek teeth that shear through tough sinews and bones. (c) Herbivores have molars that continuously grind fibrous plant foods. (d) Omnivores, such as ourselves, have molars that roll and crush a variety of foods.

Patterns in nature 75

mouth and mouth cavity (pH 6–8)

epiglottis oesophagus

trachea (windpipe)

liver

small duodenum intestine (pH 7–9) ileum

FIGURE 2.52 Some bats have an almost liquid diet. Flying-foxes and fruit-bats feed on soft fruits, nectar, pollen and small seeds. Flattened molar teeth and a ridged palate (roof of the mouth) crush the fruit; the juice is swallowed and the fibre spat out. Blossom bats (pictured) feed on pollen and nectar, using a brush-tipped tongue to reach deep into flowers. The digestive tract in these animals is very short and unspecialised, and food passes through it very quickly.

muscular wall of oesophagus

food bolus movement of food

FIGURE 2.53 A rhythmic contraction of the oesophagus wall, called peristalsis, moves food from the mouth to the stomach.

caecum

gall bladder

stomach (pH 1–3) pancreas

large intestine

rectum

appendix anus FIGURE 2.51 The human digestive system. Mouth and mouth cavity: teeth mechanically break food into pieces, saliva lubricates food, amylase digests starch into maltose Epiglottis: closes off the trachea so food goes down oesophagus Oesophagus: peristalsis carries food to stomach Stomach: proteases begin the digestion of proteins, and food is churned Pancreas: produces enzymes and neutralises acid Liver: produces bile which emulsifies fats, regulates and stores some products of digestion Gall bladder: stores bile Small intestine: digestion is completed by enzymes from the pancreas and the small intestine itself; nutrients and water are absorbed Large intestine: water is absorbed with soluble compounds like vitamins and minerals; undigested food leaves body as faeces via the anus

Oesophagus The tube between mouth and stomach is called the oesophagus. The food is pushed along the oesophagus and along the length of the digestive tract by rhythmic muscle contractions known as peristalsis.

Stomach In the stomach, chemical digestion of food, particularly proteins, begins. Mechanical digestion also continues through the churning of food. The length of time food spends in the stomach of a mammal is related to the type of diet. Carnivores have simple stomachs; herbivores with fore-gut digestion of cellulose may have complex stomachs and food may remain there for a long time. 76

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caecum oesophagus stomach

large intestine

caecum oesophagus stomach

small intestine small intestine

(a)

oesophagus

stomach

large intestine

small intestine

large intestine

caecum

(c)

(b)

FIGURE 2.54 The digestive systems of (a) a carnivore (dog), (b) a herbivore (cow), and (c) an omnivore (human).

Small intestine Much chemical digestion occurs in the small intestine and as soon as food is broken down into small enough molecules these are absorbed through the walls of the digestive system into the blood to be transported to the body cells. Most absorption occurs in the small intestine. The inside of the small intestine is constructed of folds called villi (singular villus) (Figure 2.55a). The epithelial cells on the surface of the villi have small finger-like projections called microvilli (Figure 2.55b). These folds and projections provide a large surface area enabling material in the small intestine to be absorbed quickly.

• Proteins are absorbed after being broken down to amino acids. • Carbohydrates are absorbed after being broken down to simple sugars (monosaccharides). • Lipids are absorbed after being broken down to fatty acids and glycerol. • Vitamins, minerals and water are absorbed directly.

BIOFACT

small intestine microvilli

Infections of the gut often result in diarrhoea (very watery faeces) because the food moves so quickly along the intestine that there is insufficient time for normal levels of uptake of digested food and the subsequent absorption of water by osmosis.

epithelium

mucusproducing cells

epithelium

villus epithelium

network of capillaries

lacteal lymphatic system

blood vessels to villus circular muscle (a)

longitudinal muscle

blood capillary (b)

FIGURE 2.55 (a) The internal surface of the small intestine is highly specialised for absorption. Villi and microvilli greatly increase the surface area, and there are many blood and lymphatic vessels to carry away the absorbed products of digestion. (b) An electronmicrograph showing microvilli on the inner surface of the intestinal epithelial cells.

Patterns in nature 77

BIOFACT Nectar feeders such as the honey possum have a diet low in fibre. They have short, simple digestive systems.

BIOFACT ● ●



An adult koala eats about 0.5 kg of leave per day. The caecum in an adult koala is 2.4 metres long — the longest of any mammal. The metabolic rate of koalas is about half the mammalian average.

Large intestine Water and salts are absorbed in the large intestine. Materials that remain undigested and unabsorbed, together with bacteria, cellular material from the walls of the intestines, and some water and salts, are eliminated from the digestive system as faeces. Herbivores usually produce greater quantities of large, more fibrous faeces than carnivores. Herbivores with hind-gut fermentation have an enlarged caecum at the start of the large intestine. Bacterial fermentation of plant material occurs here to break down cellulose fibres. Some absorption of the products of this digestion can then occur in the large intestine. To ensure as much advantage as possible is taken of these nutrients, some hind-gut fermenters, such as possums and rabbits, produce two types of faeces. During the day, dry, fibrous faeces are released. At night, soft, moist faeces are released from the caecum. These are re-eaten by the animal. This ensures that the nutrients released by the action of the bacteria can be absorbed by the animal more effectively in the small intestine. Carnivores have a much shorter large intestine and a small caecum. Their food requires little if any fermentation.

All gummed up: koalas and digestion The koala is notable for its exclusive diet; it eats only the leaves of eucalypt trees (gum leaves). Of around 700 species of Eucalyptus, only about 20 are eaten and, in a particular area, three or four species will be favoured. Although more than 90% of Australia’s forest trees are eucalypts, only four marsupials (the koala, greater glider, ringtail possum and brushtail possum) include them as a significant component of their diet, and only the koala eats them exclusively. Gum leaves contain only about 50% water, very little nitrogen, large amounts of fibre (including components that are difficult or impossible to digest), and potentially toxic oils. How do koalas cope? They prefer young, soft leaves and chew these to a fine paste. Most of the digestible contents are absorbed in the stomach. Both the caecum and the first part of the colon of the koala are greatly enlarged to allow for a high level of bacterial fermentation, but even so, very little energy is released from eucalypt cell walls because they are so indigestible. In fact, koalas get only about 10% of their energy from fermentation, and they rely heavily on cell contents for nutrition. Bulky, indigestible fibre is selectively and speedily egested from the gut to make way for more food. Because of the energy and nutrient costs of detoxifying some of the leaf oils, the less food a koala has to eat to survive, the better. As a group, marsupials have a lower metabolic rate than placental mammals, so the koala’s basal energy requirements are less. Also the characteristic slow and ‘lazy’ behaviour of the koalas mean that they do not use up much energy for physical activity.

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FIGURE 2.56 The koala’s digestive system is specially adapted to its diet of eucalypt leaves.

Guts: the inside story Plant matter is more difficult to digest than animal tissues. Plant cells have tough cellulose cell walls that must be broken down before the cell contents can be released. Animals are not able to do this unaided. Herbivores use micro-organisms that live symbiotically in their digestive systems to help them. The breakdown of cellulose occurs during a fermentation process in a specialised part in the digestive tract. These structures are found in either the fore-gut or the hind-gut of different parts of the digestive system (see Figure 2.57): ● Fore-gut fermenters carry out digestion in a chamber that is before the stomach. In cattle, this is called the rumen. Sheep, kangaroos and wallabies are other examples of fore-gut fermenters. ● Hind-gut fermenters carry out cellulose digestion in the caecum, a chamber that comes after the small intestine. Horses, rabbits, possums and koalas are examples of hind-gut fermenters. Because animal cells do not have a cell wall, they can be digested more rapidly. Muscle (meat) contains a lot of protein, so carnivores do not have to consume the large quantity of materials that herbivores do to obtain the same amount of nutrients. Plant cells also Koala small intestine

provide less energy than meat per gram eaten, and their breakdown time in the digestive system is much longer. In contrast, animal foods release a higher proportion of energy per gram, and they can be digested much more quickly. As a result, carnivore digestive systems are shorter and less complex than those of herbivores. The structures and functions found within the digestive systems of different animals can usually be clearly related to their dietary patterns; that is, to the types of food they eat. Food does not spend long in a carnivore’s mouth and is quickly gulped down. A dog, for example, may spend only 15 minutes per day eating. The intestines are short and unspecialised compared with those of herbivores. Carnivores in the wild are hunters and eat irregularly. Herbivores, such as as cows and kangaroos, spend much of the day chewing grass and other soft plants. Omnivores usually eat for a moderate amount of time each day; humans typically eat for about 90 minutes each day. Our digestive systems are intermediate in length between herbivores and carnivores and do not show particular specialisation. We cannot digest cellulose—it is passed through as ‘roughage’.

Wombat oesophagus stomach

Kangaroo

small intestine stomach

oesophagus

oesophagus

small intestine

caecum caecum stomach first part of large intestine first part of large intestine

remainder of large intestine

caecum

large intestine remainder of large intestine

FIGURE 2.57 Koalas, wombats and kangaroos are herbivores and use symbiotic bacteria for the digestion of cellulose. Koalas and wombats are hind-gut fermenters, whereas kangaroos are fore-gut fermenters.

1

Give a reason why carnivores need to eat less regularly than herbivores.

2

Explain why food spends longer in the herbivore’s digestive system than it does in a carnivore’s digestive system.

Patterns in nature 79

Questions 1

Define the following terms, and provide examples of each: a tissue b organ c system.

2

Cell specialisation in multicellular organisms is an adaptation that allows all of the organism’s requirements to be met. Complete the table below, showing some different kinds of specialised cells in multicellular organisms and their functions.

Cell type

Plant/animal

Diagram

Function

How structure re l a t e s t o f u n c t i o n

nerve cell

lining cell of small intestine root hair cell

leaf mesophyll cell

3

Distinguish between the autotrophic and heterotrophic nature of plants and animals.

4

Explain the following statement: ‘All living things depend on plants.’

5

Write out the word equation that summarises the process of photosynthesis.

6

a Compare aquatic and terrestrial plants in relation to the site of water and mineral uptake. b Terrestrial plants need a large surface area in order to maximise water uptake. How is this achieved?

7

8

80

Explain how the structure of typical leaves and the distribution of specialised tissue within them are adaptations to photosynthesis. Use clearly labelled diagrams in your answer. a Explain the difference between mechanical and chemical digestion. b Teeth are involved in the process of mechanical digestion. Describe the significance of their role in terms of preparing foods for chemical digestion.

Heinemann Biology

9 Identify the structures labelled W, X, Y and Z in the diagram of the herbivore digestive system shown. What is the major function of each structure you have identified? Z

Y W

X

10 Outline the importance of villi and microvilli in the digestive system. 11 a Compare the digestive systems of dogs (Figure 2.54) and koalas (Figure 2.57). Include a description of their respective digestive systems, commenting on length, complexity and role of different structures. b Make a statement relating the length and complexity of the digestive systems of dogs and koalas to the chemical composition of their respective diets.

F u r ther questions 1

2

3

Visit your local nursery or consult an encyclopaedia of plants to find out the answers to each of the following. a What is the difference between macronutrients and micronutrients? b Packets of fertiliser purchased from nurseries often list ‘NPK’ as major ingredients. i For what nutrients are these letters an abbreviation? ii How are each of these nutrients important to plants? A green pot-plant was placed in a position to receive plenty of sunlight during the day. It was watered regularly. Very little new growth occurred, and the plant soon began to lose its leaves and turn brown. Suggest a possible explanation for this observation. Complete the table, which summarises actions of enzymes.

Food

Type of enzyme

Final product of chemical digestion

carbohydrate lipid protein

4

a What is happening to the rate of photosynthesis between the points X and Y? Explain. b Why does the graph level off after point Y? c What is the limiting factor in this experiment? Why do you think so? d List two factors that are being controlled in this experiment. 5

Design a simple experiment to test the effect of temperature on photosynthesis.

6

Explain the importance of the liver in relation to the products of digestion.

7

Use a reference book to find out about the appendix in humans. What role, if any, does it play? How does it compare to similar structures in herbivorous animals? What does the presence of the appendix in humans suggest about our relationship to other species?

8

Prepare a poster that would appeal to your classmates, providing information about diet. Include ●

a summary of what is meant by the term ‘balanced diet’.



the five major food groups, with examples of the kinds of foods offered by each and the recommended daily allowance of each.

Examine the following graph, which shows the rate of photosynthesis occurring at a fixed level of light intensity.

Y rate of photosynthesis X concentration of CO2

Patterns in nature 81

2.4 2

Cell formation Exchanging gases OBJECTIVES When you you have havecompleted completedthis this section section you you should should be ableable be to: to: ● compare distinguish thebetween roles ofabiotic respiratory, and biotic circulatory factorsand in the environment excretory systems ● identify the features of the respiratory surfaces of animals that allow efficient exchange of gases ● compare the gaseous exchange surfaces of a fish, an insect, a mammal and a frog ● explain the need for transport systems to meet the requirements of multicellular plants and animals ● outline the tissues involved in the transport systems of plants, including root hair cells, xylem, phloem, stomates and lenticels ● compare open and closed circulatory systems in a vertebrate and an invertebrate.

activities ● ●



Transpiration Investigating the movement of materials in plants The uses of radioisotopes

In plants and animals, chemicals are moved within the internal environment, and between the internal and external environments, in specialised exchange and transport systems. Respiratory systems have gaseous exchange surfaces to transfer oxygen into organisms and carbon dioxide out into the environment. Circulatory systems move chemicals around the internal environment of animals. Excretory systems transfer chemical wastes from the internal environment to the external environment. (See Chapter 5, pp. 227–249.) These three systems work together to ensure that cells are supplied with the nutrients they need and that wastes do not accumulate.

Gas exchange in animals All organisms respire; that is, they take in oxygen from their external environment and release carbon dioxide. To obtain the oxygen required by cells in respiration and to get rid of unwanted carbon dioxide produced, multicellular animals have developed specialised gas exchange or respiratory surfaces.

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Respiratory surfaces must be thin and moist with a large surface area so that the gases can diffuse through. They must also bring together the internal and external environments so that gases can be exchanged between the cells and the organism’s surroundings. The respiratory surfaces of aquatic animals are more exposed to the external environment than those of land animals. Land animals have internal respiratory surfaces that they can keep moist without too much loss of water from evaporation.

Insects Insects have a system of interconnecting tubes called tracheae within their body (Figure 2.58). These tracheae are lined with cuticle and open to the external environment through pores or spiracles on the segments of the abdomen. In many insects, especially those that fly, the tracheal system includes inflatable air sacs that hold a reserve of air that can be pumped to tissues needing an extra supply, such as wing muscles. The spiracles are divided into air-intake spiracles and air-expiration spiracles, and can be closed to prevent water loss. The cuticle lining the tracheae lacks the cement and wax layers of normal cuticle, and it is therefore permeable to water. The tracheae branch throughout the tissues of the insect, bringing air directly to the cells. The endings of the tracheae, called tracheoles, are normally filled with fluid. The volume occupied by the tracheal system is relatively large (up to half the total body volume), so the surface area of the tracheae is sufficient to supply all the body cells with oxygen and remove carbon dioxide. Aquatic insects, including the aquatic larval stage of many terrestrial insects, usually have a modified system of spiracles or gas gills that take in air directly, either at the water surface or from bubbles of air trapped against the body. In some groups, such as dragonflies and mayflies, the aquatic larvae have feathery gills that take oxygen from the water by diffusion. CO2

O2

spiracle

tracheoles

air sacs

trachea tracheole

Insects have a system of branching tubes called tracheae within their body. The tracheae branch throughout the tissues of the insect, bringing air directly to the cells.

tracheae

O2 CO2

water (a)

(b)

spiracles

(c)

FIGURE 2.58 (a) Insects do not have lungs or blood vessels. They have a system of air-filled tubes called tracheae and finer tubes called tracheoles, which penetrate every tissue, bringing air into close contact with their cells. Air enters the tracheae through spiracles. (b) Some insects, such as grasshoppers, also have air sacs that can be pumped like bellows to move air through the system. (c) A mayfly nymph, with its spiny gills visible along the length of the abdomen.

Patterns in nature 83

gill arches (under operculum)

operculum

water water mouth cavity lamella blood flow

gill arch

water

water flow

water

(a)

(b)

two rows of filaments

FIGURE 2.59 (a) Water flows across fish gills in one direction, through the mouth and pharynx, past the gills and out under the operculum. Each gill arch has many finger-like gill filaments and each of these has a row of closely packed, flat, leaf-like lamellae across which gas exchange occurs. The lamellae provide a large surface area for exchange and are visibly red because they contain many blood vessels. Water and blood flow in opposite directions through the lamellae, which increases the amount of gas exchanged. (b) A row of lamellae along the gill filament of a rainbow trout.

Fish

O2

In fish, the respiratory surfaces are usually gills over which water flows (Figure 2.59). Some animals have exposed gills (e.g. sharks) and others have gills covered by an operculum (e.g. bony fish). Gills are usually finely divided, and the incoming water flows over a large surface area at the one time. There is a plentiful blood supply to transport the gases to and from the gills (Figure 2.60).

CO2 CO2 O2 FIGURE 2.60 Gas exchange in a simple gill.

Frogs

lungs in abdominal cavity O2 oxygen absorbed through skin

oesophagus nostrils

tongue

Frogs have two respiratory surfaces: the lungs and the skin. There is a very well developed blood supply to the skin. Oxygen from the air diffuses into the moist skin (subcutaneous breathing) and is transported by the blood to the heart, from where it is sent directly to the body. The lungs are simpler structures with a smaller surface area than those of mammals. They hang in the frog’s abdominal cavity and air is passed in and out by the pumping movement of the floor of the mouth (known as the buccal pump) and the opening and closing of the nostrils (Figure 2.61).

O2 glottis

floor of mouth (buccal pump)

FIGURE 2.61 The respiratory system of a frog.

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Mammals In mammals, gases are exchanged in the lungs (Figure 2.62). These surfaces are protected from desiccation by being inside the body’s waterproof covering. The surface area of contact between the blood and air is increased by the convolution of the lungs into lobes, by the branching of the bronchioles into smaller and smaller tubules, and by the division of the tubules into clusters of tiny air sacs called alveoli. There is a plentiful blood supply to transport gases to and from the lungs.

nasal cavity

bronchiole

alveolus (terminal air sac)

ring of cartilage epiglottis

left bronchus

trachea right lung

bronchiole

from pulmonary artery capillary

ribs

diaphragm

to pulmonary vein

airway

area occupied by heart O2 capillary

CO2

direction of blood flow

film of moisture alveolar epithelium

alveolus

red blood cells

FIGURE 2.62 The human respiratory system. Air moves in small volumes into and out of the lungs. Gas exchange occurs across thin alveolar walls.

Transpor t systems in flowering plants The need for transport systems To ensure that cells are supplied with the nutrients they need and that wastes do not accumulate, multicellular organisms have transport systems that enable substances to be moved to and from the internal body cells. These transport systems are specialised to exchange materials between the internal environment of the cells and the external environment. In flowering plants the transport system is called vascular or conducting tissue. It runs in long tubes through the roots, stems and leaves. It consists of two separate transport systems: xylem and phloem. These two systems run close to each other throughout the plant (Figure 2.64). In the root the xylem and phloem tubes form a central cylinder known as the stele. In young stems they form a ring of vascular bundles. In the leaf the vascular bundles are visible as veins. In flowering plants, no plant cell is far from vascular tissue.

FIGURE 2.63 A stem cross-section showing xylem and phloem (×400).

Patterns in nature 85

upper epidermis

xylem

vascular phloem bundle

vascular bundles

lower epidermis vascular bundle

epidermis phloem

stem xylem sieve tube xylem vessel

pith

vascular tissue

epidermis

root hair

phloem tap root

lateral root

vascular bundle xylem

FIGURE 2.64 Transport systems in plants are continuous through roots, stems and leaves. The vascular tissues, xylem and phloem, are tubular pathways through which fluids travel.

vascular bundle epidermis cortex pith cambium xylem phloem

FIGURE 2.65 The internal structure of a stem.

Water and mineral ions are transported upwards in the xylem and organic materials are transported both up and down the plant in the phloem. Compare the arrangement of the vascular tissue in stems and roots (Figure 2.64) to see how they differ. Biologists suggest that this arrangement is structurally the most effective in counteracting the bending stresses and strains on the plant when it is blown by the wind.

Water transport Water and mineral ions flow through plants in an upward stream. They enter via the roots and travel upwards in the xylem tubes. Water is lost from the plant by transpiration.

Root hair cells Water is taken in by plants through the roots from the soil. The water in the soil has a low concentration of solutes (soluble substances) and is therefore a dilute solution. Inside the plant, water in the cytoplasm of cells has a high concentration of solutes and is therefore a more concentrated solution. Water moves by diffusion down its concentration gradient by osmosis from the soil into the root cells (see pp. 71–73). The root hairs provide a large surface area for the uptake of water (see p. 72). The water moves through to the centre of the root and enters the xylem.

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Xylem vessels consist of dead cells, thickened with woody material, whose cross-walls have broken down, forming a continuous system of tubes.

cells cut open

cross wall xylem spongy mesophyll cross wall disappears leaving continuous tube (a)

(b)

FIGURE 2.66 (a) Xylem vessels form when the cross-walls break down, leaving a continuous tube. (b) Longitudinal section of xylem. Note the spiral thickenings of the cellulose wall, which add support.

water vessels much of the water travels along the cell walls

stoma

Xylem The main conducting cells are known as xylem vessels. They consist of dead cells, thickened with woody material, whose cross-walls have broken down, forming a continuous system of tubes. Xylem tubes may be up to 1 metre long. Lying alongside the xylem vessels are strengthening fibres and other conducting cells. Xylem gives support, strength and rigidity to the stem. Water entering the xylem in the roots is transported upwards through the stem to the leaves.

FIGURE 2.67 Transpiration in leaf cells.

The diffusion of water from a plant is called transpiration.

Transpiration When the stomates of a leaf are open, carbon dioxide, which is required for photosynthesis, can enter a leaf by diffusion. But at the same time, water molecules are lost through diffusion because of the higher water concentration inside the plant. Water evaporates from the cell surfaces, diffuses through the intercellular spaces, and leaves through the stomates. This diffusion of water from a plant is called transpiration. Water loss by transpiration is unavoidable when a plant has its stomates open. Water lost in this way needs to be replaced by water taken in through the roots. The constant upward flow of water through a plant is known as the transpiration stream (Figure 2.68). If water loss exceeds water uptake, the stomates close and cells lose their turgidity. The stems and leaves wilt and the plant may die. When stomates are closed the transpiration rate drops and diffusion occurs at a much slower rate through the cuticle. Normally stomates are open during the day for the exchange of gases in photosynthesis and closed at night. Some plants have special features (adaptations) to reduce the transpiration rate. These may include a very thick cuticle, sunken stomates, hairs, or reduced leaf size. In plants in hot dry environments, it may include the closure of the stomates during the middle of the day.

evaporation into atmosphere from leaf surface

External factors affecting transpiration

water absorbed from soil by roots

Temperature In high temperatures diffusion is more rapid (warm air holds more water than cold air).

water passes up trunk

FIGURE 2.68 The transpiration stream.

Patterns in nature 87

companion cell pits

Humidity If the atmosphere is saturated with water vapour (conditions of high humidity) the transpiration is decreased. Wind Moving air increases the transpiration rate. Water vapour is carried away from the leaf and a high diffusion gradient is maintained. Light Light intensity affects stomatal opening and this in turn affects the transpiration rate. Soil The water content of the soil and the solute concentration affect the rate at which water can be taken up by a plant.

sieve tube sieve plate

FIGURE 2.69 Sieve tubes and companion cells.

Phloem Sugars produced in photosynthesis, mostly in the leaves, are stored as starch in the leaf during the day. At night they are converted back to sugars and transported to other plant tissues in the phloem. Phloem consists of long columns of living sieve tube cells with perforated end walls, so that the cytoplasm is continuous between one cell and the next. Lying alongside the sieve-tube cells are companion cells and other supporting tissue. Organic materials in the phloem are transported both upwards and downwards within the plant. This movement is called translocation (see Chapter 5, p. 238).

Gaseous exchange in plants Plants exchange gases (oxygen and carbon dioxide) with the environment for both respiration and photosynthesis. In aquatic plants such as algae, gases simply diffuse across any exposed surfaces. In land plants, diffusion can occur directly across most surfaces, but both the leaf and stem have specialised structures that help increase the rate at which gases can be exchanged.

Stomates Stomates are pores in the leaf through which gases can diffuse. The number and arrangement of stomates vary in each plant species

The leaf is the specialised site of photosynthesis for the plant. Leaves are usually flat and thin, with a large surface area, and covered with stomates. Stomates are pores in the leaf through which gases can diffuse. TABLE 2.7 The distribution of stomates on the leaves of different species.

Av e r a g e n u m b e r o f s t o m a t e s p e r m m2 Species

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Upper e p i d e rm i s

Lower e p i d e rm i s

Runner bean (Phaseolus vulgaris)

40

281

Geranium (Pelargonium sp.)

19

59

Corn or maize (Zea mays)

52

68

Nasturtium (Tropaeolum majus)

0

130

Sunflower (Helianthus annuus)

85

156

The number and arrangement of stomates vary in each plant species (Table 2.7). They may be present on both upper and lower epidermal surfaces or only on one, usually the lower surface. Stomates can open and close (Figure 2.72). When open, gases can diffuse freely and the rate of photosynthesis increases. When closed, the rate of photosynthesis slows. The transpiration rate (see p. 87) is also controlled by the opening and closing of stomates. Guard cells control the opening and closing of the stomates. When stomates are open, the guard cells are distended with water and are said to be turgid. When stomates are closed, the guard cells are less distended. The mechanism for opening and closing therefore depends on the change in shape of the guard cells as a result of a change in their water content or turgidity. This change is brought about by the concentration of potassium ions in the cell. As the potassium concentration increases, water is drawn into the cell by osmosis. As potassium levels decrease, water leaves the cell. The two guard cells are next to each other, joined at either end, and have thickened cell walls between them. The arrangement of the bands of inelastic cellulose microfibrils around the cells maximises the opening of the pore as water enters and the cells expand. Stomates are usually open during the day and closed at night. Guard cells open and close in response to a variety of internal and external stimuli:

FIGURE 2.70 Stomates surrounded by guard cells (surface view, ×400).

Light Guard cells have a blue-light receptor. Light stimulates the entry of potassium ions into the guard cells, causing the stomates to open. Low carbon dioxide levels When photosynthesis begins, carbon dioxide is used by the cells. Low carbon dioxide levels in the air spaces of the leaf will trigger the opening of stomates. If a plant is placed in the dark in an atmosphere free of carbon dioxide, the stomates open. An internal clock The stomates open and close in a daily rhythm. The regular opening and closing occurs even if a plant is kept in the dark. Water deficiency If the plant is unable to replace the water it is losing by evaporation, the stomates will close.

FIGURE 2.71 Electronmicrograph of a stomate and its guard cells (longitudinal section, ×400).

OPEN

High temperatures Hot conditions cause the stomates to close.

Lenticels Lenticels are pores in the woody stems of plants. Gases needed for respiration by the cells of the stem are exchanged by diffusion. Oxygen diffuses in and carbon dioxide diffuses out.

water enters guard cells CLOSED

water leaves guard cells

FIGURE 2.73 Hairs on leaves help to reduce evaporation from leaf surfaces.

FIGURE 2.74 Cross-section through a stem, showing a lenticel.

FIGURE 2.72 The opening and closing of a stomate.

Patterns in nature 89

Radioisotopes Radioactive forms of certain elements, called radioactive isotopes, may be used to trace biochemical pathways. Radioactive isotopes can participate in reactions in the same way as the non-radioactive elements. For example, autoradiography is a technique used to trace the movements of certain substances around plants. It involves the following steps: 1 Carbon-14, a radioactive isotope, is added to the carbon dioxide supply of a plant, to study the movement of the products of photosynthesis. 2 The plant takes up the radioactive isotope, carbon-14. 3 Carbon-14 is used by the plant in the process of photosynthesis. 4 The movement of the radioactive isotope can be traced by taking an autoradiograph. An autoradiograph is produced when the plant is placed against photographic film. The areas of the plant in which the carbon-14 has accumulated can be seen as dark shadows on the film. Radioactive isotopes can also be used as diagnostic tools in medicine. Thallium-201 is used to detect damaged heart muscle after a heart attack. This isotope will accumulate only in normal, undamaged heart muscle. This allows doctors to determine where and to what extent a heart has been damaged. Technetium-99 is an isotope of the artificial element technetium. It emits gamma radiation and is useful in scintigraphy—a method of scanning that is used to examine the organs of the body in the diagnosis of cancer and other diseases. 1

How does carbon-14 differ from carbon-12? Explain why they can both take part in the same reactions.

2

FIGURE 2.75 An autoradiograph. The dark areas contain accumulated carbon-14.

What compounds would carbon-14 become part of in a leaf?

Circulator y systems Transport systems are needed by multicellular organisms to ensure that all the requirements of their cells are met.

Multicellular animals need to transport or circulate materials around their bodies to supply cells with nutrients and remove wastes. The flow of materials is usually maintained by a pumping system. Circulatory systems may be open or closed.

Open circulatory systems Invertebrates such as arthropods and molluscs have open circulatory systems (Figure 2.76a).

Insects In insects the circulation of body fluid, known as haemolymph, around the body is achieved by a simple pumping system consisting of one or 90

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heart

heart

small branch vessels in each organ

haemolymph

interstitial fluid (a)

(b)

accessory hearts aorta

FIGURE 2.76 (a) A simple representation of an open circulatory system. (b) A simple representation of a closed circulatory system.

more tubular hearts. Haemolymph bathes the tissues and accumulates in large spaces within the insect. Any vessels that assist the transport of the fluid are open at each end. Fluid is sucked into the tubular heart(s) through small holes. It is then pumped forwards to the front end of the insect and flows slowly backwards through the spaces surrounding the various organs. The pressure in an open system is low, so the body fluid circulates slowly. Open circulatory systems suit the needs of smaller animals. In insects they do not have to transport respiratory gases, but only distribute and collect food and wastes, and sometimes store them temporarily.

Closed circulatory systems Large active animals such as vertebrates and squids have closed circulatory systems (Figure 2.76b). In humans, the circulatory system consists of a muscular pump (the heart) that forces a fluid (blood) through a closed system of tubes (blood vessels), which carry materials rapidly throughout the body. No cell in the body is very far from a blood vessel. Nutrients, wastes and gases exchanged in respiration are carried in the blood. To reach the body cells, materials in the blood must pass into the surrounding body fluid before reaching the cells. (See also Figure 5.24, page 230.) Fluid from the blood may pass through the blood vessel walls to become part of the interstitial or body fluid which bathes all cells and keeps them moist. Some of this body fluid is reabsorbed in the blood, and the rest drains slowly into other small tubes, the lymphatic vessels. These join up to form larger vessels which eventually return the fluid they contain, now called lymph, to the blood. Both the lymphatic system and the blood system are concerned with transporting materials and maintaining the circulation of fluids within the body. Closed circulatory systems meet the needs of large, active animals. They provide nutrients and oxygen to cells and carry away wastes and carbon dioxide. However, they use more energy to provide the faster service required.

heart

FIGURE 2.77 Insects have an open circulation system. It consists of a dorsal longitudinal vessel (the heart) that contracts, forcing the fluid out at the anterior end. This fluid flows through tissue spaces and finally re-enters the dorsal vessel through a series of openings. Accessory hearts may help supply fluid to the wings and other distant structures.

jugular veins

carotid arteries pulmonary artery

brachial vein

heart aorta

brachial artery

vena cava femoral artery femoral vein

FIGURE 2.78 The human circulatory system.

Patterns in nature 91

Questions 1 Name and describe the roles of three systems used to transport chemicals between the external and internal environments of plants and animals. 2 Identify the gases exchanged in both plants and animals and state the process for which each gas is used.

O rganism

Name of respiratory surface

3 Most organisms require oxygen for respiration. This oxygen is obtained from the environment. Insects, fish, frogs and mammals all have different structures to facilitate the exchange of gases with the external environment. a Complete the following table that summarises the structure and function of respiratory systems in different kinds of animals. b List the features that all respiratory surfaces have in common. Explain how each feature facilitates gaseous exchange.

Diagram of respiratory system

Description of process by which gas exchange occurs

insect fish frog mammal

4 a Label the following diagram of a stomate. b Describe the role of stomates. c Explain the mechanism by which stomates open and close. X

W

5 a Define ‘lenticel’. b Outline the role of lenticels. 6 Explain why multicellular organisms require specialised transport systems.

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7 Prepare a cross-sectional diagram of a stem showing the arrangement of xylem and phloem tissue in vascular bundles. 8 Prepare a chart comparing xylem and phloem tissue. Include substances transported, direction of flow and description of the tissues. 9 a Define ‘transpiration’. b Describe the process of transpiration that occurs in plants from the entry point of water at the root hairs, its route through the plant, to its exit at the stomates. c Discuss the factors that affect the rate of transpiration. 10 Describe a key difference in the role of the circulatory system between insects and vertebrates. 11 Compare the efficiency of open and closed circulatory systems.

F u r ther questions 1

When we inflate a balloon we push air into it; when the balloon deflates, air is squeezed out. Explain how the activity of breathing can be compared with this.

2

The mechanism of gas exchange that occurs at the respiratory surface in all organisms is diffusion.

O2 capillary direction of blood flow

CO2

b Consider eucalypt species. Do you think that the distribution of stomates on upper and lower epidermis would follow a similar pattern? Explain your answer. 5

Water conservation is critical for plants living in arid environments. Research a selected plant species adapted to desert conditions. a Discuss the distribution and density of stomates. b What are the consequences of water conservation on gas exchange, the rates of respiration and photosynthesis, and on growth for desert-adapted plants? Explain.

6

It is important not to pile soil or compost around the base of a tree, because it might kill the tree. Explain why this is so.

7

Marine kelps are large multicellular organisms, yet they do not have specialised transport systems. Explain why this is so.

8

a Outline a simple experiment you could set up, using a carnation flower and food dye, to test the hypothesis that ‘water travels through xylem in an upward direction’. b Describe the experimental results that would support the hypothesis. (You will need a microscope for part of this answer.)

9

Research the use of radioactive isotopes in medicine. ● Make a list of elements that are used. ● For what medical conditions are radioactive isotopes useful? ● How are the isotopes used in the body? ● What can they tell us? ● What, if any, side effects may result from the use of radioactive isotopes? ● Evaluate the advantages and disadvantages of using radioactive isotopes as a diagnostic tool.

film of moisture alveolar epithelium red blood cells

a Use the diagram of a human alveolus with blood supply to explain why i oxygen diffuses from the alveolar air into the blood, and ii carbon dioxide diffuses from the blood into the alveolar air. b What would you expect to happen to the diffusion rates of oxygen and carbon dioxide for someone breathing into a paper bag? Explain. 3

Carbon monoxide is a poisonous gas present in cigarette smoke and the exhaust fumes of cars. Explain why carbon monoxide is dangerous.

4

Study Table 2.7, which illustrates the distribution of stomates on the leaves of different species. a Suggest an explanation for the distribution of stomates on the lower epidermis of leaves of different species compared with the upper epidermis.

Patterns in nature 93

2.5

Growth and repair OBJECTIVES When you have completed this section you should be able to: ● outline the importance of cell division in the growth, repair and reproduction of multicellular organisms ● outline the relationship between chromosomes, genes and DNA ● recall the role of chromosomes in the transfer of information when cells reproduce ● define mitosis and explain its role ● describe the process of cytokinesis and explain its role ● describe the different stages of the cell cycle ● recognise the sites at which mitosis occurs in mammals, insects and plants ● recall that DNA is also contained in mitochondria and chloroplasts.

Mitosis Chromosomes appear as dark-stained threads in the nucleus prior to cell division. They are composed of proteins and the nucleic acid DNA. Chromosomes carry the cell’s genetic information. Each type of organism has a particular constant number of chromosomes in each of its cells.

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Mitosis is the process by which a multicellular organism grows, repairs, maintains and reproduces itself. It is a type of cell division that results in the production of cells which are identical to the original cell. It really takes place as two separate processes. Mitosis itself is a process of division of the nucleus, and cytokinesis is the division of the cytoplasm. The nucleus contains inherited structures (chromosomes) that carry the information which controls all the cell’s activities. The single cell from which you developed had 46 chromosomes. In fact, each of your body’s cells has this number of chromosomes. Chromosomes become visible under the light microscope when a cell is about to divide. Prior to mitosis the original set of 46 chromosomes is copied (duplicated), and in mitosis one copy of each chromosome is distributed to each new cell as it forms. The period when the cells are not dividing is called interphase. During interphase the chromosomes are duplicating, but they are not visible.

The stages of mitosis Prophase (early stage) Each chromosome is visible as two identical, joined strands, called chromatids. The nuclear membrane breaks down and disappears by late prophase. Metaphase (middle stage) A tapered system of microtubules stretches across the cell, forming a spindle. The chromosomes line up at the centre of the cell, attached to the spindle fibres at the point known as the centromere. The chromatids separate. (Note: in animal cells, the centriole is involved in spindle formation. Centrioles are absent in plants, the spindle attaches to the plant cell wall.) Anaphase (towards end stage) The chromatids, now referred to as single-stranded chromosomes, move towards opposite poles, carried on the spindle fibres. Telophase (end phase) The spindle disappears. New nuclear membranes form around the two sets of chromosomes.

(a)

(c)

Mitosis is a type of cell division that results in the production of cells which are identical to the original cell. Chromatids are the daughter strands of a replicated chromosome. On separation in mitosis, each becomes a daughter chromosome.

(b)

(d)

FIGURE 2.79 Mitosis in onion root-tip cells: (a) prophase, (b) metaphase, (c) anaphase, (d) telophase.

The need for cytokinesis Division of the cytoplasm usually occurs immediately after mitosis (Figure 2.80). This is necessary to ensure that the chromosome number in each cell remains constant. The chromosome number doubles in mitosis and one cell now contains two sets of chromosomes. Division of the cytoplasm (cytokinesis) results in two cells, each with a set of chromosomes. In animal cells, the cytoplasm constricts at the centre in a process called cleavage. A ring of microfilaments forms in the centre of the cell. As they constrict, the cell begins to ‘pinch’ or cleave into two. The cell surfaces show very active movement or ‘bubbling’ as they separate. In plant cells a dividing plate (the cell plate) forms across the centre of the cell, separating the two new cells, called daughter cells. A new cell wall is built from this plate. Patterns in nature 95

interphase chromosomes are fine threads replication occurs

prophase chromosomes condense and become visible

cell membrane nucleus

metaphase chromatids apparent nuclear membrane breaks down spindle forms chromosomes align at equator two chromatids centromere

nuclear membrane centriole

'daughter' cell

spindle fibres single strand chromosome

cytokinesis division of cytoplasm occurs

telophase nuclear membranes form around new nuclei

anaphase centromeres split single strand chromosomes drawn to opposite poles

FIGURE 2.80 Mitosis and cytokinesis in an animal cell containing four chromosomes.

The cell cycle

G2 second growth stage S DNA synthesis

M mitosis

G1 first growth stage

G0 quiescence (non-division)

FIGURE 2.81 The cell cycle.

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The names prophase, metaphase, anaphase and telophase were used in the 1880s by the first scientists to describe mitosis. Because chromosomes are not visible during interphase, it was not known what was happening to them between divisions. Today, using new stains, autoradiography and measurements of cell components, we know that interphase can be divided into three periods, called G1, S and G2. G1 = the time gap between the end of mitosis and the start of chromosome duplication S = the synthesis phase when the chromosomes become doublestranded G2 = the time gap between the end of synthesis and the start of mitosis M = mitosis, the separation of the duplicated chromosomes to form two daughter nuclei G0 = state where cell division no longer occurs The length of this cycle varies for different types of cells. In cells which are actively growing and dividing, the cycle is short. Mammal cells grown in the laboratory can repeat this sequence every 16 hours. The G1 phase is the most variable. Cells may spend days, months or years in this phase. Some cells which lose the power to divide as they become older and more specialised, such as nerve cells, are arrested in the G1 phase and remain in the G0 state.

Permanent cells In the tissues of some animals there is no turnover of cells. Sufficient numbers of specialised cells are produced during development to last the entire life of the organism. These permanent cells include: ● nerve cells, which can repair themselves to a certain extent. Appropriate nerve connections form during development in an interactive way as the organs form. It would be impossible for a new nerve cell to reconnect specific parts of the nervous system that are often widely separated in the adult. ● retinal cells, which develop specific links between the eye and the brain during development which cannot be replaced. ● the cells of the lens of the eye, which are transparent. ● cardiac muscle cells, which can become larger through exercise but do not increase in number. Not surprisingly, most permanent cells are able to repair themselves to a considerable extent by the replacement of various organelles. For example, rods and cones in the retina replace the photoreceptive membranes which contain the receptors that respond to light. In replacing a severed hand, microsurgeons sew the ends of nerve bundles together so that, if the nerves are able to recover, the sheaths around the nerve bundles guide the regenerating nerve terminals to the right sites.

Sites of mitosis All living organisms commence life as a single cell. They grow and develop, become mature, then age and die. Young multicellular organisms grow rapidly due to mitosis, which increases the number of cells. Mitosis continues in adult organisms in certain areas. It provides a ‘repair and maintenance’ service to old and damaged body cells. In unicellular organisms, mitosis is the means of reproduction: one cell becomes two, two cells become four, and so on.

Plants In mature plants, mitosis occurs in the tips of roots and stems, causing an increase in length. Mitosis in special cells in the stem results in an increase in its width. Plants continue to grow throughout their life from cells capable of mitosis known as meristematic cells. Primary growth occurs from apical meristems. An apical meristem is the growing point where mitosis occurs at the tips of roots and stems. Stages in this growth are shown in Figure 2.82. There is a region of rapid mitosis followed by cell enlargement and then differentiation of mature cells. In the root this results in growth in length as the root pushes through the soil. In the stem this results in growth in length of the above-ground shoots. Patterns in nature 97

cortex vascular cambium primary xylem primary phloem

Buds are regions of potential growth, containing meristematic cells, in leaf axils (where the leaves join the stem). They might or might not develop further. If the tip of the stem is cut off or damaged, one or more buds will form new shoots. Secondary growth is an increase in thickness or diameter of the plant. Cambium cells are meristemic cells. Cell division occurs in the vascular cambium of roots and shoots, and in the cork cambium of stems (Figure 2.83). Tissues formed as a result of these cell divisions are called secondary tissues. Secondary xylem is known as wood.

primary phloem secondary phloem

; ;

primary xylem secondary xylem

cortex vascular cambium cork cambium FIGURE 2.83 Secondary growth in woody plants arises from secondary meristem tissues, the vascular cambium and cork cambium.

;; ;;

;;; ; ;; ; ; ;

;; ;;

;;

zone of cell maturation

apical meristem (active cell division) young leaves

zone of elongation apical meristem (active cell division)

axillary buds

(a)

root cap

(b)

FIGURE 2.82 The primary growth of a seedling is caused by rapid cell division in the apical meristems of the shoot and root. (a) Shoot tip, showing regions of active cell division (apical meristem) and developing leaves. (b) Root tip, showing protective root cap and zones of active cell division (apical meristem), elongation and cell maturation.

Insects Insects usually have complex life histories, passing from immature to mature forms. Most insects have immature forms known as larvae. When larvae hatch from the egg, they grow and increase in size, but this is a result of cell enlargement, not cell division. In the pupal form, the larval cells break down, and previously inactive groups of cells known as imaginal discs start to divide. They grow in both size and number to form the adult insect or imago. The female mosquito lays her eggs on the surface of the water.

The adults mate; initial fertilisation.

breathing tubes fresh water

FIGURE 2.84 The life cycle of a mosquito. Many insects have internal fertilisation, but the female then lays her eggs in water and leaves them to develop externally.

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The adult emerges from the pupal case. larva

pupa

Mammals In young mammals, mitosis rates are high and growth is rapid in all areas of the body. At maturity growth ceases but the repair and maintenance of cells continues. In adult mammals, mitosis occurs in the skin, replacing the cells that continually flake off, and causing hair/fur and nails/claws to grow. New blood cells are made every day in the bone marrow, and the cells lining the digestive system are constantly being replaced.

Cell growth and differentiation Cell growth Cell growth or enlargement precedes each cell division. If this did not happen, cells would become smaller and smaller after each division. The rate of growth depends on the cell’s environment (e.g. the temperature and the amount of food available) and the cell’s own activity. Cells grow only to a certain size that is governed by surface area to volume ratio considerations (see p. 64) and the type of cell they become.

Cell differentiation Cell differentiation is the process in multicellular organisms by which similar cells become dissimilar and specialised; they are no longer identical to the original parent cell in structure or function. The single cell from which you developed had 46 chromosomes, carrying all the information needed to produce a human being. All daughter cells formed by mitosis carry this information, but the total amount of information is not expressed in every cell. Some cells become blood cells, others become nerve cells, and so on. In other words, the cells undergo differentiation. The first visible sign of differentiation is that the structure of a cell begins to change. At the molecular level, proteins specific to a particular type of cell begin to form. This implies that some genes on the chromosomes are being activated, with other genes remaining inactive or being switched off. The location of the cell is also important—the type of cell that develops depends on where it is in the body. The same types of tissue, such as bone or muscle in mammals, are found throughout the organism, but the arrangement or pattern of tissues depends on its position in the body. How differentiation and development occur is still not entirely understood.

Uncontrolled cell growth In healthy tissues, mitosis stops when damaged cells have been replaced. As the new cells grow,

white blood cell

red blood cell **NCB 1.46**

nerve cell

epithelial cells FIGURE 2.85 Some differentiated cells.

they differentiate and function correctly and have a limited life span. They also adhere to similar cells. Uncontrolled cell division produces a mass of abnormal cells known as a tumour. A benign tumour is surrounded by a capsule and does not invade healthy tissue. In cancers the cells divide in an unlimited and disordered way, forming tumours whose abnormal cells will adhere to any nearby cells (see Chapter 7). Some cells may break off and spread to other parts of the body (malignant tumour). Cancer cells never mature and most do not differentiate, or do not do so normally. Some cancers may be curable when differentiation is understood completely, so that it can be induced and regulated. Other cancers may be curable when cell division can be regulated.

Patterns in nature 99

DNA Chromosomes are composed of proteins and the nucleic acid DNA. They are the genetic material found in the nucleus of cells and contain the coded chemical instructions that direct the growth, differentiation and functioning of a cell. The separate instructions that code particular activities or characteristics are called genes. Genes are short lengths of a chromosome and consist of DNA. This information is transferred via the chromosomes from cell to cell in the process of mitosis. The nucleus is not the only part of a cell to contain DNA. DNA is also found in mitochondria and chloroplasts.

Questions 1

Outline the significance of cell division in multicellular organisms.

6

Discuss some of the outcomes of uncontrolled cell growth in multicellular organisms.

2

a Define ‘mitosis’. b Use diagrams to illustrate the activities of chromosomes during the different phases of mitosis. c What is cytokinesis?

7

Identify the stages of mitosis illustrated in the following photographs.

3

Describe what happens during the four phases of the cell cycle. Use diagrams to illustrate your answer.

4

Interphase is often referred to as a resting phase for the cell nucleus because the chromosomes are not visible and seen to be active. Is this a correct description of the interphase nucleus? Explain your answer.

5

a Define ‘cell differentiation’. b Outline the importance of cell differentiation in multicellular organisms.

(a)

8

(b)

a What is the role of the nucleus in cells? b What is DNA? How is it important in the functioning of cells?

F u r ther questions 1

100

Identify the sites in which mitosis occurs from the following list: a skin of mammals b tips of hair and fur c red blood cells d white blood cells e root tips of flowering plants f nerve cells

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g h i j k m n

cardiac muscle production of egg and sperm repair of skeletal muscle larval cells of insects retinal cells embryo development stem tips of land plants

b What does the graph tell you about the amount of chromosome material in a cell at different stages in the cell cycle? c Describe the activities that occur during each of the different phases.

Study the following graph, showing the different phases of the cell cycle. chromosome material

2

G1

S

G2

M

3

Most of your body cells have 46 chromosomes. If all of these body cells contain the same genetic information, coding for the same instructions, explain how it is that cells in different tissues of the body are different from one another.

4

Draw a concept map to demonstrate your understanding of the key ideas in this section. Include the following terms:

G1

time

a Mitosis is the phase of the cell cycle during which chromosomes are most obviously active. Use the graph to estimate the percentage of time of a cell cycle that mitosis takes up.

chromosomes genes mitosis cytokinesis growth repair reproduction cell differentiation

Patterns in nature 101

Chapter summar y Practical activities 2.1 2.2



The light microscope



Observing cells



Chemical substances found in tissues



Diffusion and osmosis Surface area to volume ratio



2.3

2.4



Photosynthesis





Mammalian digestive systems Gaseous exchange



Transpiration



Investigating the movement of materials in plants The use of radioisotopes



2.5



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Mitosis

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2.1 • The cell theory states that (1) cells are the smallest units of life, (2) all living organisms are made up of cells, and (3) all cells come from pre-existing cells. • The cell theory was developed over several hundred years with contributions from many scientists, including Robert Hooke (1665) and Robert Brown (1831). • Observations of the microscopic structure of living organisms provide evidence to support the cell theory. • Technological advances in the development of microscopes and tissue preparation techniques have led to further development in our understanding of cell structure. • Cell organelles that can be seen with present-day light and electron microscopes include the nucleus, mitochondria, lysosomes, microtubules, endoplasmic reticulum, ribosomes, centrioles, Golgi bodies and chloroplasts. • Each type of organelle has a particular function in the cell, and its structure can be related to its function. 2.2 • The major groups of substances found in living cells are carbohydrates, lipids, proteins and nucleic acids, and inorganic minerals and ions. All these substances are used by the cell. Carbohydrates are energy sources, lipids are energy stores, proteins determine the nature and activity of a cell, and nucleic acids contain inherited information. • Selectively permeable membranes separate the internal contents of cells from the external environment. Molecules move in and out of cells across the membranes. • The fluid mosaic model of membrane structure can account for the movement of substances in and out of cells by both passive and active processes. • The double layer of phospholipids and the membrane proteins allow some small molecules to diffuse through, but many other molecules must be actively transported through the membrane. • Diffusion and osmosis are two passive processes by which substances move in and out of cells. Osmosis is the movement of water through the selectively permeable membrane. • The surface area to volume ratio affects the rate of movement of substances in and out of cells. Cells maintain a large surface area compared to their volume. This enables them to exchange materials efficiently with their environment. 2.3 • Multicellular organisms are made up of cells, tissues, organs and organ systems. The structural and functional relationship between these parts ensures that the organism functions in a coordinated way. • Autotrophs obtain their nutrients from simple inorganic materials in their environment. Heterotrophs obtain their nutrients from complex organic substances found in other living organisms.

• Autotrophs require carbon dioxide and water for the process of photosynthesis. All ecosystems depend on photosynthesis by autotrophs to provide the energy to sustain them. • In photosynthesis, light energy is trapped and used to combine carbon dioxide and water to form sugars. The overall equation for photosynthesis is a summary of a chain of biochemical reactions that occur in this process. • The structures used by plants to obtain water and minerals require a large surface area for absorption. In land plants this is provided by the structure of roots and root hair cells. • The thin, flat shape of leaves and the distribution of tissues, including stomates, ensure a large surface area for the absorption of light and the exchange of gases in photosynthesis. • The teeth of mammals break food into smaller particles to increase the surface area that is exposed to digestive chemicals. • The length and complexity of vertebrate digestive systems is related to the food the vertebrate eats. Herbivores with a diet high in cellulose have a longer and more complex digestive system than carnivores, which have a high protein diet. 2.4 • Respiratory systems exchange gases between the internal and external environment. Circulatory systems transport materials around the internal environment of animals. Excretory systems remove chemical wastes from the internal to the external environment. • Animals have specialised surfaces for exchanging gases with the environment. Insects have tracheae, fish have gills, mammals have lungs and frogs use both lungs and their skin for the exchange of respiratory gases. • In a multicellular organism, cells require a supply of nutrients and gases and the removal of waste products. Transport systems transfer these chemicals to and from cells within the organism. • The transport systems of plants are the xylem and phloem. Xylem transports water and mineral ions upwards from the roots to the leaves. The water is pulled upwards in the transpiration stream. Phloem transports nutrients and other substances upwards and downwards in a plant. Root hair cells, stomates and lenticels provide exchange areas between the plant and the external environment. • Animal circulatory systems transport nutrients, gases and wastes. They may be open or closed systems. 2.5 • Mitosis is a process of nuclear division. Its role is to provide new cells for organisms in growth, repair and maintenance, and reproduction. • The sites of mitosis vary in plants, insects and mammals and are also related to the age of the organism. • Mitosisis is followed by cytokinesis, which is the division of the cytoplasm to form two new cells. • DNA is found in nuclei and also in mitochondria and chloroplasts.

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EXAM - STYLE QUESTIONS Multiple choice 1 Which of the following three structures are visible with the light microscope? A nucleus, chloroplast, Golgi apparatus B nucleus, vacuole, chloroplast C mitochondrion, vacuole, microtubule D centriole, ribosome, chloroplast

6 Which of the following animals have both lungs and blood vessels? A fish and insects B fish and frogs C insects and humans D frogs and humans

2 What is the function of the organelle shown?

7 Identify the function of xylem tissue in a plant stem. A to transport water and mineral ions up and down the stem B to transport organic substances up and down the stem C to transport water and mineral ions upwards to the leaves D to transport organic substances downwards to the roots

A B C D

photosynthesis to act as the site of protein synthesis to package cellular secretions respiration

3 Which of the following compounds are made up of carbon, hydrogen and oxygen only? A carbohydrates and lipids B lipids and enzymes C carbohydrates and nucleic acids D proteins and glucose 4 Which statement best describes what happens during the process of osmosis in cells? A Energy is expended. B Substances such as salts move from an area of high concentration to an area of low concentration. C Water moves across the selectively permeable membrane from the more dilute solution to the more concentrated one. D Carrier proteins are involved in the transport of substances across the cell membrane. 5 What happens to the surface area to volume ratio as a cell grows? A It increases. B It decreases. C It remains the same. D It disappears.

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8 A large surface area, thin walls and a good blood supply allow the rapid and efficient transfer of materials to meet the body’s needs. Which two areas in a mammalian body have this structure? A heart and lungs B small intestine and kidney nephrons C skin surface and capillary network D lungs and small intestine 9 Which statement best describes closed circulatory systems? A They are found in large, active animals such as mammals and birds. B They allow blood to leave the blood vessels and bathe the body cells directly. C They transport digested nutrients and waste materials, but not respiratory gases. D They use less energy than open circulatory systems. 10 Which statement best describes the process of mitosis? A It occurs in the maintenance of permanent cells. B It results in variation in the cells of the next generation. C It is important in growth, repair and reproduction in multicellular organisms. D It is the key activity of the nucleus during interphase in the cell cycle.

Short answers 1

a State the cell theory. b Use an example to describe how technological advances have supported the cell theory.

2

a Name the major groups of organic substances found in cells. b Describe the role of each of these organic substances in cells.

3

Starch is an example of an organic molecule found in plant cells. It turns purple-black in the presence of iodine. Outline a simple experiment using cellulose tubing, water, starch and iodine to demonstrate that starch molecules are too large to pass across a selectively permeable membrane.

a Identify the function of this cell. b Describe the structural adaptation of this cell and explain how this feature helps the cell to undertake its function. 5

O

Consider the following diagram of a specialised plant cell.

root hair cell

The following diagram shows what happens to a plant cell when immersed in a salt solution that is of a different concentration than the cell cytoplasm.

a Is the solution in which the cell has been placed more or less concentrated than the cell cytoplasm? Give reasons for your answer. b Name the structure labelled Q. c Name the process that has occurred. d i When a red blood cell is placed in distilled water it will eventually burst. Explain why this is so. ii When a plant cell is placed in distilled water it will not burst. Why not? 7

The following graph shows the rate of photosynthesis in a sunflower leaf exposed to two different oxygen concentrations in an environment of increasing carbon dioxide concentration. The level of light intensity remained constant throughout the experiment.

The outline diagrams shown are taken from an electronmicrograph of a plant cell and an animal cell.

Q

M N O

P

a Identify structures M, N, O, P, Q. b Outline the role of each of these structures. c Which cell represents the animal cell and which represents the plant cell? Give three reasons why you think so.

10% oxygen concentration rate of photosynthesis

4

6

30% oxygen concentration

increased concentration of CO2

a Write out the balanced chemical equation for the process of photosynthesis. b What happened to the rate of photosynthesis with increasing carbon dioxide concentration? Account for this observation. c Compare the rate of photosynthesis at the two different oxygen concentrations.

Patterns in nature 105

d It has been observed that oxygen and carbon dioxide compete for the active site of an enzyme involved in photosynthesis. Use this information to account for the graphs at the two different oxygen concentrations. 8 Study the simplified diagrams comparing the digestive systems of dogs and koalas. Koala small intestine

oesophagus

Feature

Function

large surface area thin, moist walls

stomach caecum

first part of large intestine

remainder of large intestine

Dog small intestine

b The respiratory surfaces of animals must have particular features that allow efficient gas exchange. Complete the table which relates the structure of respiratory surfaces to the function of gas exchange.

oesophagus

stomach caecum

large intestine

rich supply of blood

10 a Outline the pathway taken by water as it enters plant at the roots, moves through the stem and is removed at the leaves. In your answer, name the process and tissues involved. b Cacti are plants adapted to conditions of high temperatures and low water availability. Stomates are often sunken into pits on stems and leaves and remain closed during the hottest part of the day. The stomates are the site of gas exchange in plants. When stomates are open water can be lost to the atmosphere through evaporation. i Outline the role of stomates in relation to gas exchange and water movement in plants. ii Explain the role of stomates in water conservation for desert-adapted plants such as cacti during the hottest part of the day. iii How would you expect this response to affect the rate of photosynthesis and plant growth? Explain your answer. 11 a Mitosis is a division of the nucleus of a cell. Explain the significance of this process for cells. b Study the following cell from a multicellular organism, which is undergoing mitosis.

Account for the differences observed between the digestive systems of these two mammals. Your explanation should include a discussion of diet and the roles of the parts of the digestive system. 9 a Name and describe the organs of gas exchange in i insects ii fish iii frogs iv mammals.

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i Identify the phase of mitosis. ii Does the cell represent a plant or animal cell? Give two reasons for your answer. c Define ‘cytokinesis’. Explain its importance in the process of cell division.

Chapter 3

LIFE ON EARTH

Life does not need ideal conditions to evolve. Living things have been found to exist in the most hostile of conditions on Earth, and Australian scientists have discovered that bacteria exist far down in the Earth’s crust, and have done so for millions of years. The early Earth was a violent and reactive place. There was no oxygen in the atmosphere, lightning was common, and ultraviolet radiation streamed down unhindered. These conditions—very different to those found today on Earth—proved ideal for the formation of the chemicals of life. Some of these formed membranes that separated organic compounds from the environment. The first primitive cells had arrived. These cells were capable of carrying out chemical reactions that allowed them to reproduce and sustain themselves. Some produced oxygen as a byproduct in the reaction of photosynthesis. The atmosphere was altered forever, and more complex life forms began to evolve. Since that time, life has existed continuously on Earth. The fossil evidence indicates many species have come and gone, yet many have remained relatively unchanged. To further their understanding of living things, scientists have developed classification systems that group organisms according to their structural or genetic similarity. We can better describe the origins, processes and evolution of life as new technologies and techniques are developed.

This chapter increases students’ understanding of the history, nature and practice of biology, and of current issues, research and developments in biology.

3.1

The origins of life OBJECTIVES When you have completed this section you should be able to: ● describe the conditions on early Earth ● name the basic chemical components necessary for the existence of life on Earth ● discuss the implications of the existence of organic molecules in the cosmos for the origin of life on Earth ● describe two scientific theories related to the evolution of the fundamental chemicals of life ● discuss the significance of Urey and Miller’s experiments in relation to our current understanding of the composition of the Earth’s early atmosphere ● identify advances in technology that have increased our understanding of the origins of life and the evolution of living things.

All living things are made up of cells. But where did cells come from? How did life originate? Different cultures have developed different ideas to try to explain the origins of life. In this section we will look at some scientific evidence and the way scientific knowledge about the evolution of life on Earth has developed.

The primeval Ear th activity ●

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Urey and Miller’s experiment

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Many thousands of millions of years ago, according to physicists, the region of space now occupied by the solar system consisted mostly of scattered interstellar gases. Then, an event occurred that would eventually enable life to exist. A nearby star exploded, sending massive shock waves through space, like the sound of a thunderclap through the air. This shock wave caused the interstellar gases to collect into much denser clouds. It is thought that these clouds condensed over eons of time, finally becoming dense enough to trigger the nuclear reactions needed to form a star—the Sun—about 4600 million years ago. The

cooling solar gases surrounding the star coalesced into fragments, which accreted to form the planets. By 4500 million years ago, our Earth had formed. Life on Earth probably originated between 3500 and 4000 million years ago. What was it like on this ‘primeval’ Earth? Our current knowledge suggests that massive oceans existed, with only the beginnings of land masses above the surface of the water. There was no ozone layer, so large amounts of ultraviolet solar radiation reached the Earth. The atmosphere contained some water vapour, hydrogen, hydrogen cyanide, abundant carbon dioxide, nitrogen and possibly ammonia and methane. There was no free oxygen. Volcanic activity produced heat and sent large amounts of gases, ash and dust into the atmosphere. Violent electrical storms were common.

Chemicals of life Were all the basic components necessary for life present on this early Earth? For life to have originated, the required chemicals (Table 3.1) need to have been formed and then come together as a self-replicating system. This system would need some form of protection for its chemical constituents, and had to be able to use an energy source to replicate itself. We know that living things do this using the structure of the cell.

The living things on Earth today have not always been here; life on Earth is constantly changing. The idea of evolution is simply one of change. The theory of biological evolution states that life on Earth has changed with time: different species have gradually developed in response to changing environments. These changes have occurred slowly—often over millions of years. The groups of organisms in different populations have changed from ancestral types to modern-day types.

A meteorite is a rock that has struck the surface of the Earth from outer space.

BIOFACT

TABLE 3.1 The chemical components of life.

Compound

BIOFACT

Elements

water

hydrogen, oxygen

carbohydrates

carbon, hydrogen, oxygen

lipids

carbon, hydrogen, oxygen

proteins (made up from building blocks of 20 amino acids)

carbon, hydrogen, oxygen, nitrogen; other common elements include phosphorus and sulfur

nucleic acids (DNA and RNA)

carbon, hydrogen, oxygen, nitrogen, phosphorus

How could cells have formed on a primitive Earth? Let us look at two very different theories.

Theory 1: The chemicals for life came from outer space—a cosmic origin Scientists believe that the Earth was heavily bombarded with meteorites during the early years of its formation. When certain types of meteorites called carbonaceous chondrites were first analysed in the 1970s, they were found to contain organic molecules, including amino acids. This provided evidence of the existence of organic molecules elsewhere in the cosmos. It is possible that meteorites falling on the early Earth could have contributed some of the organic molecules required by living systems.

Meteorites have been hitting the Earth since its formation. Before the Earth formed an atmosphere there was nothing to stop a rock from space hitting the Earth’s surface. Today, most of the rocks that enter the Earth’s atmosphere burn up before they reach the ground, because of the great heat produced by the friction with the air. But many are so large that they do not disintegrate completely before hitting the Earth’s surface. The Murchison meteorite, which struck the Earth near the Victorian town of Murchison in 1969, is a carbon-containing meteorite. Chemical analysis identified 92 amino acids. Nineteen of these were similar to those found on Earth, but the remainder were unlike any found on Earth.

FIGURE 3.1 Fragments of the Murchison meteorite.

Life on Earth 109

Panspermia: life from outer space? In the early 1900s a Swedish chemist, Svante Arrhenius, developed the theory of panspermia (which means ‘seeds everywhere’). Arrhenius theorised that individual bacterial spores could drift across space, propelled by the pressure of light, and that these were the seeds of life on Earth. The panspermia theory has been used by others to develop ideas about evolution and the origins of life on Earth. Generally, theories of cosmic ancestry state that life on Earth was seeded from space, and that the evolution of life is based on genetic programs and systems that have come from other worlds.

Evidence in meteorites In 1996, the US National Aeronautics and Space Administration (NASA) announced that fossilised evidence of ancient life was found in a meteorite that had originated on Mars. But scientists could not prove that the microscopic formations were really the remains of living organisms, because no comparisons could be made with anything living or dead on Earth. In March 1999, NASA scientists announced that even stronger fossilised evidence for past life on Mars had been found in two other meteorites. These findings have inspired NASA to prepare space probes 1

Name the major building blocks for living cells.

that will collect soil, rock and dust samples from space in perfectly sterile conditions, so that it can be analysed for microscopic life. The first probe, called Stardust, passed within 100 km of Comet Wild-2 in 2004 to gather dust particles from the comet’s tail and return them to Earth for analysis.

Nanobes Soon after the Martian meteorite discovery, geologist Dr Philippa Unwins, from the University of Queensland, discovered what might prove to be evidence that the fossilised organisms found in the Martian meteorite could have come from ultramicroscopic organisms similar to those on Earth. The growths that she found on rock samples from the Earth were named ‘nanobes’. Dr Unwins’ analysis showed that, like single-celled organisms, nanobes have a membranous wall and a nuclear core. They also contain the major building blocks for life: oxygen, nitrogen and carbon. An intriguing question remains: Are the organisms found in the Martian meteorites the remnants of past Martian life, and therefore perhaps the origins of life on Earth?

2

Do you think life on Earth could have developed from molecules from space?

Theory 2: The chemicals of life were formed on Earth Oparin and Haldane In the 1920s, Russian scientist Alexander I. Oparin (1894–1980) and British scientist John B. S. Haldane (1892–1964) both suggested that the early atmosphere of Earth contained all the necessary basic chemical components for life. They hypothesised that more complex organic molecules could have been created in slow spontaneous reactions using energy from ultraviolet radiation or lightning discharges. Oparin suggested that organic molecules slowly collected in the surface layers of the oceans, forming an organic ‘soup’. These molecules could than have combined to form larger structures, eventually forming cells.

Urey and Miller’s experiment Oparin and Haldane’s theories remained untested until the 1950s, when two American scientists, Harold Urey and Stanley Miller, performed the following experiment.

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A closed system was set up and powerful electrical sparks were discharged into a glass chamber containing ammonia (NH3), hydrogen (H2) and methane (CH4) (Figure 3.2). After only one week of continuous electrical discharges and recycling steam through the apparatus, the condensed water in the flask became red and turbid. When Urey and Miller analysed the liquid using paper chromatography techniques they found it contained a number of amino acids. Urey and Miller’s experiment was significant because it supported Oparin and Haldane’s theories and demonstrated that complex organic molecules such as amino acids can be produced naturally from more basic chemical components. water vapour electrode H2O N2 NH3

'ocean'

CH4 CO2 H2

'atmosphere'

cold water

H2O

heat

sample of cooled water containing organic compound

FIGURE 3.2 The design of the apparatus used in Urey and Miller’s experiment, in which they showed that organic molecules could form in the environment of the early Earth.

Current knowledge Since Urey and Miller’s classic experiment, scientists have further developed Oparin and Haldane’s theories on the origin of life. Recent experiments have demonstrated that amino acids can be produced under conditions more like those of early Earth, using ultraviolet light instead of electrical discharges, and carbon dioxide instead of ammonia and methane. If hydrogen cyanide (HCN) is included, a greater variety of amino acids is produced.

Changes in technology Our ability to describe the origins, processes and evolution of living things has been made possible by advances in science and technology, particularly in molecular biology and biochemistry. Scientists and engineers have developed many techniques to find out more about the Earth, its history and the living organisms that occupy it. Developments in engineering have enabled both space and deep sea exploration. Samples of all types of materials can now be analysed to the molecular level by different techniques, including chemical analysis and X-ray crystallography. Chemical separation techniques such as chromatography help to isolate molecules for further study. The ages of rocks and fossils can be dated by radiometric dating methods. Developments in microscopy, particularly the electron microscope, have led to a new

BIOFACT Nanotechnology is the research, development and manufacture of objects at the molecular level. It uses microanalytical techniques from chemistry and engineering to study and manipulate atomic and molecular structures from both physical and biological systems. Nanotechnologists usually work with materials that are smaller than 100 nanometres, or 10–7 metres — the average size of a virus. Within the next 20 years nanotechnology will have a huge impact on the way things are designed and constructed in our society. Some surgery will be conducted at the cellular level; computers and other machines will be smaller, more powerful and more precise; and the artificial manufacture of any type of material will be possible.

Life on Earth 111

understanding of structures at the molecular level. Biochemical analysis, particularly of DNA, have enabled scientists to undertake comparative studies of different organisms. Genetic engineering techniques continue to help scientists to understand how change can take place in living organisms and thus we can better understand the relationship between organisms and their possible evolutionary pathways. The use of these new technologies has led to a better understanding of the origins of life and the evolution of living things. Examples of all the techniques named above can be found in this book. In your reading and research, both in this book and elsewhere, you will find examples of how advances in science go hand-in-hand with advances in technology.

Theories about membrane formation Some types of organic molecules can form a protective layer which maintains a separate internal environment. This could explain how cell membranes were first formed. Coacervates Complex organic molecules suspended in water will spontaneously form separate droplets. Each droplet has a boundary layer of water molecules which separates the molecules inside the droplet from the external environment. Proteinoids Proteinoids are formed when dry amino acids are heated to 130˚C and then cooled in

water. This produces very small round shapes (microspheres) similar in size to bacteria. They have a selectively permeable boundary layer and can divide in a way that resembles budding (see p. 193). Liposomes Complex organic molecules known as phospholipids can combine in water to form spherical structures with a boundary layer similar to cell membranes. This layer acts as a selectively permeable membrane and absorbs organic molecules from water.

Questions 1

Describe the conditions on the early Earth, and the types of chemicals that probably occurred there.

2

Define the term ‘evolution’.

3

Why is it possible that there are other forms of life in the universe?

4

Outline two scientific theories that attempt to account for the appearance of organic compounds on Earth.

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5

Describe Urey and Miller’s experiment, and explain the significance of their results.

6

Identify two technological advances that have had an impact on our knowledge of the origins of life, and explain how they had such an impact.

F u r ther questions 1

The following compounds are the important chemical components of life:

4

Discuss the significance of the discovery of ‘nanobes’ in Martian meteorites.

water carbohydrates lipids proteins nucleic acids a Divide the list into organic and inorganic compounds. Outline the criteria you used to place the different chemicals into each group. b Describe the role of each of these compounds in living things.

5

a Use a flow chart to summarise the sequence of events that are thought to have produced organic compounds on the early Earth. b Compare this sequence of events with the theory of Haldane and Oparin and the experimental work of Urey and Miller. c While the work of these scientists is significant to our understanding of the origin of life on Earth, it does not explain how life actually began. What other key events are yet to be explained to complete our understanding of the origin of life?

2

Outline the evidence that supports the idea that there may be other forms of life in the Universe.

3

NASA is working on a project that involves collecting soil, rock and dust samples from space. The samples must be collected and analysed under perfectly sterile conditions. a What are NASA researchers looking for? b Explain why sterile conditions are critical in this project.

Life on Earth 113

3.2

Fossils and the evolution of life OBJECTIVES When you have completed this section you should be able to: ● describe the key steps in the evolution of life, including the development of organic molecules, membranes, procaryotic and eucaryotic organisms, colonial cells and multicellular organisms ● identify evidence that present-day organisms have evolved from ancestral organisms ● identify the geological and palaeontological evidence that suggests when the earliest lifeforms appeared on Earth ● explain the importance of the change from an anoxic to an oxic atmosphere on the evolution of living things ● outline how scientific knowledge may be in conflict with cultural understandings in relation to the origins of life.

Major stages in the evolution of life activities ● ● ●

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A timeline for life on Earth Looking at fossils Learning about the past

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The following steps are believed to be the major stages in the evolution of life on Earth: 1 The formation of organic molecules Complex organic molecules formed in water on the early Earth. 2 The formation of membranes A membrane developed to isolate and protect the system of large complex organic molecules which evolved to include nucleic acids and became capable of self-replication. 3 Procaryotic cells The first cells were simple structures known as procaryotes (see p. 124). They are the earliest type of fossil found.

4 Eucaryotic organisms Cells containing a membrane-bound nucleus and cell organelles developed (see Chapter 2, pages 49–55). 5 Colonial cells Multicellular organisms may have originated when daughter cells became bound together after cell division to form an aggregation of similar cells or colony. Stromatolites (p. 117) provide an example of this both as fossils and as present-day colonial cells. 6 Multicellular organisms Multicellular organisms containing cells which show specialisation of function evolved. Each cell has its own particular function and is dependent on others. Each organism, however, functions as a coordinated whole.

The evolution of organisms The study of fossils shows us that there has been a great diversity of living organisms since Precambian times (Table 3.2, page 120). The types and abundance of organisms, however, have changed over long periods of time. With a knowledge of present-day diversity we can trace back the evolutionary pathways of living organisms. Some appear to have changed greatly over time, others have remained the same for millions of years, and some forms have become extinct and are no longer found today (Figure 3.3). We can relate the fossil record to the time over which living things have been evolving on Earth.

change over time horses

remains unchanged horseshoe crab elephants

extinct dinosaur

FIGURE 3.3 Some examples of evolutionary pathways. Most species, such as elephants and horses, have evolved from other species; some, such as horseshoe crabs, have remained the same for millions of years; and many species, such as all the dinosaurs, have become extinct.

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Fossils The study of fossils is called palaeontology. Fossils are any preserved remains or traces of past life found in sedimentary rocks or other material of different ages. Fossilisation is a rare event and it is rare for a complete fossil to be found. Hard parts such as bones, teeth and shells of animals and wood, pollen and spores of plants are most commonly preserved. Traces of organisms such as footprints, trails, burrows and even animal excreta (called coprolites) can also tell us something about the organisms that made them. Occasionally fossils, particularly more recent ones, may be preserved whole as a result of an unusual event. These fossils are particularly useful for comparison with living forms. For example: ● Woolly mammoths more than 10 000 years old have been found frozen in the permanently frozen soil of Siberia. ● Resin from coniferous trees may trap insects and preserve them whole when the resin hardens to form amber. ● Peat bogs may preserve whole organisms in a mummified condition. These remains are often called ‘subfossils’.

(a) living fish

sediment from river

(b)

fish skeleton partly buried by sediment (c) recent sedimentary rock

older sedimentary rock

land raised above water level

FIGURE 3.4 Ginkgos were once known only from fossil leaves like this one, which was found at Koonwarra in southern Victoria. But in the 1930s the ginkgo or maidenhair tree, Ginkgo biloba, was discovered as a living tree in central Asia, and is now widely cultivated in gardens. It is called a ‘living fossil’, because it is the last survivor of a group of plants that has all but died out.

more recent sediment collecting

older sedimentary rock (d)

fish skeleton fossilised

geological fault exposes fossil skeleton

FIGURE 3.5 The formation of a typical fossil. (a) The dead fish is quickly buried by sediment. (b) The sediment is covered by more layers, and the pressure consolidates it into soft sedimentary rock. As this happens, the bones of the fish disintegrate and the spaces left behind are replaced by chemicals that form a hard rock. (c) The rocks containing the fossil are raised above sea level by Earth movements. (d) Weathering (or a palaeontologist’s hammer) exposes the fossil.

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Palaeontological and geological evidence of early life The study of rocks and fossils can also provide evidence of early lifeforms and their activities. The oldest sedimentary rocks on Earth are about 3800 million years old.

Palaeontology is the study of life in ancient (geological) times, through the fossil record.

Palaeontological evidence Fossil evidence in very ancient rocks is scarce compared with the abundance of fossils found in rocks over the past 600 million years. The earliest fossils found are of two types: microfossils which are similar to present-day single-celled anaerobic procaryotic organisms, and stromatolites, which are layered mats of photosynthetic procaryotic cells called cyanobacteria (see p. 129) whose modern-day descendants can still be found in Western Australia. These two types of fossils are found in rocks 3400 to 3500 million years old from the Warrawoona Group in Western Australia. Stromatolites are also found in the 2800 to 3000 million-year-old Fig Tree Group of rocks in South Africa and in 2000 million-year-old Gunflint Chert rock found on the shores of Lake Superior in North America.

(a)

(b)

Geological evidence The first primitive cells were heterotrophic: they obtained energy by consuming other organic compounds. Then cells containing pigments developed. These cells were able to capture light energy from the Sun and use it to convert carbon dioxide to more complex organic compounds for their own use—in other words, they photosynthesised. As a byproduct of this process, oxygen was produced. The evolution of photosynthesis had a dramatic effect on the environment of Earth. It led to an explosion in the abundance of photosynthetic organisms. These organisms used up carbon dioxide which gradually reduced the levels present in the atmosphere. The oxygen produced did not at first build up in the atmosphere but was taken up by rocks. These oxidised rocks can be seen today in the ancient banded iron and red bed rock formations (Figure 3.7).

FIGURE 3.6 (a) A section through a fossil stromatolite. Stromatolites are rock-like structures built by procaryotic cells called cyanobacteria. They were once thought to be extinct— only fossil stromatolites had been found. (b) In the early 1990s, living stromatolites were discovered in Shark Bay, Western Australia. The cells form a mat which traps a layer of sediment. The cyanobacteria grow up through the sediment to form a new mat layer. The Shark Bay stromatolites grow at the rate of about 1 mm per year.

Geology is the study of rocks and the Earth’s crust.

Life on Earth 117

How old is that rock? Igneous rocks are formed by volcanic activity. The age of igneous rocks is estimated using radiometric dating methods. These methods are based on the principle that radioactive elements decay to different forms (for example, uranium to lead, rubidium to strontium) at rates that are constant for each element. This rate of decay is independent of the nature of the rocks or the environmental conditions to which they are exposed. Uranium has a half-life of 4500 million years. This is the time taken for half of the atoms in a sample of uranium to decay to lead. By comparing the relative ratio of uranium to lead, the age of the rock can be calculated. This can be done only with igneous rocks because, as they cooled, those that contained uranium formed crystals that contained no lead. Similar calculations can be made using other radioactive elements in rocks, such as strontium. Because igneous rocks are formed by cooling of molten rock that reaches the surface of the Earth, they do not contain fossils. But they can be used as reference points to compare the age of sedimentary rocks above (younger) or below (older) the igneous

rock layer. For example, if a sedimentary layer containing fossils lies below igneous rock that was dated at 200 million years old, then the fossils must be at least that age or older. Fossils that contain carbon can be analysed by radiocarbon dating. The unstable carbon isotope 14C has a half-life of 5730 years and decays to 12C. This ratio is constant in the CO2 of the atmosphere, but once CO2 is incorporated into organic compounds such as plant cellulose, the ratio decreases steadily. This is because no new 14C is taken into the plant. So the ratio of 14C to 14C can be used to find out how long ago a plant fossil formed. This method is limited to samples no older than about 50 000 years, because by that age there is not enough 14C left to measure. Sometimes the only way to determine the age of a fossil bed is by ‘indicator fossils’. A coal deposit at Anglesea in Victoria contains many important flowering plant fossils, including relatives of modern banksias. The coal is dated at 40 million years old by the presence of certain fossils such as pollen grains whose age is well known from geological strata found elsewhere.

1

What is meant by ‘half-life’?

3

2

Which radioactive isotope is useful for dating human remains?

Which radioactive isotope is useful for dating ancient rocks?

The changing atmosphere

FIGURE 3.7 Red bed rock formations were formed when the oxygen produced by early organisms was taken up by rocks.

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When all oxidisable surface rock had been saturated with oxygen, oxygen began to build up as a gas in the atmosphere. Ultraviolet radiation from the Sun reacted with some of the oxygen gas to form ozone (Figure 3.8). As the amount of oxygen in the atmosphere increased, more and more could be converted to ozone, until an ozone layer formed around the Earth, high in the atmosphere. This ozone layer acted as a shield, absorbing ultraviolet radiation so that less reached the surface of Earth. The significance of the change from an anoxic atmosphere to an oxic atmosphere was that anaerobic organisms declined. As oxygen levels rose, photosynthetic organisms became more abundant, while the growth and metabolism of anaerobic organisms were inhibited by the presence of oxygen. Today anaerobic organisms survive only where there is a very low oxygen concentration, such as in the mud in swamps and bogs, and deep underground or in deep ocean hydrothermal vents (see p. 128). The ozone layer protected living things from the harmful effects of ultraviolet radiation. This protection enabled living things to colonise the land.

As oxygen levels rose in the atmosphere, living systems developed ways to use oxygen directly to produce chemical energy. Aerobic organisms evolved that produced energy more efficiently by the process of respiration (see pp. 14–15). Greater metabolic activity became possible and organisms could be more active. The result was an increase in the size and complexity of organisms. Eucaryotic cells evolved, as did multicellular plants and animals. With the evolution of photosynthesis and respiration, life on Earth has changed forever. The conditions under which life was originally formed have been replaced by new conditions. The presence of oxygen in the atmosphere inhibits the formation of complex organic molecules such as amino acids, and the abundance of basic chemical components has also been depleted and locked up in existing life-forms. The abundance of life-forms competing for survival would now not allow the slow rise of living systems as occurred on the early Earth.

Sun

Conditions needed for life as we know it today are: • protection from ultraviolet radiation • free oxygen in the atmosphere • liquid water. A steady state cycle of gases in the atmosphere now exists between photosynthesising organisms which use carbon dioxide and produce oxygen, and aerobic organisms which use oxygen and produce carbon dioxide. The restriction that respiration places on aerobic organisms is their absolute dependency on the availablity of oxygen, either as free oxygen in the atmosphere or as dissolved oxygen in water. Without oxygen, aerobic organisms do not survive.

BIOFACT

ultraviolet radiation O2

O

oxygen molecule

O

+

O2

O

2 oxygen atoms

O3 ozone molecule

Banded iron rock formations (2000–2800 million years old) have a high oxygen concentration present as magnetite (Fe3O4). This type of rock is no longer formed. Red bed rock formations are also highly oxidised and contain oxygen present as hematite (Fe2O3). FIGURE 3.8 The formation of ozone. Ultraviolet light splits oxygen molecules (O2). Each atom combines with an O2 molecule to form ozone (O3). The free oxygen atoms are called ‘oxygen-derived free radicals’. They are extremely reactive and toxic to living cells.

Other ideas about the origins of life Different cultures have developed different ideas to try to explain the origins of life. Some of these ideas are many thousands of years old. They are very often linked to the religious and spiritual beliefs of a culture. The development of scientific knowledge about the origins of life may conflict with some of these ideas. For example, some people believe that all the types of creatures that have ever existed on the Earth were created when the Earth was formed, and that none has descended or evolved from any other. Life on Earth 119

Many people believe in biblical creationism: the idea that the Earth, and everything on it, was created by God in the first six days of time, rather than by a gradual evolution. In some places creationism must be taught in schools, alongside the theory of evolution. But some biblical scholars believe that the theory of evolution and our existing scientific knowledge of the Earth’s history does not necessarily conflict with the Bible’s account, because there are different ways of interpreting the words in the Bible.

TABLE 3.2 A time scale for the evolution of plants and animals.

Period

Cainozoic

Quaternary Recent Pleistocene Tertiary sub-era

Era

Mesozoic

Palaeozoic

Precambrian

Started (mya)* 0.1 1.8

Neogene Pliocene Miocene

5.32 23.8

Palaeogene Oligocene Eocene Palaeocene

33.7 54.8 65

Cretaceous

141

Jurassic

205

Triassic

251

Permian

298

Carboniferous

354

Devonian

410

Silurian

434

Ordovician

490

Cambrian

545

680 2600 3050 4600

* millions of years ago Note: The Neogene and Palaeogene periods together make up the old Tertiary period, which is now a ‘sub-era’. The Precambrian is sometimes divided into three separate eras. 120

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Evolution of plants

Evolution of animals



rise of human civilisations first Homo species



first hominids (human ancestors)



land dominated by mammals (including apes), birds and insects radiation of mammals

● ●



increase in number of herbs

land dominated by angiosperms

● ●

angiosperms (flowering plants) arise and expand as gymnosperms decline

● ●

last of the dinosaurs; second great expansion of insects dinosaurs abundant; first birds



last of the seeds ferns



first dinosaurs, first mammals



land dominated by gymnosperms



rise of reptiles; first land vertebrates



great coal-forming forests; beginning of evolution of ferns and gymnosperms



age of amphibians; first reptiles; first great expansion of insect species



expansion of primitive vascular plants; origin of first seed plants towards end of period; first liverworts



age of fishes; first amphibians and insects; corals dominant



invasion of land by first vascular plants towards end of period



invasion of land by a few arthropods



marine algae abundant



invertebrates dominant; first corals; first vertebrates (fish)



primitive marine algae



marine invertebrates abundant, including representatives of most modern phyla



first invertebrates



first eucaryotes



first procaryotes



formation of the Earth

Life on Earth 121

Creation and cultures Ancient cultures developed explanations of the origins of the Universe and everything in it. These explanations are called creation myths or cosmogenies. They are among the earliest attempts by many cultures to explain some of the most profound questions about the nature and origin of the Universe, and of life. To the Egyptians and Babylonians, Creation occurred not long before they had come into existence. To these cultures, gods were responsible for all aspects of the world. According to the most common cosmogeny in Chinese culture, the first living being was P’an Ku, who evolved inside a gigantic cosmic egg. This egg contained all the elements of the Universe, totally intermingled. P’an Ku grew about 3 metres every day. During this process he separated the Earth and the sky within the egg, and gradually separated nature’s opposites: male and female, light and dark, and so on. As he grew, P’an Ku also created the first humans. The egg hatched after 18 000 years, and P’an Ku died from this massive effort of Creation. Eventually, from his sweat, rain and dew appeared; from his eyes, the Sun and the Moon; from his voice, thunder; and from his body, Earth’s natural surface and features were created. Australian Aboriginal people believe in a creation time, called the alcheringa or Dreaming. The cosmogenies of different Aboriginal peoples vary. Generally, the Dreaming was the time when ancestral beings emerged from beneath the ground, and the time when humans, animals, plants, the world, the Sun, Moon and stars were created. According to one tradition, two superhuman beings came down to the Earth from the west. These beings formed humans from strange creatures that had themselves evolved from animals and plants. Greek cosmogenies were more definite and methodical than those of many other cultures. Aristotle’s theory of Creation lasted for more than 2000 years in Western cultures. He based his idea on systematic and common-sense observations. He thought that everything in existence was positioned in an immense and complex hierarchy that did not change. This hierarchy ranged from rocks, up through plants and animals, to humans, and finally to God. Lucretius, a Roman philosopher and poet who died around 50 BC, believed that the Universe could not have been created by a divine power,

because it was so imperfect. He thought that the Earth formed first by the gradual coming together of particles, and that the rest of the Universe was formed by the lighter particles, including fire and water, that were squeezed out as the Earth formed. He thought that, although this process started long ago, the Earth was formed very recently. He said that plants grew from the Earth like the feathers from a bird, and that the ancestors of all the animals had been born from the mother Earth, and these species remained the same ever since, although some had perished.

FIGURE 3.9 According to the Old Testament of the Bible, the creation of the Earth by God was followed by His creation of humans (Adam and Eve) and all the creatures and plants of the Earth.

1

Name four cultures that have unique creation ideas.

2

Give an example of ways in which current scientific knowledge conflicts with creation ideas in one of these cultures.

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Questions 4

Describe two pieces of evidence that support the theory that life existed on Earth 3000 million years ago.

a Define the difference between oxic and anoxic conditions. b Explain the significance of the change from an anoxic atmosphere on Earth to an oxic one in the evolution of organisms.

5

a Define ‘fossil’. b Explain what the fossil record tells us about life in the past.

Explain why the development of photosynthesis was so important for the evolution of living things on Earth.

6

Different cultures have different theories about the origins of life. Why do these often conflict with developments in scientific knowledge?

3

Investigate some of the different theories that have been suggested to account for the extinction of the dinosaurs. Display your findings in a way suitable for giving a class presentation; for example, overhead transparencies, a computer display, such as Powerpoint, or a poster.

4

Research the ideas about the origins of life from an ancient culture. Present these ideas as pictures or a story for children.

1

List the major stages in the evolution of living things. Next to each stage, write a brief sentence about its significance, giving an example in each case.

2

3

F u r ther questions 1

A newspaper report claims that dinosaur fossils have been found in the red bed rock of a Sydney railway cutting. Write a letter to the editor explaining why this could not be true.

2

a Explain why igneous rock does not contain fossils. b How is igneous rock important to the interpretation of the fossil record?

Life on Earth 123

3.3

Procaryotes: the first living things OBJECTIVES When you have completed this section you should be able to: ● describe advances in technology that have enhanced our knowledge and understanding of procaryotic organisms ● describe the main features of the environment occupied by one of the following: archaea, eubacteria, cyanobacteria, nitrogen-fixing bacteria, methanogens, or deep-sea bacteria ● identify the role of a named procaryotic organism in its ecosystem.

What are procar yotic organisms? activities ● ●

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Scientists believe that procaryotic organisms were the first type of cells to evolve about 3050 million years ago, and that for about 550 million years they were the only living things on Earth. Procaryotic cells are still the most abundant life-form on Earth and play an important role in all ecosystems. Procaryotic cells differ fundamentally in structure from eucaryotic cells (Table 3.3). All procaryotes are independent unicellular organisms, although some may aggregate in colonies or as long filaments. Sometimes this is because the cells secrete a slimy sheath around themselves. Procaryotic cells are small, typically about 1 µm (micrometre) in diameter. They lack a nuclear membrane and membrane-bound internal organelles. Structurally, three regions can be identified (Figure 3.11).

TABLE 3.3 Procaryotic and eucaryotic cells compared.

Feature

P rocaryotic cells

Eucaryotic cells

Examples: archaea and eubacteria

Examples: plants, animals and fungi

Membrane-bound organelles

Absent

Present

Chromosome

Single strand of DNA, often circular

More than one chromosome

Nucleolus

Absent

Present

Ribosomes

Present but small

Present

Endoplasmic reticulum

Absent

Present

Microtubules

Absent

Present

Enzymes of respiration

Attached in infoldings of cell membrane

Attached to internal membranes of mitochondria

Photosynthetic pigments

Attached in infoldings of cell membrane

Attached to internal membranes of chloroplasts (plants only)

Cellular nature

Unicellular

Unicellular or multicellular

Cell division

Not by mitosis

By mitosis

Average cell size

1 micrometre

10–100 micrometres

Appendages may be attached to the cell surface. These may be flagella or pili. There is a cell envelope consisting of a cell membrane, a cell wall, and in many procaryotes an additional protective layer known as a capsule. On the cell membrane, which may have some inward folds, are found the sites for respiration and photosynthesis. The cytoplasmic region contains a single DNA chromosome, ribosomes and various inclusions (see Figure 3.10). The single chromosome is circular and has no membrane around it. Many procaryotes also have small circular DNA pieces called plasmids. Plasmids are self-replicating and are not essential for the survival of the cell. They are often used in genetic engineering experiments (see p. 312). When procaryotic cells reproduce, the DNA strand replicates. Each strand attaches to the cell membrane. The membrane between the two strands elongates and pinches inwards, forming two new cells. There are no microtubules or centromeres present in procaryotic cells, so no spindle is formed, unlike normal mitosis. Ribosomes in procaryotic cells are smaller than those of eucaryotes. Inclusions are normally food reserves but may sometimes be collections of pigments or enzymes. Technological advances in electron microscopy have increased our knowledge of procaryotic organisms. In particular, Carl Woese discovered the existence of two fundamentally different types of procaryotes, using comparative sequencing (see p. 134).

The electron microscope revealed that cells could be of two types: eucaryotic and procaryotic. All multicellular organisms have eucaryotic cells. Eucaryotic cells all have a nuclear membrane and membrane-bound organelles.

BIOFACT In the oceans, procaryotes make up 90% of the weight of all marine organisms. On land, a single gram of soil may contain 109 procaryotic cells.

Life on Earth 125

In cyanobacteria the cell membrane folds inwards and contains photosynthetic pigments

cell wall plasmid

cell membrane

DNA

mesosome In some bacteria the cell membrane folds inwards and contains the enzymes of respiration.

ribosome

food reserve

(a)

capsule or slime layer

pili flagellum

Pili are protein rods projecting from the cell wall. They help the cell to 'stick' to other cells or surfaces FIGURE 3.10 The structure of procaryotic cells as revealed by the electron microscope. (a) A generalised diagram of a procaryotic cell. (b) Structures that can be present outside the cell wall of procaryotic cells.

An outer capsule or slime layer protects the cell. It may help colonies of bacteria to stick together.

One or more flagella may be present. The beating of the flagella enables the cell to move. (b)

Classification of procar yotes Procaryotes can be divided into two distinct groups, archaea and eubacteria. Table 3.4 lists the differences between them. Note that all these differences are at the molecular level; they are not visible structural differences. The cell walls of Eubacteria are made of a unique disaccharide– amino acid complex known as murein.

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TABLE 3.4 Differences between archaea and eubacteria.

Feature

A rchaea

Eubacteria

Murein

No murein in cell wall

Cell wall composed of murein

Lipids in cell membrane

Branched

Unbranched

RNA synthesis

1 large enzyme involved

1 small enzyme involved

Sensitivity to the antibiotics streptomycin and chloramphenicol

Not sensitive

Sensitive

Procar yotic organisms Procaryotic organisms occupy every environment on Earth, including the most extreme. They are found in water, whether salt or fresh, hot or cold. They are found on land and in the air. Their major role in ecosystems is to recycle nutrients. Procaryotes are the major contributors to the recycling of the world’s oxygen, carbon, nitrogen and sulfur. They may be found in ecosystems as producers, consumers, decomposers, symbionts and parasites.

Archaea are microscopic, single-celled organisms with no membrane-bound organelles

Archaea There are three groups of archaea: methanogens, halophiles and thermophiles. They are all found in unusual or extreme environments.

Methanogens Environment Methanogens are found in bogs and deep soils, in marine and freshwater sediments, in the intestinal tracts of herbivores, and in sewage treatment facilities. They are anaerobic and cannot tolerate exposure to oxygen. In a unique process, they use hydrogen (H2) as an energy source and carbon dioxide (CO2) as a carbon source for growth, producing methane gas (CH4) in the process. Methanogen activity created most of the Earth’s natural gas deposits. Cattle have methanogens in their stomach and belch about 50 litres of methane a day when chewing the cud.

Role in ecosystem Methanogens play a role in the recycling of carbon and are important decomposers. The methane they release into the atmosphere contributes to the carbon cycle. Where methanogens are found in symbiotic relationships, such as in the gut of cattle or termites, their role is to assist the breakdown of cellulose and thus aid digestion for their hosts. There has been a large rise in the amount of methane in the Earth’s atmosphere in recent years, mainly because of the increase in domestic cattle kept for human consumption.

FIGURE 3.11 Photomicrograph of Methanobrevibacter, a methanogen species.

Life on Earth 127

Halophiles

BIOFACT Deep sea hydrothermal vents occur along mid-ocean ridges. Hot, mineral-rich water flows out from rocks and cracks in the ocean floor. The minerals are precipitated on contact with the cold ocean water, forming large deposits. The mounds formed are rich in minerals such as sulfides, lead, copper, cobalt and zinc. Chimney-like structures called black smokers are created when sulfide minerals crystallise from the hot water. The crystals form hollow chimneys through which hot water continues to flow. When the hot water flowing out mixes with the cold ocean water, the minerals are precipitated as tiny particles that make the water appear black and give black smokers their name.

Environment Halophiles are found in environments where the salt concentration is very high, such as the Dead Sea in the Middle East, the Great Salt Lake in the USA, and evaporating ponds of saline water. They are all aerobic. Halophiles are aerobic organisms, but they also have another system of producing energy. Their red colour is caused by a unique pigment called bacteriorhodopsin, which enables them to photosynthesise and produce energy without using oxygen.

Role in ecosystem In their ecosystem, halophiles are part of the food chain and are consumed by filter feeders. Very little is known about their role in ecosystems, and further research is needed.

Thermophiles Environment Thermophiles require high temperatures for growth (80–105˚C). They are found in areas of volcanic activity such as hot springs, geysers, and hydrothermal vents and cracks in the ocean floor. The thermophiles that live in the hydrothermal vents in the depths of the ocean are sometimes called deep-sea bacteria. Thermophiles use sulfur as an energy source.

BIOFACT The giant tube worm Riftia pachyptila, which lives in hydrothermal vents, can grow to over a metre in length and has no mouth, gut or anus. It obtains all its nutrition from sulfur bacteria that live inside it in a special organ called a trophosome.

Role in ecosystem The deep-sea sulfur-oxidising bacteria in hydrothermal vents are the primary producers (chemoautotrophs) in the deep sea food web and support an amazing community. While the bacteria may form large mats on the sea floor and some organisms feed on them directly, other vent organisms have developed a symbiotic relationship with the bacteria. The organism provides the shelter and the bacteria provide the nutrients.

Eubacteria sphere (coccus)

rod (bacillus)

bent rod (vibrio)

spiral (spirillum)

FIGURE 3.12 Some common shapes of eubacteria.

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Environment Eubacteria (Figures 3.12, 3.13) are found everywhere: in soil, water, air, and decaying organic material, and inside other living organisms. With fungi they play an important role as decomposers of organic matter for recycling in nature. While most eubacteria are harmless, some may cause disease in other organisms. Human diseases caused by eubacteria include cholera, typhoid, tuberculosis and boils. Eubacteria can be classified into a number of different groups, including green bacteria, purple bacteria, spirochaetes, Gram-positive bacteria and cyanobacteria.

Role in ecosystem As decomposers, soil bacteria secrete enzymes on dead organic material. Once material is broken down to simpler compounds, the bacteria absorb those they require. Decomposers play a large role in maintaining fertility.

cell wall

cytoplasm

nuclear material

FIGURE 3.13 This is the bacterium Escherichia coli, which is naturally present in the human intestine. An unusually high level of this bacterium in water may be an indication of faecal contamination. The photograph shows the bacterium magnified about 30 000 times its normal size.

Cyanobacteria Environment Cyanobacteria (Figures 3.14, 3.15) were once called blue-green algae and classified with the simple algae. Recent studies have found that they are more closely related to the bacteria. All cyanobacteria contain a blue pigment called phycocyanin, from which they derive their name. They occur naturally in wet or damp situations: ponds, streams, wet rocks and soil. They flourish in warm conditions, particularly where the water contains dissolved organic material. Like bacteria, they are small. Many form individual filaments, while others form slimy masses. Some cyanobacteria produce toxins, and can reproduce so rapidly that they form a vast, slimy mat (a ‘bloom’) that poisons the water in lakes and slow-flowing streams.

polyedral body (carboxysome)

nuclear material

Anabaena

Gloeocapsa

Oscillatoria

FIGURE 3.14 Some common shapes of cyanobacteria.

FIGURE 3.15 Cyanobacteria flourish in water containing high levels of dissolved nutrients. The cyanobacterium Oscillatoria brevis, magnified about 25 000 times its normal size.

cell wall

thylakoids

Life on Earth 129

Role in ecosystem Cyanobacteria are photosynthetic organisms. They are primary producers at the base of the food chain within their ecosystem. Cyanobacteria in the Earth’s oceans generate large quantities of oxygen, and they are sometimes called the ‘grass of the sea’ because of their role as primary producers. Some cyanobacteria also fix nitrogen from the air (see below). Stromatolites are cyanobacteria first discovered in fossils more than 3000 million years old (see p. 117). Their abundance and carbonfixation ability in photosynthesis over millions of years contributed to the reduction in carbon dioxide and increase in free oxygen in the Earth’s atmosphere.

Nitrogen-fixing bacteria Environment Most nitrogen-fixing bacteria occur in the soil. They may be aerobic or anaerobic, free-living or symbiotic. Symbiotic nitrogen-fixing bacteria occur in nodules on the root systems of many plants, for example, legumes, she-oaks and acacias.

Role in ecosystem

BIOFACT About 90% of the nitrogen that enters the atmosphere comes from nitrogen fixation by procaryotes. The remaining 10% comes from the combustion of fossil fuels and from lightning strikes.

FIGURE 3.16 Root nodules (arrowed) on legumes contain Rhizobium bacteria, which fix nitrogen. The nitrogen is taken up by the host plant, and any excess is released into the soil.

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Nitrogen-fixing bacteria play an important role in the nitrogen cycle, and their activity is the only way that atmospheric nitrogen is made available for use by living things. Nitrogen fixation is the transformation of nitrogen from the air to ammonia. After being fixed, the nitrogen is then incorporated into organic compounds such as proteins and nucleic acids in micro-organisms and plants and from there, in the food chain, to animals. Nitrogen-fixing bacteria, often found in a symbiotic relationship in plant roots, take nitrogen from the air and convert it into more complex compounds such as nitrates which can be used by plants. This allows them to grow in nutrient-poor soils.

Questions 1

Outline the main differences between procaryotic and eucaryotic cells.

2

Explain how a the electron microscope, and b advances in molecular biology have increased our knowledge and understanding of procaryotic organisms.

3

What is a plasmid? What are the only kinds of organisms in which plasmids are found?

4

a Name the two groups into which procaryotes are divided. b Describe the distinguishing features for each group.

5

For any one named type of procaryotic organism a describe the main features of its environment b identify the role of this organism in its ecosystem.

6

Stromatolites that exist today are also represented by fossil remains that date back more than 3000 million years. a What kind of organism is a stromatolite? b Explain how stromatolites contributed to change in environmental conditions over millions of years.

2

a Nitrogen is abundant in the atmosphere as a gas (N2), but plants and animals cannot use it in this form. i Why is nitrogen an important nutrient for plants and animals? ii Explain how plants obtain nitrogen. iii How do animals obtain nitrogen? b How are legumes such as peas, beans and lentils different from other plants? c Legumes are beneficial plants to grow in domestic vegetable gardens and in larger scale farming practice. Explain the benefits offered by growing legumes.

F u r ther questions 1

Identify the procaryotic and eucaryotic cells from the following illustrations.

(a)

(b)

(d)

(e)

(c)

(f)

Life on Earth 131

3.4

Taxonomy: classifying organisms OBJECTIVES When you have completed this section you should be able to: ● explain the need for a classification scheme of living organisms ● describe different criteria used to classify organisms ● summarise the advantages and disadvantages of different classification systems ● explain how different levels of organisation in hierarchical systems assist the classification process ● discuss how changes in technology have impacted on the development and revision of classification systems ● outline the problems that arise in the classification of extinct organisms ● understand the binomial system of naming organisms ● use simple dichotomous keys in the identification of organisms ● explain how the classification of organisms can enhance our understanding of the relationships between past and present life-forms.

Why classify? activities ● ●

Constructing and using a key Identifying plants and animals

BIOFACT Most taxonomists work in herbariums, botanic gardens, museums or universities. The results of their work are usually published in scientific journals. When a taxonomist publishes new ideas about the classification of organisms, other taxonomists may decide to accept or reject the ideas. Sometimes, a new idea takes many years to become accepted.

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About 1.8 million different types of living things on Earth have been found and described in detail. Each year this number increases. In order to make the study of this enormous diversity of life easier, biologists group together organisms that have similar characteristics. The way that the organisms are grouped is called a classification system. Classification systems help biologists to understand the relationships between organisms, and to talk to other biologists about organisms without having to describe the organisms in detail (see p. 141). The science of classifying organisms is known as taxonomy. Classification systems aim to reflect our current knowledge of the world of living things. To cope with this huge volume of knowledge, biologists are continually devising and changing classification schemes. Classification systems are considered to be arbitrary (uncertain), because each one reflects the views of the biologist who devised it. Although most biologists use an internationally agreed classification system, there are many variations, and the system is always changing to allow for new knowledge about organisms and the relationships between them.

Features used to classify organisms In any system of classification, organisms need to be studied closely. Organisms may differ in the following ways: anatomy (structure), physiology (functioning), behaviour (doing) and biochemistry (molecular activity). At the genetic level there may be differences in chromosome number and structure. While all of these criteria can be used, the most practical one for use in the field is anatomical structure. Structure is easily observed in an organism, whether it is living or dead or found as a fossil. Structure is more constant in an organism’s lifetime than other features that may change seasonally or with maturity. The classification of animals is based mainly on structure, and the classification of plants is based on both structure and means of reproduction. The means of reproduction, however, can usually be inferred from the structures present. Increasingly, molecular structure— similarities in DNA and proteins—is being used by scientists to help classify organisms. (See boxes on pages 135 and 138.)

Classification systems There have been many ways that living things have been classified at the kingdom level, the first step in classification (Table 3.5). In 1735 Carolus Linnaeus first divided the world into animal, vegetable and mineral kingdoms, in the first edition of his great work Systema Naturae. The French preferred their own classification systems, which were developed later by Georges Cuvier (for animals) and Antoine-Laurent de Jussieu (for plants). In 1866 a German biologist, Ernst Haeckel, created a kingdom for all microscopic unicellular organisms, which he called Protista. The development of the electron microscope in the 1950s revealed procaryotic cells. These were placed in a separate kingdom, Monera. In 1967, R. H. Whittaker’s classification system identified the Fungi as a separate multicellular eucaryotic kingdom. In 1977, Carl Woese discovered a further fundamental division within the Monera, which became divided into the Archaea and Eubacteria (see p. 134). Some scientists consider these to be separate kingdoms.

BIOFACT Until the 19th century, Latin was the universal language of science, so most scientists gave themselves Latin names. Linnaeus published his work under the name of Carolus Linnaeus. His real name was Carl von Linné.

TABLE 3.5 Living things can be classified into up to six different groups at the kingdom level.

2 kingdoms

plants, animals

3 kingdoms

protists, plants, animals

4 kingdoms

monera, protists, plants, animals

5 kingdoms

fungi, monera, protists, plants, animals

6 kingdoms

fungi, archaea, eubacteria, protists, plants, animals

Life on Earth 133

The following is the most common classification system, which recognises five kingdoms: Plants (kingdom Plantae) are those organisms which contain chlorophyll and make their own food. Their cells are eucaryotic and are surrounded by a cellulose cell wall. Animals (kingdom Animalia) are those organisms which do not contain chlorophyll and cannot make their own food. Their cells are eucaryotic and have no cell wall. Animals can be unicellular or multicellular. Protists (kingdom Protista) are single-celled organisms whose cells are eucaryotic; for example, protozoans and some algae. Monera (kingdom Monera) are single-celled organisms whose cells are procaryotic, for example, bacteria and cyanobacteria.

FIGURE 3.17 Although lichens are a symbiosis between fungi and algae, they are currently classified in the kingdom Fungi, according to their fungal symbiont. This cross-section of a lichen thallus shows the layer of darker, green algae (a) above an open network of fungal hyphae (f).

protists Bacteria cyanobacteria

Eucaryotes plants

fungi

animals

bacteria

Archaea halophiles methanogens thermophiles

FIGURE 3.18 Classification into three domains.

Fungi (kingdom Fungi) are organisms which do not contain chlorophyll, but their cells are eucaryotic and surrounded by a cellulose cell wall. Some fungi are unicellular (yeasts) while others appear to be multicellular. All systems of classification have advantages and disadvantages. Because they are human constructs (that is, they are made up by people), the features selected to separate organisms into groups may lead to difficulties in classifying some organisms. Fungi are a problem for taxonomists. Should they be classified as plants or protists, or put in a group by themselves? Depending on the criteria used, algae may be classified as protist or as plant. Lichens consist of a fungus and an alga: which group should they belong to? Is it best to classify organisms at the cellular level? The five kingdoms model recognises two major cellular structural divisions within living things — procaryotes and eucaryotes. Procaryotes are now identified as two structurally different groups — the Eubacteria and Archaea. Woese, in recognition of this, proposed a scheme of classification at the cellular level. He divided living things into three domains: Bacteria, Archaea and Eucaryotes (Figure 3.18). Viruses and prions (see Chapter 7) still remain outside any of these classification schemes. There is plenty of opportunity left for classification schemes to be further revised!

Levels of organisation Classification systems use a hierarchical system in which organisms are placed into groups, at different levels, according to the features they share. A branching pattern or ‘tree’ emerges which assists us to see the relationships between organisms. Figure 3.19 shows the hierarchical system used for both plants and animals. Organisms become more similar as we move through the levels from kingdoms to species. KINGDOM PHYLUM King Phil

CLASS Classed

ORDER Ordinary

FAMILY Families as

GENUS Generous and SPECIES Special

FIGURE 3.19 The classification hierarchy developed by Linnaeus, which is still used today. ‘King Phil’ will help you remember the order of the different classification levels.

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Modern tools for classifying organisms The science of taxonomy became more precise when evolution and the theory of natural selection were explained. Organisms that have a common ancestor are thought to be related, so they are grouped together. The fossil record and homologous structures (see p. 270) have been used traditionally by taxonomists as evidence for classification. But fossils and homologous structures can sometimes be misleading or inconclusive, and the presence of structures with a similar function can result from convergent evolution (see pp. 273) instead of ancestral relationships. Over time, other evidence has come to be used.

Embryonic development As microscopes were improved, taxonomists began to use the new information they provided to identify relationships among organisms. For example, comparative embryology has revealed that the major group to which vertebrates, including humans, belong is closely related to the group that includes the starfish and its relatives.

Classifying by molecular structure Studies at the molecular level play a large part in determining evolutionary relationships. Analyses of proteins and DNA can reveal relationships that could not be determined in any other way. Organisms that have many proteins or bases in common are closely related. DNA does not only reveal close evolutionary relationships. It can also be used to determine when two related organisms began to evolve from a common ancestor. Mitochondrial DNA is used in this type of study. Scientists believe that mitochondrial DNA mutates at predictable rates, so it is like a molecular clock that can be used to date evolutionary events. By comparing the differences in the base sequences of DNA, and using the predicted mutation rates, it is possible to determine the time over which two types of organisms have been diverging. For example, this sort of analysis indicates that chimpanzees and humans began to evolve from a common ancestor about 5 million years ago.

1

Explain why homologous structures may not give an accurate idea of evolutionary relationships.

2

What is the implication from studies of organisms that share many common genes?

Advances through new technology Biological classification systems are continually revised because of the impact of technology. The development of the light microscope first revealed that living things were made up of cells. Improved light microscopes and then the electron microscope revealed more and more levels of detail of their internal structure. This knowledge expanded the number of kingdoms of living things from two to five. Advances in molecular biology and biochemistry are causing further revisions. For example, Woese’s discovery of two major groups within the Monera was based largely on differences at the molecular level, not on anatomical structure. Recent advances in molecular techniques, such as comparing the sequences of amino acids in proteins or the sequencing of DNA bases along short lengths of chromosomes (genes) from related organisms, have considerably added to our knowledge of evolutionary relationships between organisms, and have enabled taxonomists to continually change and refine the existing classification system.

BIOFACT Similar sequences of amino acids are evidence of a common ancestry. One technique compares a protein called cytochrome C that is present in the mitochondria of almost all organisms. The amino acid sequences of this protein in humans and chimpanzees are identical for all 104 positions. Human cytochrome C differs from that of dogs by 13 amino acid sequences.

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(b) (a) FIGURE 3.20 Examples of organisms from the five kingdoms. (a) Monera: Anabaena, a cyanobacterium. (b) Protista: a desmid alga, Cosmarium sp. (c) Fungi: red-star fungus, Aseroe rubra. (d) Plantae: a moss, Rosulabryum billardierei. (e) Fungi: a lichen, Hypogymnia subphysodes. (f) Plantae: rough tree-fern, Cyathea australis. (g) Plantae: mountain gentian, Chionogentias muelleriana. (h) Plantae: drooping she-oak, Allocasuarina verticillata. (i) Animalia: marbled gecko, Phyllodactylus marmoratus. (j) Animalia: eustheniid stonefly, Eusthenia venosa. (c)

(d)

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(e)

(g)

(f)

(i)

(h)

(j)

Life on Earth 137

(b) FIGURE 3.21 These two species grow in vastly different places, but are closely related. (a) Common heath (Epacris impressa) from south-eastern Australia. (b) A rhododendron (Rhododendron fragrantissimum) from central Asia. (a)

A new classification system for flowering plants In 1998 an international group of botanists published a new classification for the families of flowering plants. It used not only the traditional structural appearance of plants but also results from the new molecular technique of gene sequencing. Plants were grouped according to similarities in the analysis of the detailed structure of their DNA. The new system has suggested some evolutionary

relationships not previously realised. For example, the Australian family Proteaceae, which includes banksias, grevilleas and waratahs, was shown to be related to the northern hemisphere family Platanaceae, the plane trees; and the family Epacridaceae, Australian native heaths, should be placed in the family Ericaceae, which includes azaleas, rhododendrons and Scottish heather.

Explain why some flowering plants have been reclassified.

The binomial system of nomenclature A binomial name consists of two words. The first is the genus name, and the second is the species name.

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Although many organisms have ‘common’ names, like Australian magpie and great white shark, these names vary greatly from place to place. Sometimes one animal can have several different common names (such as the striated fieldwren, which is also called the cocktail, fieldlark, reedlark and sandplain-wren); and sometimes one common name can be used for several animals (such as the name swamp gum, which is used in Tasmania for one eucalypt, but in Victoria for another eucalypt). To overcome this problem, scientists use a binomial system to give every type of organism only one name, which is called the scientific name. This system was developed by Linnaeus in the 18th century, but has been extended by many other people since.

In the binomial system of classification, an organism is given a name consisting of two words. The first word has a capital letter and represents the genus to which the organism belongs. The second word represents the species within the genus to which the organism belongs. Both words are always printed in italics. The species is the basic group in classification. All organisms in a species closely resemble one another, although there are usually differences between individuals. They can interbreed and produce fertile offspring. The animal shown in Figure 3.22a is Petauroides volans. The genus name Petauroides comes from the Greek word for a rope-dancer or tightrope walker, petaurista. The species name volans means ‘flying’ in Latin. This is the greater glider, a very agile flying mammal of wet forests in south-eastern Australia. Many organisms are named after the person who first discovered them. The plant shown in Figure 3.22b is Banksia coccinea. The genus name Banksia tells us it was named after Joseph Banks, a famous botanist who came to Australia with James Cook on board the Endeavour. The species name coccinea is Latin for scarlet, referring to the brilliant colour of the flowers.

(a)

(b)

FIGURE 3.22 (a) Petauroides volans, the greater glider, depicted in an early lithograph by John Gould. (b) Scarlet banksia, Banksia coccinea.

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Keys Names and descriptions of all known living organisms can be found in reference books, scientific journals and in the collections of various scientific institutions. Searching through such a vast literature in order to identify an unknown specimen is slow and tedious. To make identification easier, keys have been developed. The most common type of key contains pairs of alternative descriptions of an unknown organism. This is called a dichotomous key. By observing the organism and deciding which descriptions are the most appropriate, you can work through the key and establish the organism’s identity.

Using a key Use the following key to identify the two adult Banksia leaves pictured on this page. By answering the questions correctly, and following the directions to the next question in sequence (the numbers on the right refer to the numbered questions on the left), an unknown individual object or specimen can be identified. The answers are at the bottom of page 143. 1a 1b 2a 2b 3a 3b 4a (a)

(b)

4b

Leaves small and narrow, 1–1.5 cm long . . . . . . . . . . . Banksia ericifolia Leaves 2–16 cm long . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Leaves 2–10 cm long, margins smooth or irregularly toothed . . . . . . . 3 Leaves 8–16 cm long, margins distinctly and evenly toothed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Banksia serrata Leaves less than 1 cm wide . . . . . . . . . . . . . . . . . . . Banksia marginata Leaves generally 1–2 cm wide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Adult leaves with smooth margins, juvenile leaves toothed . . . . . . . . . . . . . . . . . . . . . . Banksia integrifolia Leaves irregularly toothed; young shoots with covering of rusty hairs . . . . . . . . . . . . . Banksia asplenifolia

Classifying extinct organisms Fossil remains of organisms may be difficult to classify. The fossil evidence may be incomplete or may not show enough detail of the organism’s structure for classification purposes. If the organism has been extinct for a long period of time there may be no similar type of living organisms with which to compare it. The rules for naming organisms allow scientists to name fossils even if they have only a part of the organism. This enables every fossil to be given a name, but it also means that one organism might end up with two or more scientific names. In that case, the first name used would be the correct name.

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Classification and under standing The classification of organisms helps us to understand present and past life on Earth in several ways. Ordering Grouping helps to bring a sense of order to an otherwise vast range of different types. It is a convenient way of organising living things to study them. Classification makes study easier by simplifying the description of living things. One group word can indicate several features. For example, by using the word bird we are saying ‘an egg-laying animal with two legs, two wings, feathers and a beak’. The single group word describes many of the organism’s characteristics. Communicating Classification helps us communicate with each other. Biologists throughout the world use the same system for naming organisms. Each living thing has an internationally accepted name and can be distinguished from any other by using that name. The words used are written in Latin, even if they come from English or another language. This international naming system is understood by scientists in all countries, so there is no confusion in communication. Relationships Classification helps to work out relationships between organisms. This is a field of study in which there is much speculation because of our increasing knowledge, at the genetic and molecular level, of the similarities and differences between organisms. Traditionally, classification schemes have shown us the diversity of present-day organisms, while at the same time attempting to show how old they are and their evolutionary relationships. Classification systems are called phylogenetic when they try to reflect the evolutionary history of the organisms that are grouped together.

Grouping helps to bring a sense of order to an otherwise vast range of different types.

• Classification helps us communicate with each other. • Classification helps to work out relationships between organisms. • Classification is the first step in learning about the relationships of organisms with their environment.

Conservation It is only by observing organisms, and describing and classifying them, that we are able to build up a picture of the range of organisms in different environments. Classification is the first step in learning about the relationships of organisms with their environment. Once endangered animals and plants have been identified, we can take action to conserve and protect them.

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Questions 1

Why do biologists try to classify organisms?

6

2

a Outline the characteristic features of organisms in kingdoms Plantae and Animalia. b List the three other kingdoms. Describe their distinguishing features, providing examples in each case.

Which level of classification contains organisms that are more similar to each other: order or species? Why is this so?

7

Advances in technology have improved our knowledge of living things. Outline two examples of how technological advances have affected classification systems.

8

Explain the difficulties that can arise in classifying extinct organisms.

9

Describe the binomial system of naming organisms. Include a specific example to illustrate your answer.

10

Classification systems are designed to do more than just help us identify living things. What else can they do?

3 4

5

Choose one classification system and discuss its advantages and disadvantages. One biologist insists that algae should be classified as plants. Another says only some algae are in the plant kingdom, others are protists. Who is right? How does this illustrate the arbitrary nature of classification systems? Give two reasons why structure is a useful characteristic in classifying organisms. Give reasons why colour, size and habitat are not usually used to classify organisms at the major grouping levels.

F u r ther questions 1

What is ‘specific’ about the members of one species? Are they identical? What do they have in common? Give an example.

2

Consider the crosses between the different kinds of organisms shown: lions and tigers ➞ ligers cats and dogs ➞ dats tomatoes and potatoes ➞ pomatoes

1a 1b 2a 2b 3a

Brown seaweed . . . . . . . . . . . . . . . . 2 Green seaweed . . . . . . . . . . . . . . . . 4 Fronds large and single . . . Laminaria Fronds branched . . . . . . . . . . . . . . . 3 Fronds with central axis, flattened, complex branching . . . . .Grystophora 3b Fronds jointed and cylindrical . . . . . . . . . . Hormosira 4a Fronds large, thin and membranous . . . . . . . . . . . . Ulva 4b Fronds fine and thread-like . . . . . . . . . . . . Cladophora

Are these ‘hybrid’ organisms considered members of a species? Explain your answer. 3

4

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A mule is the offspring from a cross between a male donkey (62 chromosomes) and a female horse (64 chromosomes). It is a sterile animal; that is, it cannot produce sex cells. When horticulturalists cross certain plants, they obtain what they call ‘mules’. What do you think they mean by this term? Use the following key to identify the seaweed shown.

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5

One morning you see a large, strange-looking creature crawling across the lawn in your backyard; it seems to be some sort of insect. Explain how you would go about identifying the creature to species level.

Chapter summar y Practical activities 3.1



Urey and Miller’s experiments

3.2



A timeline for life on Earth



Looking at fossils



Learning about the past



Bacterial environments



Environments of living things, past and present

3.3

3.4



Constructing and using a key



Identifying plants and animals

3.1 • Conditions on early Earth were very different from those found today. The composition of the atmosphere, the high levels of ultraviolet radiation and violent electrical storms could have provided the conditions necessary for the production of organic molecules. • Organic molecules found in meteorites from space imply that life may not be unique to Earth. • Two theories to explain the origins of life on Earth are that organic molecules (a) arrived from space on meteorites, or (b) were formed as a result of conditions on the early Earth. • Urey and Miller’s experiment demonstrated that the composition of the primitive atmosphere on early Earth could have produced organic molecules. • As new technologies have been developed, they have increased our understanding of the origin of life and the evolution of living things. 3.2 • Major stages in the evolution of life on Earth have been identified. Cell formation, and the later evolution of autotrophic cells, were critical stages. Life evolved from single cells to multicellular organisms. • Palaeontological (fossil) and geological evidence has been used to construct a time scale for the evolution of living things spanning about 4500 million years. • The change from an anoxic to an oxic atmosphere on Earth was significant in a number of ways. The formation of the ozone layer protected living things from harmful radiation. Anaerobic organisms declined, and the process of respiration that used oxygen directly provided more energy for living organisms, leading to the evolution of larger, more complex organisms. • Scientific knowledge usually differs from traditional cultural beliefs about the origins of life. 3.3 • Technological advances in the development of microscopy, particularly the development of the electron microscope, have increased our knowledge of procaryotic organisms. • Different types of procaryotes, such as archaea, eubacteria and cyanobacteria, have identified roles in the varying environments they occupy. For example, the role of a cyanobacterium is as a producer in any ecosystem in which it is found.

Answers to key question, p. 140 a is Banksia integrifolia — 1b, 2a, 3b, 4a b is Banksia marginata — 1b, 2a, 3a

3.4 • Scientists classify organisms in order to better describe and study them. Classification systems are internationally recognised. This aids communication between scientists. • Different selection criteria are used in different classification systems. There are advantages and disadvantages with each system. Structural characteristics are still the most useful for sorting organisms into groups.

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• Levels of organisation in a hierarchical system help to show relationships between groups. The levels are kingdom, phylum, class, order, family, genus and species. • New discoveries and changes in technology, for example in molecular biology, are causing continual development and revision of classification systems. • The bionomial (two-name) system is used for naming organisms. A genus name and a species name are given to each organism that are unique identifiers for that type of organism. The use of keys makes the identification of organisms easier. • Extinct organisms found only as fossils may be difficult to classify, particularly if they are incomplete or their structural details are not clear. • Classifying organisms helps us to understand present and past life on Earth.

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EEXXAAM M-S- STTYYLLEEQQUUEESSTTIO IONNSS Multiple choice 1 Which statement best describes the conditions on Earth when the first life-forms are believed to have appeared? A abundant free oxygen and carbon dioxide as well as water vapour, hydrogen, hydrogen cyanide; violent electrical storms and volcanic activity; a well-developed ozone layer B abundant carbon dioxide as well as water vapour, hydrogen, hydrogen cyanide, but no free oxygen; violent electrical storms and volcanic activity; a well-developed ozone layer C abundant carbon dioxide as well as water vapour, hydrogen, hydrogen cyanide, but no free oxygen; violent electrical storms and volcanic activity; no ozone layer D abundant water vapour, hydrogen, hydrogen cyanide, but no free oxygen or carbon dioxide; violent electrical storms and volcanic activity; no ozone layer 2 Where does the evidence come from that some of the fundamental chemicals of life originated from elsewhere in the Universe rather than on Earth? A telescopic examination of space matter B meteorite analysis that indicates the presence of amino acids C the belief that bacterial spores drifting across space ‘seeded’ the Earth D none of the above 3 What is studied in palaeontology? A fossilised plants B life-forms that existed during the Palaeozoic era C fossil-bearing rocks D fossils 4 When does the fossilisation of an organism occur? A When the remains do not decay. B When the remains are buried quickly. C When the remains are undisturbed. D all of the above 5 Which of the following represents the correct sequence of major stages in the evolution of life on Earth? A organic molecule formation, membrane formation, eucaryotic cells, procaryotic cells, colonial cells, multicellular organisms B organic molecule formation, membrane formation, procaryotic cells, eucaryotic cells, colonial cells, multicellular organisms C membrane formation, organic molecule formation, procaryotic cells, eucaryotic cells, colonial cells, multicellular organisms

D multicellular organisms, colonial cells, eucaryotic cells, procaryotic cells, membrane formation, organic molecule formation 6 Which statement best describes procaryotic organisms? A They are believed to be the first forms of life on Earth. B They are strictly independent, unicellular organisms. C They lack a nuclear membrane and other membrane-bound organelles. D all of the above 7 Which statement best describes the role of procaryotic organisms? A They play an important role in recycling of materials within ecosystems. B They are the main producer organisms in ecosystems. C They are the only decomposer organisms in ecosystems. D They are usually parasitic on other organisms. 8 Which characteristic is most useful when classifying organisms? A colour B shape C structure D size 9 When are classification systems least useful? A When we are sorting out the diversity of living things. B When we only have structures, not functions, of living things to study. C When we are studying the relationships between organisms. D When we wish to find out the age of a fossil. 10 Classification systems are revised from time to time when new information illuminates our understanding of the relationships between organisms. Advances in technology have led to changes in classification groupings of organisms. Which of the following does NOT represent a technological advance that has affected classification systems? A comparison of anatomical features B comparisons of DNA sequences C the development of the light microscope D the development of the electron microscope

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Short answers 1

One scientific theory relating to the evolution of the basic chemicals of life suggests that the chemicals for life originated from outer space. A second theory suggests that they were formed on Earth. a List three organic compounds necessary for life. b Outline one piece of evidence that supports each of the two theories.

2

5

Locality one

E B F C

G

a Write down the letters corresponding to each fossil type in order from most recent to oldest. b Radiometric carbon dating is used to date fossils that contain carbon. Fossil F has been assessed as being about 2.5 million years old using radiometric dating techniques. Why can fossils be dated using radioactive carbon?

A possible classification scheme would be to place all single-celled organisms, such as the ones shown below, in a group together. Paramecium a protozoan

Locality two D

A

a Explain why soft-bodied organisms are rarely fossilised compared to organisms that have some hard parts, such as shell or bone. b Outline three conditions needed for fossilisation to occur.

3

Study the rock strata shown, which represent fossils discovered at two different locations.

6

Consider the following tree diagram of procaryotes. METHANOGENS

yeast

E. coli rod-shaped bacteria

ARCHAEA PROCARYOTES

1a 1b 2a 2b 3a 3b 4a 4b 5a 5b

146

In the 1920s Alexander Oparin and John Haldane hypothesised that the Earth’s early atmosphere contained all of the basic chemicals of life and that more complex organic molecules were produced in conditions of ultraviolet radiation or lightning discharge. a Describe the experiment conducted by Harold Urey and Stanley Miller in the 1950s that followed up Oparin and Haldane’s hypothesis. b What were the results of the experiment conducted by Urey and Miller? c Outline the significance of their results.

GREEN BACTERIA PURPLE BACTERIA

For each of the three organisms, name a characteristic it possesses which it does not share with the other organism. 4

HALOPHILES THERMOPHILES

EUBACTERIA

SPIROCHAETES GRAM-POSITIVE BACTERIA CYANOBACTERIA

a Describe the common features shared by archaea and eubacteria. b How are the archaea and eubacteria different from eucaryotic organisms? c Describe the environment of a named procaryotic organism. d Outline the role in its ecosystem of the procaryote you named in part (c). 7

Use the partial key provided below to classify the winged insect shown. Write down each of the steps you have taken in reaching your decision.

Adults with wings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 Adults without wings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24 All wings membranous, and usually transparent or wings with (powdery) scales or hairs covering them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 One or both pairs of wings partly or wholly hardened or parchment-like and opaque (not transparent) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 Forewings forming a pair of hard covers meeting in a straight line down the centre of the back. Mouth parts of biting type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4 Forewings overlapping, not meeting in a straight line down the back. Hindwings membranous and often fan-like . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Forewings very short, covering less than half the abdomen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 Forewings covering all or most of the abdomen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Order Coleoptera Abdomen with two hardened pincer-like or forceps-like extensions behind. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Order Dermaptera Abdomen without a pair of pincer-like or forceps-like extensions behind (but often with a pair of short segmented processes) . . . . . . . . . . . . . . . . . . . .some Order Coleoptera

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Chapter 4

EVOLUTION OF AUSTRALIAN BIOTA

Australian ecosystems are unique. They contain animals and plants unlike any others in the world. Why is this so? Can we find evidence to help explain modern Australia? About 160 million years ago, in the Jurassic period, all continents on Earth consisted of one immense landmass—Pangaea. This split into two large continents, Laurasia to the north and Gondwana to the south. About 80 million years ago Gondwana began to break apart, with Australia eventually separating from South America and Antarctica. Australia was then isolated for the next 30 million years, and during this time fauna and flora unique to Australia evolved. Throughout this period of isolation, a number of global climatic changes affected Australian ecosystems. The effects are reflected in the fossil record, which indicates slow but continual changes in living things. Species have come and gone, including megafauna such as giant kangaroos and wombats, ferocious marsupial lions and giant pythons. Many modern-day organisms seem just as strange to us when we first encounter them. Australia’s changing environment has made it the driest inhabited continent on Earth. As Australia became warmer and drier, both plants and animals evolved with adaptations that enabled them to survive this increasing aridity and extremes of temperature variation. Knowledge gained from the study of past environments helps us to better understand present-day ecosystems and allows us to more accurately predict the effects of our actions on the environment.

This chapter increases students’ understanding of the applications and uses of biology, the implications for society and the environment, and current issues, research and developments in biology.

2.1 4 2

Gondwana: Cell formation ancient supercontinent OBJECTIVES When you have completed this section you should be able to: • outline the theory of plate tectonics • identify and describe geological and biological evidence supporting the theory that Australia was once part of the supercontinent called Gondwana • describe evidence that supports the idea that present-day organisms developed from ancestral species that existed in the past • identify factors that have contributed to the evolution of Australia’s endemic plants and animals • discuss research that indicates evolutionary relationships between extinct and present-day species • outline reasons that account for the disappearance of Australia’s megafauna.

The formation of Australia activities ● ●

A moving Australian continent Changing ideas

Gondwana comprised the presentday landmasses of South America, Africa, Madagascar, India, Antarctica, Australia and New Zealand.

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Australia became a separate continent about the 45 million years ago. There is evidence that it was originally part of an ancient supercontinent, Pangaea. About 160 million years ago, in the Jurassic period, Pangaea split into two large continents, Laurasia to the north and Gondwana to the south (Figure 4.1). Laurasia consisted of today’s North America, Europe and Asia (except India); Gondwana comprised the current landmasses of South America, Africa, Madagascar, India, Antarctica, Australia and New Zealand. By the end of the Jurassic period, crustal plate movement had separated Africa and South America, and India was drifting northwards. During the Cretaceous period, Madagascar split from Africa and New Zealand split from Australia. About 60 million years ago, during the Palaeogene period, South America separated from Antarctica and Australia. Australia drifted northwards, and about 55 million years ago it began to break away from Antarctica (Figure 4.2). This separation was not complete until about 45 million years ago. Australia is still drifting north at the rate of about 6 cm each year.

˚N

30

FIGURE 4.1 Geological evidence has enabled scientists to work out how the present-day continents might have fitted together. This shows how Gondwana might have looked in the early Cretaceous, about 140 million years ago. The dotted lines indicate the possible continental boundary.

60˚S

30˚S



former coastline today's coastline

65 million years ago

45 million years ago

40˚

40˚

60˚

60˚

polar easterly winds 80˚

(a)

80˚

limited ice development

South Pole

South Pole

(b)

30 million years ago

20˚

7 million years ago 20˚

40˚ 40˚

60˚

60˚ first evidence of sea-level ice; extent of ice cap not known 80˚

80˚

(c)

South Pole

(d)

South Pole

FIGURE 4.2 (a) 65 million years ago, Australia was still connected to Antarctica. (b) 45 million years ago the two continents were separating. (c) 30 million years ago the separation was complete and a large icecap had developed over Antarctica. (d) 7 million years ago the ice-cap was at about its present size.

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Plate tectonics The development of the theories of continental drift in the early 1900s and plate tectonics some four decades ago gave scientists, for the first time, a sensible way to explain and understand the creation of mountains, volcanoes and oceans. The theories enabled them to explain how mineral and fossil fuel resources originated and where such fuels may be found, and, in recent years, to explain the evolution of different groups of organisms. The idea of continental drift was first proposed in 1912 by a German astronomer, Alfred Wegener. He theorised that the continents as they exist today were once all attached in a single landmass. He called this concept Pangaea. This landmass was originally surrounded by a global ocean, but then split up into smaller landmasses which drifted to separate places around the Earth.

Eurasian plate

The theory of plate tectonics, first developed by geologist Harry Hess in the 1960s, holds that continents and oceans are carried on the large crustal plates of the Earth’s surface which move on top of the Earth’s semi-molten interior. This movement of pushing parts of the Earth’s crust together or pulling them apart creates a ridge marking the edge of two separate plates. As a result, mountain ranges and oceans are created, and devastating earthquakes occur. The plates move only a few centimetres every year, but when measured over tens of millions of years, great distances are covered and great forces are generated. Scientists believe that the Indian plate’s push northward into Asia (at a rate of 19 centimetres a year) uplifted the Himalayas.

North American plate Pacific plate

African plate

Indo-Australian plate

Nazca plate

South American plate

FIGURE 4.3 The Earth’s continental plates. Australia lies on the Indo–Australian plate.

Antarctic plate

Southern connections BIOFACT The name Gondwana means ‘forest of the Gonds’. The Gonds were people who lived in the region of India where the first Glossopteris fossil was discovered.

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How do we know that Australia was once part of Gondwana? The answer comes mostly from our knowledge of geology and biology.

Geological evidence Geologists studying the rock strata around the continental margins have found that they match perfectly in many places, such as between southern Australia and one section of Antarctica, and between the west coast of Africa and the east coast of South America.

Deep-sea surveys have discovered mid-ocean ridges where the plates are moving apart, allowing mantle material to well up and create new ocean floor. The surveys found that the rock that forms the ocean floor is increasingly older the farther it is from these ridges, indicating that the plates have been moving apart steadily for a long time. Areas of new ocean floor forming between continental plates are called ‘spreading zones’. All this evidence supports the theory that Australia was once connected to other southern continents as part of the supercontinent of Gondwana.

Biological evidence The fossil record and the present-day distribution of plants and animals provide biological evidence that Australia was once part of Gondwana.

Fossil evidence Glossopteris and Gangamopteris are fossil plants found in rocks of the same age in Africa, Australia, India, South America, Antarctica and New Zealand. Alfred Wegener used Glossopteris as one piece of evidence to support his theory of continental drift.

Marsupial mammals are widespread in Australia. They also have a long fossil record on the continent. Only one marsupial, the opossum, is still found in North and South America, but the fossil record shows marsupials were present on all the continents that we believe made up Gondwana. Fossil plants and animals found in Antarctica, including marsupials, are the same as those found in Australia in rocks of the same age.

FIGURE 4.4 A fossil plant called Glossopteris is found in rocks of the same age in India, South America, Antarctica, Australia and New Zealand. This fossil specimen is from New South Wales and is about 200 million years old.

Fossil plants and animals found in Antarctica, including marsupials, are the same as those found in Australia in rocks of the same age.

Extant organisms Nothofagus, the southern beech trees, are found today in forests in Australia, New Guinea, New Zealand and South America (Figure 4.5). Fossil specimens are also found in Antarctica. Many plants and animals are found only in the isolated areas where Nothofagus is still found, and nowhere else; for example, a parasitic fungus, a liverwort, and small invertebrates which depend on the liverwort (Figure 4.5). Many groups of animals have close relatives in South America, Africa, India and New Zealand, but not in northern Asia, Europe or North America. They include parrots, the ratites or flightless birds

‘Extant’ means still in existence.

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FIGURE 4.5 The Nothofagus rainforests in south-eastern Australia and New Zealand are considered to be relicts of ancient Gondwanan rainforests. They include many species not found in any other habitat, such as the liverwort Gackstroemia weindorferi (left).

cassowary Africa South America

New Guinea kiwis

rhea emu ostrich

Australia New Zealand

FIGURE 4.6 The flightless birds called ratites occur only in the southern hemisphere. Comparisons of their DNA have given us an idea of their family tree. The Australian emu is most closely related to the cassowary of north-eastern Australia and New Guinea. The New Zealand kiwis are ‘second cousins’. The rhea from South America and the ostrich from Africa are more distant relatives—‘third cousins’.

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(Figure 4.6), marsupial mammals, chelid turtles, some geckoes, many families of earthworms, terrestrial molluscs, spiders and insects, and the scorpion genus Cercophonius. These biological examples, when considered together with the geological evidence, help to support the theory that landmasses in the southern hemisphere, including Australia, were once connected.A large proportion of Australian animals and plants are unique to Australia, but their ancestry can be traced back to extinct Gondwanan species. The fossil history of Australia shows slow but continual changes in living things. Since Australia became a separate continent, many of the plants and animals that were present then have continued to evolve and spread out to occupy a wide variety of habitats. Current research continues to provide further evidence about the evolutionary relationships between Australia’s fossil record and present-day species (see Section 4.2). The long isolation of the Australian continent has produced a large number of plants and animals which are endemic to Australia, and has resulted in species richness. Endemic species are those which are unique to a region. In Australia, 85% of flowering plants, 82% of land mammals, 89% of reptile species and 93% of frog species are endemic. Species richness refers to the high number of species in a particular area. For example, Queensland rainforests and the Great Barrier Reef show species richness. Species richness in the Great Barrier Reef includes: 4000 species of molluscs, 1500 species of fish, 400 species of corals, 215 species of birds, 23 species of mammals, and 6 species of turtles.

FIGURE 4.7 Reef and fringing rainforest at Cape Tribulation, North Queensland.

Australia’s megafauna Megafauna are large animals. Today there are only a few types found on Earth, such as elephants and whales, but in the past they were more common. Over the last 50 000 years most of the world’s megafauna have become extinct. Two main theories have been put forward to explain this. The first is climate change. Many extinctions occurred at about the end of the last Ice Age, in the Pleistocene (see Table 3.2, p. 120). As the weather warmed and conditions became drier, ecosystems changed and habitats were gained and lost. In Australia, the climate changed from cold and dry to warm and dry, and water became scarce. The second theory is human expansion. The megafauna were big and slow and therefore vulnerable to hunting—in particular to the arrival of skilled hunters. It is thought that the extinction of many of Australia’s megafauna occurred at about the time that humans arrived on the continent for the first time. It is likely that the disappearance of most of Australia’s megafauna was a consequence of both factors: initially climatic change but later also hunting by humans. Today, many smaller relatives of Australia’s megafauna survive. The fossil record, particularly for marsupials, shows a decline in size since the late Pleistocene. Both the eastern grey kangaroo (Macropus giganteus) and the red kangaroo (Macropus rufus) can be considered megafauna survivors, but they are up to 30% smaller than they were thousands of years ago.

The long isolation of the Australian continent has produced a large number of plants and animals which are endemic to Australia, and has resulted in species richness.

BIOFACT Australia’s extinct megafauna include: • marsupial lion • diprotodon (a large wombat-like marsupial) • giant kangaroos • giant echidna • giant Tasmanian devil • giant Australian python • giant goanna • giant Australian megapode (a bird similar to the malleefowl) • Genyornis (a giant flightless bird).

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short-faced giant kangaroo Procoptodon pusio

Diprotodon optatum

giant wombat Phascolonus gigas

FIGURE 4.8 Some examples of Australia’s extinct megafauna, with a human for size comparison.

common wombat Vombatus ursinus

Living fossils—extant species The evolution of most living organisms can be traced to common ancestors, through their distribution and their fossil record. But some present-day organisms have survived apparently unchanged since ancient times. We call these ‘extant species’ or ‘living fossils’. Extant species are usually found in areas where the environmental conditions have not changed in millions of years. Australia has many examples of these. Stromatolites, found today only in Western Australia, are the oldest living fossils known (see p. 117).

Plants Many plants are found in isolated areas today but are also known from their extensive fossil record. These include plum pines (Podocarpus), celery pines (Phyllocladus), huon pine (Lagarostrobus), kauri pine (Agathis australis), the primitive seed-bearing cycads, the Northern Territory palm Livistona mariae and southern beech trees (Nothofagus) (see p. 152). Another species, the Wollemi pine (Wollemia nobilis), was discovered in 1994 in an isolated rainforest gorge in the Wollemi National Park, 150 kilometres north-west of Sydney. It belongs to a group of plants known previously only from fossils dating back 150 million years.

FIGURE 4.9 The Wollemi pine was discovered by a park ranger in 1994 in an isolated gorge of the Wollemi National Park. It belongs to a group of plants known previously only from fossils.

What environmental conditions would favour the survival of ‘living fossils’ over millions of years?

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Animals Onychophorans or velvet-worms inhabit damp rainforest floors in only a few locations in the world. They have characteristics of two present-day groups of animals, the arthropods and annelids. They are known from the fossil record from the Cambrian period, over 500 million years ago. The Queensland lungfish (Neoceratodus forsteri) belongs to a group of ancient bony fishes with fossils known from 400 million years ago. The present-day Queensland lungfish appears identical to fossils found in New South Wales that are 100 million years old. Two species of crocodiles occur today in Australia: the freshwater crocodile (Crocodylus johnstoni)

and the saltwater crocodile (Crocodylus porosus). Crocodiles are known as fossils from the Triassic period, 220–250 million years ago. Monotremes (the platypus and echidnas), although highly specialised in their ecological niches today, represent descendents of the earliest mammals, which laid eggs and produced milk for their young. Today Australia’s plants and animals, many of them unique and with origins dating back millions of years, are often referred to by scientists as Australia’s ‘biological ark’. They are an important heritage for all Australians and significant to anyone who studies the evolution of life on Earth.

FIGURE 4.10 The short-beaked echidna, Tachyglossus aculeatus, is one of three present-day monotreme species — descendants of the earliest mammals.

Suggest two reasons for the survival of Australia’s unique mammals.

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Questions 1

Draw up a timeline that illustrates the key events in the formation and position of the Australian continent, from its supercontinent origins to the present.

2

Describe the geographical and biological evidence that supports the theory that Australia was once connected to the supercontinent called Gondwana.

3

a Make a list of extinct Australian megafauna. b Outline the two theories that have been proposed to account for the extinction of these organisms.

4

Outline some of the similarities and differences between present-day Australian native animals and their ancient relatives.

5

a Explain what is meant by ‘extant Australian species’. b Identify some plant and animal examples of living fossils that are endemic to Australia.

5

The cassowary (northern Australian and New Guinea), Australian emu, New Zealand kiwis, South American rhea and African ostrich are all flightless birds that are native to different parts of the southern hemisphere. DNA testing suggests a genetic relationship between the different species. a What conclusions can be drawn from this information? b Use the information accompanying Figure 4.6 to draw an evolutionary tree for these flightless birds.

6

Explain how Australia’s isolation from other land masses can be used to account for the large number of endemic species of plants and animals.

7

What does the existence of living fossils suggest about the environment in which they live?

8

Research the work of Tim Flannery and/or Michael Archer in relation to Australia’s fossil history.

F u r ther questions 1

a What do we mean when we say that Australia is an ‘island continent’? b Discuss the significance of this idea in terms of the evolution of Australian flora and fauna.

2

a Use a current topographical map of the world to mark the positions of mid-ocean ridges and spreading zones. b Explain how mid-ocean ridges and spreading zones are formed. c Outline the relationship between these two different features.

3

a Based on our current understanding of the movement of tectonic plates, outline the predicted future positions of global topographical features, including the continent of Australia. b Suggest the changes that are likely to occur to Australia’s climate as a result of its changing position. c What impacts might this have on Australia’s flora and fauna?

4

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Find out why Australia does not experience many severe earthquakes.

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4.2

Changes in Australian flora and fauna OBJECTIVES When you have completed this section you should be able to: ● recall that variation exists within a species and be able to give examples of variation ● explain how variation can affect the chances of survival of species during environmental change ● describe evidence that indicates change in the Australian environment over millions of years ● explain how changes in vegetation, such as the contraction of rainforests and the increase in grasslands and sclerophyll forests, have affected animal and other plant species ● identify regions of the Australian continent that are subject to significant changes in water availability and/or temperature ● outline changes in the distribution and abundance of Australian organisms as suggested by the fossil record ● discuss theories that attempt to explain observed changes in distribution and abundance of species over time ● explain how Darwin interpreted his observation of Australian flora and fauna in terms of his theory of evolution.

Evolution and variation in species It was the observation of variations between species and individuals by scientists in the 19th century which led to the proposal by Darwin and Wallace called the theory of natural selection. This theory explains how changes over time, or evolution, can occur in living things.

activities ● ● ●

The theory of natural selection



A timeline for Australia Australian fossils The great evolution debate Variation in a species

The theory of natural selection proposes that it is the environment that selects favourable variations and eliminates harmful ones. After many generations of selection, the characteristics of a population may change and the population becomes adapted to the environment.

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The theory of natural selection explains how changes over time, or evolution, can occur in living things.

BIOFACT Alfred Wallace (1823–1913) came to the same conclusions about evolution that Charles Darwin had reached. Their first articles on this subject were published in England in 1858, in the same issue of the Journal of the Linnaean Society.

Differences between organisms belonging to the same species are called variations.

Darwin’s theory of evolution by natural selection has four main points: 1 In any population there are variations; all the members of one species are not identical. 2 In any generation there are offspring that do not reach maturity and reproduce; the characteristics of these organisms are removed from the population. 3 Those organisms that survive and reproduce are well adapted to that environment; they have favourable variations (survival of the fittest). 4 Favourable variations are passed on to offspring; they become more and more common in the population. The main contributions by Darwin to the theory of evolution were (a) the idea that species can change over time, and (b) the mechanism of natural selection to explain how change takes place. The native plants and animals present in Australia today are largely the result of changes over millions of years. But how did these changes come about?

Variation within a species

race lunatus

races whitlocki and chloropsis (a)

(b)

When we look at individuals belonging to the same species—just think of yourself and other people you know—we can see differences. These small differences between organisms belonging to the same species are called variations. They include differences in features such as size and colour of various body parts. Today we can also identify biochemical differences between individuals. These differences become important in evolutionary terms when environmental change occurs. If there is variation in a population then there is the chance that some members will be able to survive. The white-naped honeyeater, Melithreptus lunatus, occurs throughout eastern Australia, and also in south-western Western Australia (Figure 4.11). However, populations in different areas show distinct differences in several features. Those in eastern Australia have a short bill and an orange-red eye-patch (Figure 4.11b). These belong to the race lunatus. But populations in Western Australia have larger bills

(c)

FIGURE 4.11 (a) Distribution of the white-naped honeyeater, Melithreptus lunatus. (b) The eastern form of the white-naped honeyeater (race lunatus). Note the eye-patch colour. (c) The western form (race chloropsis).

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and white or green eye-patches (Figure 4.11c). These belong to the races whitlocki and chloropsis. Because the races in Western Australia cannot interbreed with eastern populations (they are geographically isolated) and are subject to different environmental conditions, they might eventually evolve into a different species. This is probably how separate but similar-looking species have evolved; for example, the crimson rosella of eastern Australia and the Adelaide rosella of South Australia. The floral emblem of Victoria is the common heath, Epacris impressa. This plant shows a remarkable variation in the colour of the flowers, from pure white to pink to deep red (Figure 4.12). This variation is so great that for a long time the different forms were classified as different species. It is still not clear what causes the different colours to occur, although soil type might be an important factor.

FIGURE 4.12 Two colour forms of the common heath, Epacris impressa.

The island continent Australia is an island continent of 5.6 million square kilometres, surrounded by ocean. Compared with other continents it is small and flat. It is geologically stable, its rocks are old and eroded, and it has some of the Earth’s poorest soils. Australia’s climatic pattern is one of increasing dryness or aridity moving inland from the coast, and of increasing temperatures from south to north.

Australia’s climatic pattern is one of increasing dryness or aridity moving inland from the coast, and of increasing temperatures from south to north.

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Arafura Sea

Torres Strait

Indian Ocean Pacific Ocean

sum

mer rain fall er ra infall

wint

FIGURE 4.13 Australia is an island continent with a diversity of habitats, plants and animals. Much of the land is below 300 m in altitude and is arid. Only the edge of the continent has enough rainfall to enable tall forests to grow. The climate of northern Australia is tropical; southern Australia is temperate.

Southern Ocean Climate type tropical temperate intermediate arid

Australia’s changing environment When Charles Darwin looked at marsupial fossils in Australia, he did not find them very different from living species. Today we have much more evidence of change in Australia, both to the climate and to living things. When environmental change occurs and the climate becomes hotter, colder, wetter or drier, the distribution and abundance of living things changes. Those organisms with favourable variations may survive the changes, and in doing so bring about a change in species by natural selection. Australia has undergone physical and environmental changes which affect its flora and fauna, and changes are still occurring.

The changing climate When Australia and Antarctica were still joined together, about 65 million years ago, the climate was cool and wet and much of the land was covered with temperate rainforest. About 45 million years ago, as Australia completed its separation from Antarctica and began to drift northwards, the wind patterns changed (see Figure 4.2, p. 149). Warm air moving south from the tropics was blocked as air and ocean currents began to circulate around Antarctica. Antarctica became colder and the ice cap slowly grew. Australia became cooler and drier. As Australia drifted northward, however, it gradually became warmer. The area of rainforest shrank and other types of vegetation increased.

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high

Rainfall

increase in frequency of fire

low

Relative importance of major vegetation type

grassland

fire-sensitive woody sclerophyll

fire-tolerant woody sclerophyll

rainforest

22 million

16 million 2 million 50 thousand 10 thousand Years before present

FIGURE 4.14 Over the last 20 million years, the area of rainforest in Australia has decreased, while the area of sclerophyll forests and grasslands has increased. This fundamental change in vegetation is related to changes in rainfall and an increase in aridity. (From M. Archer and G. Clayton, Vertebrate Zoogeography and Evolution in Australasia.)

As Australia moved north of the Tropic of Capricorn, the climate in the northern part became tropical. There have been many climatic fluctuations since the late Neogene period, reflecting global cycles. There have been warmer, wetter periods when forests expanded, and cooler, drier periods when grasslands increased. Overall, Australia has become warmer and drier, particularly inland. The current Quaternary period has had dramatic temperature fluctuations, including many ice ages. The last 120 000 years has been a warm period during which fire has become a significant environmental feature.

The changing landscape While climate has been the major source of ecosystem change in Australia since it separated from Antarctica, there have been other events that have changed ecosystems. For example, sea levels have risen and fallen—mainland Australia and Tasmania have been joined and parted at least eight times in the past 30 million years. Erosion has continued, lowering the land surface to make Australia the flattest continent on Earth. About 35 million years ago, volcanic activity created extensive lava flows around the east coast of Australia, and about 20 million years ago the Eastern Highlands (which had been raised during the Devonian, 370 million years ago) were slightly uplifted.

The Australian environment By the 1840s the fertile coastal regions of Australia were well known to European explorers and settlers. But the dry interior had been seen by only a few white people, so that as late as the 1890s they still thought

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that fertile valleys full of woods and forests might exist amongst the ranges in the centre of Australia. The naturalist Baldwin Spencer was one of the first to recognise the truth, during the Horn Expedition of 1894: Once in the Centre and among the dry and thinly wooded plains forming the lower Steppe-lands, later on in the desert country around Lake Amadeus and, lastly, among the higher Steppe-lands in the Macdonnell Ranges, any illusions we had cherished as to spectacular discoveries, either botanical or zoological, vanished once and for all.

Schlerophyll (‘hard leaves’) refers to plants with tough, evergreen leaves such as eucalypts.

January maximum temperature

The increasing aridity as Australia drifted north had created vast inland deserts. Only a few tiny remnants of ancient vegetation survived, such as Palm Valley in the Macdonnell Ranges of the Northern Territory. As climatic conditions changed on the Australian continent, so did the Australian biota. Increasing heat and aridity favoured the plants and animals that could tolerate such conditions, and these spread out and increased in numbers. Those that could not became restricted in both distribution and abundance. Rainforests contracted and all tall forests became restricted to the edge of the continent. Sclerophyll communities and grasslands increased over time. The face of Australia changed completely, particularly in the centre of the continent. Size Australia is an island continent surrounded by ocean. The greatest width of land occurs along the Tropic of Capricorn. Compared with other continents it covers a relatively small area (5.6 million km2).

very hot hot warm temperate cool

July minimum temperature

over 16˚C 10˚ – 16˚C 5˚ – 10˚C 1˚ – 5˚C

FIGURE 4.15 Summer maximum and winter minimum temperatures in Australia.

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Temperature Australia is a hot continent. The January (summer) maximum and July (winter) minimum temperatures are shown in Figure 4.15. No place in Australia suffers intense cold. Inland, the temperature range is wide because the heat from the day is rapidly lost at night. Most of Australia has tropical or subtropical temperatures. Only in the south-east and south-west are there milder, temperate conditions. Temperature variation in Australian inland deserts and grasslands can be very great. Rapid heating in the day can raise the temperature to over 40˚C in summer, but rapid cooling at night drops the temperature quickly. In winter, daytime temperatures may be over 20˚C but at night can drop to below freezing. Rainfall Australia is the world’s driest continent. Three-quarters of Australia receives less than 800 mm annual rainfall (Figure 4.16). Inland Australia and most of the west coast have a low, unreliable rainfall; all seasons are dry. A common pattern over large areas is for droughts and erratic rainfall followed by floods. Along the east coast, rainfall occurs throughout the year. Water availability is usually a problem over large areas of inland Australia. Many bodies of water are temporary and the levels and flow of rivers and creeks are unpredictable. Floods followed by drought is a common pattern. Large dry air masses (anticyclones) are generally situated over the interior. They move south during the summer, attracting warm moist air from equatorial regions in the form of monsoons and tropical depressions. This gives the northern part of Australia its ‘wet’ season. In winter the anticyclones move north and a westerly air stream from the Southern Ocean brings winter rains to the south of the continent. Aridity Lack of rainfall combined with high temperatures means that much of inland Australia is arid. One-third of mainland Australia is desert with an annual rainfall of less than 250 mm. A further third is semi-arid with an annual rainfall between 250 and 500 mm. Overall, Australia’s climatic pattern is one of increasing dryness or aridity

mean annual rainfall

abundant average little FIGURE 4.16 The distribution of rainfall in Australia.

moving inland from the coast, and of increasing temperatures from south to north. Topography Australia is a comparatively low, flat continent. Rainfall and temperature are modified significantly only by the eastern highlands, which produce a definite rain-shadow effect along the east coast and Tasmania.

land over 600 m

FIGURE 4.17 Australia is a relatively low, flat continent; few areas are above 600 m in altitude.

forest woodland shrubland grassland desert

FIGURE 4.18 The distribution of major vegetation types in Australia.

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Ancient soils The soils are leached and minerals are impoverished over much of the continent. Most soils are deficient in phosphorus, and there is usually only a thin surface layer of nutrients. Fire Wildfire is a relatively common event in the Australian environment. Many plants have the ability to survive regular burning. It has been estimated that grasslands experience fires every 5–7 years, dry eucalypt forests every 11 years, and tall wet eucalypt forests every 40–50 years. Vegetation The pattern of increasing aridity inland from the coast is shown in the change from forest ecosystems through to scrub and grasslands. woodland desert

shrubland

1600 inland

woodland

1200

forest

800

rainforest forest

400

distance in kilometres

0 coast

FIGURE 4.19 Changes in aridity from the coast to the inland in eastern Australia are reflected in vegetation changes.

Changes in the distribution of species BIOFACT Some examples of Australian marsupials with various food preferences are: Herbivores—koalas, kangaroos and possums Carnivores—quolls Insectivores—planigales Carrion-eaters—Tasmanian devil Nectar-eaters—pygmy-possums Omnivores—bandicoots

As Australia became warmer and drier, both plants and animals evolved with adaptations to enable them to survive this increasing aridity.

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As Australia became warmer and drier, both plants and animals evolved with adaptations to enable them to survive this increasing aridity. Organisms which could survive warmer and drier conditions flourished and radiated out to colonise new ecosystems. Those that could not, either died out or became confined to areas which had suitable environmental conditions. Adaptive radiation is the change in a species from its original form to a different form adapted to different environments or ways of life. Australia’s vegetation is dominated by two groups of flowering plants which have successfully evolved to colonise a wide variety of habitats: the acacias (wattles) and eucalypts. Both groups of plants have evolved forms which can survive well in hot, arid conditions. The 950 species of acacias are found almost everywhere, from temperate and tropical rainforest to woodlands, arid and semi-arid areas, and coastal heaths and sand dunes. The 800 species of eucalypts are found mainly in forests and open woodlands. Another group of flowering plants, the Proteaceae, has diversified widely since its origins in Gondwana. Thirty-five types are endemic to Australia, including banksias, grevilleas, hakeas and waratahs.

When Australia became a separate continent, it contained three types of mammals—placentals, marsupials and monotremes. The fossil record shows us that the placental mammals, apart from bats, died out. Placental mammals did not reach Australia until Australia drifted close enough to Asia for colonisation to occur from the north. But marsupials diversified, and as the climate became warmer and drier, they spread out to occupy many ecosystems. Today there are 141 species of marsupials in Australia, usually grouped according to their food preferences. Australian frogs evolved in an interesting way as Australia became warmer and drier. They too have diversified to occupy a wide variety of habitats, including inland arid areas. Compared with amphibians in other areas of the Earth they show a lack of dependence on permanent water. Many of them (such as the water-holding frog, Cyclorana platycephala) breed in temporary pools and have a relatively brief tadpole stage of development.

FIGURE 4.20 Many acacias have lost their leaves and have flattened photosynthetic leaf stalks called phyllodes.

Theories about changes to Australian species At two places in Australia—Riversleigh in north-western Queensland and Naracoorte in South Australia—fossils have been found that help give us a clearer picture of the evolution of Australian ecosystems to warmer and drier conditions. Riversleigh demonstrates the change from rainforest to dry habitats through its fossil collection dating from the Miocene to the Pliocene period. Fossils found at Naracoorte, together with pollen data from nearby Wylie Swamp, indicate that, during the Quaternary, inland lakes dried up and vegetation changed from forest to open woodland. Riversleigh and Naracoorte are World Heritage Areas. The fossils already found there and the continuing research will make a great contribution to our knowledge of the evolution of Australian biota. The fossil evidence about changes in distribution and abundance of Australia’s species shows us that the two main factors that contribute to the changes are climate change and human impact. Climate change Increasing temperature and a decline in the availability of water are shown in the contraction of rainforests and the rise of woodland and grassland communities. The rise and fall of Australian mammals and the radiation of marsupial groups in terms of their numbers, types and their geographic location can also be shown to be related to climate.

FIGURE 4.21 Despite its fierce looks, the Tasmanian devil (Sarcophilus harrisii) feeds only on carrion.

Human impact Humans have come as invaders to Australia. The first wave of humans, the Aborigines, may have contributed to the extinction of Australia’s megafauna (see p. 153). Their use of fire as part of their hunting practices also modified the environment. The arrival of Europeans changed local ecosystems extensively, particularly through their agricultural methods and the introduction of new species. Tim Flannery, an Australian zoologist and author of the book The Future Eaters, suggests that humans have upset the balance of life in Australia so much that we threaten the land that supports us and consequently our own survival. He argues that Australia’s ecology is unique compared to the rest of the world. With our poor soils and high proportion of arid lands, the human population that can be supported must be carefully controlled.

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Charles Darwin in Australia Darwin’s work

FIGURE 4.22 Charles Darwin was only 22 years old when he left England to explore the southern hemisphere on the Beagle.

Darwin had abandoned studies in medicine and theology to follow a career as a naturalist. In 1831 he left England on a ship called the Beagle, which travelled around the world, mainly to chart the coast of South America. The Beagle was away for 5 years. During this time Darwin observed and collected examples of plants and animals in South America, the Galapagos Islands and Australia. He examined fossils as well as living specimens and studied the events of the Earth’s geological history. As he puzzled over the variety of organisms he saw, and the adaptive nature of this variety, Darwin began to question the theory of special creation he had been taught. He tried to explain the similarity between groups he saw: similarities between animals on the Galapagos Islands and those on the South American mainland; similarities between living forms and fossils; and similarities between adjacent species as he travelled south on one continent. He became convinced that new species can develop from an ancestral type. He described an evolutionary ‘tree’, with a stem that originated 3000 million years ago, and branches to represent the diversity of new species that gradually developed in different environments. Darwin visited Australia from January to March 1836. He visited New South Wales, staying in Sydney and visiting the area around Bathurst, and also visited Hobart in Tasmania, and Albany in Western Australia. In his letters and diaries, and later his books, he used examples taken from his observations of Australian flora and fauna to support his theory of evolution by means of natural selection. When staying on a sheep station at Wallerawang, Darwin observed ‘crows like our English jackdaws were not uncommon, and another bird something like the magpie’. He saw a rat-kangaroo (a potoroo) which he compared to a European rabbit both in size and behaviour. He watched platypuses playing and diving in the Cox’s River ‘like so many water rats.’ He watched a fly and an ant fall into the conical trap of an ant-lion and wrote ‘without doubt this predacious larva belongs to the same genus, but to a different species from the European one’. In his diary he recorded some of his early thoughts regarding what later developed into his ideas about evolution: Earlier in the evening I had been lying on a sunny bank & reflecting on the strange character of the Animals in this country as compared to the rest of the World. A Disbeliever in everything beyond his own reason, might exclaim, ‘Surely two distinct Creators must have been at work’; their object however has been the same & certainly in each case the end is complete.

About the ant-lion, he wrote ‘Would any two workmen ever hit on so beautiful, so simple & yet so artificial a contrivance? I cannot think so.’ Darwin had been looking at different organisms that lived in similar environments and showed similar adaptations. When discussing how animals and plants may become diversified for different ‘habits of life’ in The Origin of Species, Darwin used Australian marsupials as an example, although he saw them as in an early stage of diversification.

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Darwin on eucalypts When Darwin first ventured into the Australian bush, he noticed how different the trees were: The trees nearly all belong to one family, and mostly have their leaves placed in an upright position instead of, as in Europe, in a nearly horizontal position: the foliage is scanty, and of a peculiar pale green tint, without the gloss; hence the woods appear light and shadowless. This, although a loss of comfort to the traveller under the scorching rays of summer, is of importance to the farmer, as it allows grass to grow where it otherwise would not. The leaves are not shed periodically; and this appears to be the case in the entire southern hemisphere, namely South America, Australia, and the Cape of Good Hope. The inhabitants of this hemisphere and of the intertropical regions thus lose, perhaps, one of the most glorious (though to our eyes common) spectacles in the world—the first bursting into full foliage of the leafless tree. They may, however, say that we pay dearly for this by having the land covered with mere naked skeletons for so many months. This is too true; but our senses thus gain a keen relish for the exquisite green of the spring, which the eyes of those living within the tropics, sated during the long year with the gorgeous productions of those glowing climates, can never experience. The greater number of trees, with the exception of some of the blue-gums, do not attain a large size; but they grow tall and tolerably straight, and stand well apart. The bark of the Eucalypti falls annually, or hangs dead in long shreds, which swing about in the wind, and give to the woods a desolate and untidy appearance. I cannot imagine a more complete contrast, in every respect, than between the forests of Valdivia or Chiloe and the woods of Australia.

FIGURE 4.23 The Sydney blue gum (Eucalyptus saligna) was a familiar sight for Darwin during his visit to New South Wales in 1836. It grows up to 50 metres tall on deep soils.

Briefly explain the reason for the differences Darwin noticed.

Evolution of Australian biota 167

The bishop and the ape Darwin’s book Origin of Species created a storm of controversy when it was published in 1859. His theory affected the prevailing beliefs about the creation of humankind, because it suggested that humans were subject to the same evolutionary rules as any other species; we were no longer a special creation in the image of God. At the very heart of the controversy stood religion. What use would religion be now if its teachings over the centuries had been lies? People either believed in the Bible in its entirety, or not at all. Defeating Darwinism was made more difficult by the discovery of primitive human remains in a cave near Dusseldorf, Germany, in 1856. These remains were named ‘Neanderthal Man’ after the valley in which they were discovered. In 1858 primitive human remains were also discovered in a cave near Brixham in Devon, England. The Church was a profound critic of Darwinism. It strove to prove that the doctrine of creation was more valid than the theory of evolution. The criticism came to a head in June 1860. On the evening of 30 June, an unexpected debate took place during the meeting of the British Association for the Advancement of Science held at Oxford University. The Bishop of Oxford, Samuel Wilberforce (1805–1873), attempted to destroy Darwin’s credibility by reading a paper in which he claimed that there was nothing in the theory of evolution: ‘rockpigeons were what rock pigeons had always been’, he said. Wilberforce had been coached in his arguments by Richard Owen, a jealous and hostile opponent of Darwin.

At the meeting was Thomas Huxley (1825– 1895), an eminent biologist who described himself as the ‘gladiator-general’ of Darwinism. Huxley often spoke at public debates and often clashed with the clergy and journalists. After finishing his paper, Bishop Wilberforce turned to Huxley and asked, was it through his grandfather or his grandmother that Huxley was descended from a monkey? Huxley later described his reply in this way: He [Bishop Wilberforce] performed the operation vulgarly and I determined to punish him—partly on that account and partly because he talked pretentious nonsense . . . If, then, said I, the question is put to me ‘would I rather have a miserable ape for a grandfather, or a man highly endowed by nature and possessed of great means and influence, and yet who employs these faculties and that influence for the mere purpose of introducing ridicule into a grave scientific discussion’, I unhesitatingly affirm my preference for the ape. Whereupon there was unextinguishable laughter among the people, and they listened to the rest of my argument with the greatest of attention . . .

Huxley was of course referring to the bishop himself. The popular version of Huxley’s remarks was that Huxley had said he preferred to have an ape for a grandfather, rather than a bishop! There was much public discussion about the debate, and Wilberforce, Huxley and Darwin were each ridiculed by different editors and journalists. But it was clear that Huxley’s arguments were the most persuasive, and Darwin’s ideas soon became firmly established in modern scientific thought.

FIGURE 4.24 The two protagonists: Thomas Huxley (left) and Bishop Samuel Wilberforce.

Why did The Origin of Species create such controversy when it was published?

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Questions 1

Use examples of an Australian animal and plant to illustrate that variation exists within populations of a species.

2

a Describe the changes that have occurred in the Australian climate over the last 65 million years. Explain why these climatic changes have occurred. b Assess the impact of these climatic changes on plant communities, in terms of both distribution and vegetation type.

3

4

Outline how Charles Darwin interpreted his observations of Australian flora and fauna in terms of his theory of evolution. Use specific examples to clarify your answer.

5

a Describe Australian environmental conditions, identifying those areas that experience wide fluctuations in temperature and water availability. b Relate the distribution of vegetation to temperature and rainfall patterns.

Describe the evidence from Riversleigh in Queensland and Naracoorte in South Australia that demonstrate change in Australian ecosystems.

F u r ther questions 1

2

3

a Define the term ‘species’. b Populations of the white-naped honeyeater, Melithreptus lunatus, occur throughout eastern Australia and in south-west Western Australia. i Describe differences between populations that inhabit different areas. ii Suggest reasons for the variations observed between the different populations. iii Explain the difference between a race and a species. iv Could the different races eventually become separate species? Explain. Variation exists within a population. Make a list of 10 characteristics that show variation within human populations. The domestic dog belongs to the species Canis familiaris. The great dane, golden retriever, cocker spaniel, fox terrier and chihuahua are different breeds within this species. a List some differences between these breeds. b List some differences within one of these breeds.

c Considering the differences in size between the largest and smallest of these breeds, explain how it is that they are all considered to belong to the same species. 4

Investigate the evolution of a selected Australian plant or animal. You can choose from the list provided, or choose another example in consultation with your teacher. thylacine, Tasmanian devil, wombat, dingo, kangaroo, echidna cycad, Nothofagus (southern beech), Huon pine, Wollemi pine Use the following points as a guide to assist you in your investigation. a Describe the features of the organism you have chosen. What kind of organism is it? b Outline any fossil evidence that suggests an evolutionary pathway for the organism. Identify the similarities and differences between current and extinct forms.

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4.3

The continuation of species OBJECTIVES When you have completed this section you should be able to: ● summarise the differences between the processes of mitosis and meiosis ● compare internal and external fertilisation and discuss their importance in terms of terrestrial and aquatic environments ● describe examples of pollination, seed dispersal and asexual reproduction in Australian plant species ● identify and discuss examples of reproductive strategies in Australian animals that enhance reproductive success and offspring survival ● relate reproductive adaptations evident in native species to their chances of survival in the Australian environment ● use Australian examples to discuss the advantages of asexual reproduction in particular conditions.

activities ● ● ●

Mitosis and meiosis compared Fertilisation in water and on land Pollination

A species continues because, during the life of an individual, offspring are produced by the process of reproduction.

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All living things must eventually die. A species continues because, during the life of an individual, offspring are produced by the process of reproduction. The same applies to other species of organisms. Reproduction may be sexual or asexual.

Meiosis Meiosis is a type of cell division that forms cells with half the number of chromosomes normally found in cells of the species. It usually occurs in the tissues of multicellular organisms that provide the cells involved in sexual reproduction. In sexual reproduction an organism produces special sex cells or gametes in the reproductive organs. Two types of gametes are produced, male and female. One male gamete and one female gamete, usually from two different individuals, the parents, come together and fuse in a process known as fertilisation. This results in a cell known as a zygote. The single-celled zygote then divides by mitosis to form the new individual. The new organism, although similar to its parents, will not be identical—it will show variation. The gametes produced by meiosis contain only half the normal number of chromosomes. If they did not, on fertilisation, the chromosome number would be doubled in the new generation. Most human cells normally contain 46 chromosomes (Figure 4.25). Forty-six is known as our diploid number. We use the symbol 2n for the diploid number. Our sex cells or gametes contain 23 chromosomes. Twenty-three is our haploid number. We use the symbol n for the haploid number. In animals meiosis occurs when sex cells are forming, resulting in haploid gametes. Like most animals, humans are unisexual: males produce haploid gametes called sperm, females produce haploid gametes called ova or eggs. Most plants are hermaphrodite: one individual produces both

Meiosis is a type of cell division that forms cells with half the number of chromosomes normally found in cells of the species.

n= 23

n= 23



2n= 46

Most human beings have a diploid number of 46: we say 2n = 46. We all result from the fertilisation of two cells with n = 23.

TABLE 4.1 Some examples of chromosome numbers.

O rganism

Diploid number

Unicellular organisms Saccharomyces cerevisiae (Brewer’s yeast) Euglena gracilis

30 90

Plants Pisum sativum (garden peas) Eucalyptus species Lycopersicum solanum (tomato) Archidium stellatum (a moss) Helianthus annuus (sunflower) Arachis hypogaea (peanut) Saccarum officinarum (sugar cane)

14 22 24 26 34 40 80

Animals Drosophila melanogaster (fruit fly) Musca domestica (housefly) Antechinus rosamondi (marsupial mouse) Phascolarctos cinereus (koala) Macropus rufus (red kangaroo) Betta splendens (Siamese fighting fish) Homo sapiens (human) Pan troglodytes (chimpanzee) Ornithorhyncus anatinus (platypus) Canis familiaris (dog) Columba livia (pigeon)

8 12 14 16 20 42 46 48 52 78 80

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male and female gametes. The function of a gamete is to unite with another gamete in fertilisation to produce the first cell of a new individual. This union of two haploid cells results in a new zygote containing the diploid number of chromosomes. Further cell division during growth and development is by mitosis (see p. 94), which maintains the diploid number.

Pairing Similar chromosomes are called homologous chromosomes. Each pair of chromosomes is made up of one chromosome contributed by each parent.

Chromosomes can be matched into pairs, as shown in Figure 4.25. Similar chromosomes are called homologous chromosomes (from the Greek homologos = agreeing). Each pair of chromosomes is made up of one chromosome contributed by each parent. Each chromosome in a pair carries information about the same inherited characteristics.

(a)

FIGURE 4.25 The chromosome pairs of (a) a human female, and (b) a human male. The chromosomes are shown as double threads. The 23rd pair in each set are the sex chromosomes—XX in females and XY in males.

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(b)

The process of meiosis Random segregation During meiosis, the homologous chromosomes are divided between separate gametes. The imaginary cell shown below has one pair of chromosomes (2n = 2). One has been inherited from the individual’s mother (the maternal chromosome) and one from the father (the paternal chromosome). maternal chromosome homologous chromosomes

meiosis paternal chromosome

In this example, meiosis results in gametes with n = 1. Each gamete contains one chromosome; one gamete contains the maternal chromosome and the other contains the paternal chromosome. The cell shown below has 2n = 4; it will form gametes with n = 2 (or two chromosomes each). One chromosome comes from each pair.

The gametes contain one long chromosome and one short chromosome. Which short one goes with which long one? There is no reason to keep the chromosomes from the mother together or those from the father together, although they could end up in the same gamete. The chromosome pairs separate independently of each other; the long ones separate:

and the short ones separate:

or

Evolution of Australian biota 173

Meiosis produces haploid cells which show variation. The variation has been produced by the random segregation of the chromosome pairs.

This process is often called random segregation of chromosomes. In our cells, with 2n = 46, random segregation could produce 223 (more than 8 billion) different gametes in each person, assuming that the homologous chromosomes are different. Just one of those gametes unites with one from the other parent. It is no wonder children in the same family are usually noticeably different. Meiosis involves two cell divisions (see Figure 4.26). The first division separates the homologous chromosomes from each other; the second division separates the daughter strands or chromatids. If we begin with a cell with four chromosomes, the first division produces two cells with two double-stranded chromosomes. By the end of the second division there are four cells with two single-stranded chromosomes. You can see how complex this is in a human cell where 2n = 46. The variety of gametes is enormous. The chromosomes carry the genes, the units of heredity. Meiosis jumbles this inherited information to produce many different combinations of physical features. This is how a baby comes to have ‘her father’s nose and her mother’s eyes’.

Mitosis and meiosis compared Both mitosis and meiosis are types of cell division. In mitosis a single division results in the formation of two genetically identical diploid daughter cells. This type of cell division in multicellular organisms brings about growth and repair. It is also the basis for asexual reproduction. In meiosis, two divisions result in the formation of four haploid daughter cells, each of which are genetically different. Meiosis is the first stage in the formation of gametes in sexual reproduction in animals and flowering plants.

Fertilisation: bringing gametes together All gametes vary genetically as a result of meiosis. We have seen that fertilisation brings together two gametes which fuse to form a diploid zygote. This new individual is a unique combination of genes, different from either of its parents. Fertilisation in sexual reproduction increases variation in a species because (a) it is sheer chance that determines which gametes will be involved in fertilisation, and (b) the chance of the same type of egg and the same type of sperm again being produced and uniting is remote. In sexual reproduction the haploid gametes produced by the parents must be brought together for fertilisation. Fertilisation—the bringing together of haploid gamates—is not as easy a task as it might appear. Firstly, careful timing is required: both male and female gametes need to be produced and ready at the same time. Secondly, arrangements must be made to bring the gametes into contact with each other; and thirdly, water is required. Male gametes must move and swim towards the female gamete. How is all this achieved? Firstly, an organism must be sexually mature. Mice can breed after they are 6 weeks old, and fruit flies just hours after emerging from the pupal stage, but humans are not sexually mature until they are about 13 174

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nuclear membrane

centromere

singlestranded chromosome centriole cytoplasm cell membrane First meiotic division a. The diploid number of chromosomes appear.

bivalent

chromatids paired chromosomes

b. Homologous chromosomes pair with each other, shorten and thicken.

c. Replication has occurred and the chromatids become visible.

spindle fibre

chiasma

centriole

d. Homologous chromosomes move apart.

e. A spindle forms and homologous chromosomes move to opposite ends.

f. Homologous chromosomes separated but not enclosed in nuclear membranes.

Second meiotic division g. Spindles form at right angles to the first one and the chromatids separate.

h. Four nuclei appear, each enclosing the haploid number of chromosomes.

i. Cytoplasm divides to form four gametes.

FIGURE 4.26 The stages in meiosis.

Evolution of Australian biota 175

years old. Many plants grow, reproduce and die in a single season. Citrus trees take about 4 years before they first flower and fruit, and macadamia trees take 7 years before they first produce nuts. Once sexual maturity is reached, gamete production and release are often stimulated by environmental factors; for example, seasonal changes such as day length and temperature.

External and internal fertilisation External fertilisation External fertlisation is a characteristic of most aquatic organisms. Gametes are shed directly into the water, fertilisation occurs, and the fertilised eggs develop into adults.

External fertilisation takes place outside the body. It is a characteristic of most aquatic organisms. Gametes are shed directly into the water, fertilisation occurs, and the fertilised eggs develop, according to that organism’s life cycle, into adults. Many millions of gametes are usually released to ensure that some will successfully meet and fertilisation will occur. While there is no control over the gametes meeting, their chances of doing so are increased by • cyclical reproductive behaviours • synchronised timing of gamete production and release • the development of courtship and mating behaviours in animals. This form of fertilisation is successful in an aquatic environment, enabling the gametes and young produced after fertilisation to spread out and colonise large bodies of water.

Internal fertilisation Internal fertilisation is a characteristic of most land organisms. After fertilisation, further development of the new organism requires water.

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Internal fertilisation occurs inside the body of the female in animals or in the female part of the plant in sexually reproducing plants. It is a characteristic of most land organisms. After fertilisation, further development of the new organism requires water. Direct transfer of gametes greatly increases the chance of successful fertilisation. Males still produce large numbers of sperm but females may produce far fewer eggs. The reproductive strategies involved include bringing the sexes together with courtship and mating behaviours and having a method of gamete transfer. Colonisation of terrestrial ecosystems has only been possible by overcoming the need for water in fertilisation. Organisms on land have developed many mechanisms to ensure successful transfer of the male gametes to the female. The female provides a moist environment for the sperm to swim to the egg. Without the need for external water for fertilisation, even the driest environments could be successfully colonised.

Sexual reproduction in flowering plants Flowers are the reproductive organs of angiosperm plants. Different parts of flowers have different functions as part of the reproductive process. The number, shape, size and colour of the different parts depend on the type of plant and are often used in plant identification and classification (see p. 133). Flowers are protected when in bud by sepals. In many flowers these are petal-like, but sometimes they are fused together into a different structure (Figure 4.27a). The petals surround the male and female reproductive organs and may be brightly coloured. Petals can be separate (Figure 4.27b) or fused into a tube (Figure 4.27c).

(a)

(b)

‘Flora’ refers to plants.

FIGURE 4.27 (a) Fused sepals form the cap that protects maturing eucalypt flowers. (b) A typical flower, showing the unfolded petals. (c) In some plants, such as this native correa, the petals are fused together to form a tube or corolla.

(c)

Male reproductive organs The male reproductive organ is called a stamen. It consists of two parts: the anther and the filament. There are usually several stamens in a flower. Some plants produce nectar from nectaries at the base of the filaments. Meiosis occurs inside the anthers and results in the formation of haploid pollen grains. A pollen grain has an outer protective wall and contains two haploid nuclei (Figures 4.29–4.31).

FIGURE 4.28 Stamens in a tea-tree flower.

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pollen sac

pollen grains

anther

FIGURE 4.29 Pollen grains form within pollen sacs in the anthers. When the pollen matures, the sacs split open, exposing the pollen for dispersal.

anther

pollen sacs split exposing pollen

filament

filament

diploid nucleus

1 pollen mother cell in anther 2 first division of meiosis produces 2 haploid cells

(a)

3 second division of meiosis produces 4 haploid pollen cells

4 thick walls secreted around pollen cells thick outer wall

haploid nuclei

5 a pollen grain

(b) FIGURE 4.30 (a) Mature pollen grains (highly magnified). (b) Pollen grains from different plant species.

FIGURE 4.31 Pollen grains form by the meiotic division of mother cells in the anthers. A mature pollen grain contains two haploid nuclei, and is protected by a thick outer wall.

thin inner wall

Female reproductive organs The female reproductive organ is called a pistil. It consists of one or more carpels in the centre of the flower. Each carpel consists of stigma, style and ovary (Figure 4.33a). Inside the ovary are several ovules. Meiosis occurs inside each ovule, resulting in the formation of eight haploid cells, one of which is the ovum or egg. 178

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1

2

one ovule; meiosis occurring

5

3

2 haploid cells

6

4

4 haploid cells 3 cells die

surviving haploid cell nucleus divides by mitosis

7

8 egg

small hole or micropyle 2 nuclei

4 nuclei

8 nuclei

one nucleus becomes the ovum (egg)

FIGURE 4.32 Development of the ovum in a flowering plant.

FIGURE 4.33 (a) Embryo sac in ovule (highly magnified). (b) The female reproductive organs in a flower.

ovule stigma carpel style ovary (b)

(a)

male nuclei

Pollination and fertilisation Pollination is the transfer of pollen from a stamen to a mature stigma. If this occurs, fertilisation can take place, as follows. A germinating pollen grain sends out a tube which grows down the style towards the ovary. The two nuclei of the pollen grain travel down this tube. One becomes the tube nucleus. The other male nucleus divides into two (Figure 4.34). The pollen tube enters an ovule through a tiny hole, the micropyle (Figure 4.35). One of the male nuclei fuses with the ovum to

pollen tube

tube nucleus

FIGURE 4.34 Germination of a pollen grain.

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germinating pollen grain stigma

form the fertilised zygote. The other fuses with the two other haploid nuclei in the ovule to form a cell containing three fused nuclei (a triploid cell).

style pollen tube ovule placenta

ovary wall (future pericarp) integument (future seed coat) male nucleus pollen tube enters micropyle female nucleus micropyle

FIGURE 4.35 The process of fertilisation.

(a)

Self-pollination and cross-pollination If pollination involves pollen and stigma from the same plant it is known as self-pollination. In cross-pollination, the pollen from one plant is transferred to a stigma on another plant of the same species. Self-pollination is prevented in some plants by having the stamens and stigma ripening, or maturing, at different times. Some plants, such as most species of the Australian she-oak family (Casuarinaceae), have male and female flowers on separate plants, so that self-pollination is impossible (Figure 4.36). Some plants are normally self-pollinating. A familiar example is the garden pea, in which pollination occurs before the flower has fully opened. Some plants, such as the dandelion, selfpollinate if cross-pollination fails to occur (Figure 4.37). If fertilisation occurs after self-pollination, the gametes will have come from a single parent. If fertilisation occurs after cross-pollination,

(b)

FIGURE 4.36 Flowers of Allocasuarina humilis, a she-oak from Western Australia: (a) male, (b) female.

stigma

stigma open for cross-pollination

pollen grain stigma bends for self-pollination

FIGURE 4.37 Pollination in the dandelion.

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anther

the gametes will have come from different parents. There will be greater variation shown by the offspring of plants which cross-pollinate because their parents are different. Cross-pollination relies on external agents such as insects, wind and birds. The amount and type of pollen produced differ according to the method used (Table 4.2). In some cases flowers and their pollinators show specific adaptations (Figure 4.38).

Pollination by animals Many Australian flowers are pollinated by insects, birds and mammals. Nectar-feeding birds such as honeyeaters (Figure 4.40) and other animals such as the tiny honey-possum (Tarsipes rostratus) act as pollinators for many Australian plant species. Fruit bats pollinate many species of eucalypts. The bush rat (Rattus fuscipes) and the brown antechinus (Antechinus stuartii) are known to pollinate Banksia ericifolia.

(b)

(a)

(c)

FIGURE 4.38 (a) The grass trigger-plant (Stylidium graminifolium) has an unusual adaptation for pollination. Only a heavy insect such as a bee can bring about pollination. In (b) the flower is open, ready to receive an insect, with the stigma bent down. When an insect lands on the flower, the stigma springs up, brushing against the insect and picking up any pollen the insect is carrying. In (c) the flower has been triggered, and the stigma is standing up.

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Insects such as ants, native bees and wasps are important pollinators of many Australian orchids. Orchids have a wide range of methods for attracting pollinators, including scents, colours, nectar, food production and mimicry. Scents vary from honey-sweet to rank and fetid, depending on the type of insect that is sought. Certain colours, especially bright yellows, attract bees seeking nectar. Other orchids produce nectar deep within the flower, forcing insects to brush against anthers and stigmas in order to reach their reward. Members of the genus Gastrodia (potato orchids) even produce insect food called ‘pseudopollen’ as a reward to pollinators that visit their flowers. It is possible that many orchids depend on specific insects for pollination, and that without them the plant series could not survive (Figure 4.40).

TABLE 4.2 Differences between wind-pollinated and insect-pollinated flowers.

Insect-pollinated

Wind-pollinated green bracts

sticky stigma in centre of flower

feathery stigma presents large surface area

anther firmly attached to filament

ovary

Callistemon flower

grass flower

large conspicuous flowers

prominent hanging anthers loosely attached to filament

feathery stigma exposed to trap blown pollen grains

brightly coloured stamens, sweet scent and nectar (to attract pollinators)

anther hangs outside flower exposing pollen to wind

pollen produced is heavy and sticky— attaches to insect

spikelets— groups of two or three flowers; small and inconspicuous; no scent or nectar Callistemon citrinus (crimson bottlebrush)

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grass inflorescence

FIGURE 4.40 As this western spinebill (Acanthorhynchus superciliosus) feeds on the nectar, it brushes against the stamens, collecting pollen that can be transferred to the stigma of the next flower the bird visits.

Growth and development Once pollination and fertilisation have occurred, the zygote divides to become the embryo. It is protected during its early development by being enclosed in the ovule and ovary of the parent plant. After fertilisation, changes take place in all parts of the flower. The petals wither and drop off. The ovule swells as the embryo grows to become the seed. The ovary swells and elongates to become the fruit. During the initial stage, the developing embryo receives its nourishment from the parent plant. The triploid cell formed at fertilisation also divides and grows rapidly to form a food store or endosperm. This provides the embryo with food later in its growth. In some plants, such as peas, the endosperm is completely absorbed and stored in the two cotyledons or seed leaves of the growing embryo (Figure 4.41). In other plants, such as corn (maize), the endosperm remains separate from the embryo.

FIGURE 4.39 To a male wasp, the flying duck orchid (Caleana major) looks and smells like a female wasp. In trying to mate with it, male wasps transfer pollen from other flying duck orchids they have visited.

two cotyledons (seed leaves)

}

embryo plant

FIGURE 4.41 A germinating bean seed, showing the embryonic plant with its two cotyledons (seed leaves). FIGURE 4.42 A plant embryo in its embryo sac.

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Seed dispersal Development proceeds only to a certain stage and then the seed becomes dormant. The types of fruits and seeds produced by flowering plants vary enormously. In some cases either the seeds or the fruit may become woody as in a nut, or soft and fleshy as in a berry. In all cases, protection, water and food are provided for the new offspring as it develops and for when it is separated from the parent plant. The seeds are dispersed from the parent plant by any of a number of mechanisms (Table 4.3). Whatever the strategy used, the purpose is the same: to disperse the offspring to a place where it is most likely to germinate and survive.

Refer to your study of a local ecosystem and make sure you include examples of pollination and seed dispersal by the plants in your area.

TABLE 4.3 Fruit and seed dispersal mechanisms.

Method of dispersal Animals

Wi n d

after being eaten, the seed passes out in faeces

parachute

lilly pilly

Water

rolling

conestick

the seed catches on the coat of a passing animal

Self

coconut

light seeds

macadamia nut

catapulting

hovea

‘galvanised burr’

FIGURE 4.43 In arid areas of Australia, ants are known to help at least 1500 species of plants disperse their seeds. Ants carry the seeds to their nests and eat the special structures (called disseminules) on the outside of the seeds. This is an example of mutualism (see Chapter 1, p. 23)—the ant obtains food and the plant has its seeds dispersed.

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‘mountain devil’

waratah fruit and seed

Sexual reproduction in animals The typical stages of the life cycle of animals are represented in Figure 4.44. The adult animal is diploid and produces haploid gametes by meiosis. Two haploid gametes fuse in fertilisation to form a diploid zygote. By mitotic division the zygote develops to become a mature adult.

Fauna refers to animals.

diploid organism (2n) mitosis

meiosis

zygote (2n)

haploid gametes (n)

fertilisation

FIGURE 4.44 The typical life cycle of an animal.

Gamete formation in animals Gamete formation involves cell division by meiosis followed by cell differentiation to produce highly specialised cells. In males the haploid gametes are called sperm. They usually have tails and swim towards the female gamete. In females the haploid gametes are called ova or eggs. They are usually much larger than sperm cells and contain food stores for the developing embryo.

Fertilisation Sexually mature individuals need to be brought together so that the gametes are close enough for the male gametes to swim towards the female gametes for fertilisation. This process is called mating. In animals, it is triggered by an environmental stimulus. The phases of the moon, changing water levels, warmer weather, and an abundance of food are examples of external stimuli known to trigger mating behaviour. In some cases, courtship rituals and recognition displays have developed so that members of the opposite sex of the same species can recognise each other and know that the other is ready for mating. For most animals on land these mating rituals need to bring the pair into close contact so that internal fertilisation can take place (Figures 4.45 and 4.46). Most aquatic animals have external fertilisation. Aquatic invertebrates usually shed their eggs and sperm directly into the water. There is no special method of transfer of gametes, so millions of them are released to increase the chances of successful fertilisation. Synchronised timing of the production and release of the gametes may also help (Figure 4.48). For example, common coral trout (Plectropomus leopardus), which inhabit the Great Barrier Reef, congregate at certain reef sites to spawn (shed their gametes) at the time of the new moon during the breeding season from September to November. Each month at this time they form groups that stay together for about five days and release their gametes before dispersing. It is thought that this reproductive behaviour is triggered by a rise in seawater temperature in spring.

FIGURE 4.45 Some birds have elaborate courtship rituals to increase their chances of reproducing successfully. Lacking showy plumage, the male satin bowerbird decorates its nest with bright blue objects in an effort to attract a female.

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FIGURE 4.46 Courtship behaviour: Australian Brolgas (Grus rubicundus) perform an elaborate dance before mating.

FIGURE 4.47 The common coral trout, Plectropomus leopardus.

BIOFACT About 48% of all shark species and 73% of all ray species are found only in Australian waters.

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Some aquatic vertebrates such as fish show courtship and mating behaviours which bring males and females close together. Although this greatly increases the chance of successful fertilisation, many thousands of gametes are still produced and released. A few fish, such as sharks, have an increased chance of successful fertilisation because the male transfers his sperm directly into the female’s body. Male sharks have a pair of claspers which they insert into the female’s urinogenital opening (cloaca), so that the sperm pass along the claspers into the female’s reproductive tract. Aquatic mammals such as whales, seals and dolphins use internal fertilisation, in which sperm is transferred directly from the male into the female. The courtship behaviour of these animals is often spectacular. For example, humpback whales and southern right whales that breed off the coast of Australia may leap, roll and slap the water surface with their flippers. Amphibians, which live most of their life on land, need to return to an aquatic environment to breed. Because fertilisation occurs in the water, they produce large numbers of gametes and have no special method of transferring the gametes. Frogs attract their mates by calling. Every frog species has a different call. The male eastern banjo frog, also called the pobblebonk, makes a banjo-like ‘plonk’ sound. This species uses specific waterholes as breeding sites. The female lays about 4000 eggs in a floating foam nest, often attached to vegetation. The male of the endangered corroboree frog, which inhabits the alpine area of New South Wales, digs a small burrow on the edge of a pool in a sphagnum bog, where mating and egg laying occur. Internal fertilisation is a characteristic of most land organisms. The male transfers his gametes directly into the female’s body. This overcomes the need for external water. Direct transfer of gametes greatly increases the chance of successful fertilisation. Internal fertilisation in animals usually occurs in the reproductive tract of the female. Eggs released from the ovary are fertilised by sperm swimming up the reproductive tract. After fertilisation, development of the zygote will continue either internally, inside the body of the female, or externally but within a shelled egg.

FIGURE 4.48 Sea urchins release millions of eggs and sperm into the water at the same time.

Animal life cycles (a) Sea urchin—indirect development

(b) Frog—indirect development

adult

adult frog

coordinated release

settles • metamorphosis

free swimming larva

sperm • 1000s

eggs • little yolk • 1000s

fertilisation • external

• few survive

mating

metamorphosis

sperm • 1000s

eggs • moderate yolk • 100s

fertilisation tadpole larva • many survive

• external

zygote

zygote

external development • hours

external development

• days

(c) Bird—direct development

(d) Placental mammal—direct development growth

adult

• parental care

adult

baby • miniature adult • most survive

growth

mating

chick • miniature adult • most survive

eggs • much yolk • few, large

sperm • 1000s fertilisation • internal

external development • incubated • weeks

internal development • nutrition through placenta • months

sperm • 1000s

eggs • little yolk • very few

fertilisation • internal zygote

large zygote in shell

embryo inplants in uterus

FIGURE 4.49 (a) Sea urchin—indirect development; (b) frog—indirect development; (c) bird—direct development; (d) placental mammal— direct development.

Evolution of Australian biota 187

larva of rock lobster Palinurus elephas (greatly enlarged)

Survival of the embryo and young External development

trochophore larva—a stage in the development of annelids and molluscs (greatly enlarged)

After fertilisation, the zygote commences development. External development in water from fertilised egg to adult is hazardous. Most animals that have external fertilisation provide little or no parental care for the development of their offspring. Environmental conditions may be unfavourable when the young are developing, causing many to die, and many may be eaten by predators. Large numbers of offspring are needed to ensure that at least some survivors will reach adulthood. Some animals that have internal fertilisation lay their fertilised eggs in water. Others such as reptiles, birds, spiders and many insects lay shelled eggs, but mammals and a few other animals retain the fertilised eggs within their own bodies for internal development. Development of offspring may involve larval stages in which the larvae live in different habitats to those of the adults. For example, the developing young of many invertebrates become part of the plankton for a while (Figure 4.50). Developing fish eggs hatch into young known as ‘fry’. They too form part of the plankton. Tadpoles emerge from frogs’ eggs (Figure 4.51) and are at first herbivorous and later carnivorous. Some aquatic animals display parental care. For example, male seahorses develop a brood pouch and guard their young as they grow (Figure 4.52).

when sexually mature, frogs return to the water to mate FIGURE 4.50 The eggs of many invertebrate animals hatch into larvae that look very much like the adults. These microscopic larvae live as part of the plankton in the upper layers of the ocean.

frog feeds and grows young frog leaves water

internal gills replaced with lungs; tail shortens frog’s egg (enlarged)

hind legs, then front legs, grow

jelly coat embryo

yolk

newly hatched larva has external gills

tadpole (larva) now has internal gills

FIGURE 4.51 The typical life cycle of a frog. Mating involves amplexus, in which the female release eggs that are immediately fertilised by the male. This fertilisation is therefore external.

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BIOFACT Two fish found in the Murray River, the golden perch (Macquaria ambigua) and the silver perch (Bidyanus bidyanus), reproduce only after the river floods. The young fish are able to grow and develop at a time when there is plenty of water and food available. If flooding does not occur, the female is able to resorb any eggs which had commenced production in her body.

FIGURE 4.52 Male seahorses guard their young in a brood pouch.

Shelled eggs Land animals such as reptiles, birds, monotremes (echidnas and platypuses), spiders and many insects lay eggs surrounded by hard shells to prevent desiccation. The shell of the egg allows gaseous exchange to occur but prevents water loss. Eggs are fluid-filled and contain a food store (the yolk) for the developing young (Figure 4.53).

Land animals such as reptiles, birds, monotremes, spiders and many insects lay eggs surrounded by hard shells to prevent desiccation.

amnion allantois —inner —embryo's membrane wastes stored here

chorion —outer membrane

shell

amniotic cavity yolk—enclosed umbilical —fluid-filled; in yolk sac provides protection cord

FIGURE 4.54 Female loggerhead turtles (Caretta caretta) take great care in constructing the nest and depositing the eggs, but then desert them to return to the ocean.

FIGURE 4.53 The shelled egg of a reptile. The allantois, chorion and amnion form the membrane systems enclosing the egg.

Evolution of Australian biota 189

Parental care Once the eggs have been laid the degree of parental care of the developing young varies. Most invertebrates lay their eggs and then leave them. They are usually laid in a position where, on hatching, the larvae will find food (Figure 4.55). Reptiles may also abandon their eggs after laying, but they are laid in a specially dug nest or burrow. This provides protection and more stable environmental conditions, particularly in relation to temperature. Crocodiles show the greatest degree of parental care in reptiles. They guard the nest and assist at hatching. After hatching they may protect the young for some months. Birds show considerable parental care. They build nests and keep the eggs warm and protected during development (Figure 4.56a). On hatching, they feed and guard the chicks until they are able to support themselves (Figure 4.56b).

(a)

(a)

egg sac (b)

egg

(c) FIGURE 4.55 (a) Insect eggs are usually laid on the plant that will provide the food source for the young when they hatch. (b) Spiders often guard their eggs until the young hatch. (c) An egg sac cut open to show the eggs.

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(b) FIGURE 4.56 (a) Male and female emperor penguins share the incubation of the egg to begin with, holding it on their feet and covering it with a flap of abdominal skin. The female then returns to the water, leaving the male to guard the egg for about two months. (b) Hatchling birds are blind and featherless, and need continual parental care to ensure they survive. This includes feeding them, protecting them from predators, and cleaning wastes from the nest.

Internal development Internal development ensures the developing young have: • protection • a supply of nutrients • a constant temperature • a constant chemical environment. In all mammals except monotremes, the young develop in the female’s uterus, receiving nutrients via the placenta. The young are born alive. This type of reproduction is termed viviparity. Other animals may have internal development of their young. In some fish, some reptiles and many insects, the fertilised egg continues to develop inside the mother and the young are born alive. This type of reproduction is termed ovoviviparity (Figure 4.57).

• In placental mammals the young develop within the mother’s uterus, receiving nutrients via the placenta. • In marsupial mammals the young develop for only a short time within the mother’s uterus, then complete development in the mother’s pouch. • In monotremes the mother lays eggs in which the young develop.

Parental care After birth, mammals suckle their young, providing milk, protection and care until the young are well established. On land a lack of water can restrict the number of offspring produced, but increased parental care assists their survival.

FIGURE 4.57 Blue-tongued lizards (Tiliqua species) can produce up to 20 live young. Their eggs are unshelled and remain in the mother’s body while the embryo develops. The young are born fully formed and must immediately fend for themselves.

Reproduction in Australian animals: three case studies Monotremes In playtpuses and echidnas the production of gametes is seasonal, and both male and female reproductive organs regress (reduce in size and stop producing gametes) outside the breeding season. Little is known of the early stages of platypus reproduction—egg-laying, incubation and hatching—because, in the wild, these activties take place in an underground burrow, and few platypuses have been bred in captivity. The platypus breeds in late winter and spring. The female digs a long nesting burrow, with a nesting chamber lined with grass and leaves. She plugs the burrow with soil whenever she enters or leaves.

The platypus usually lays two oval eggs less than 20 mm long. The amount of yolk in monotreme eggs is large compared to the eggs of marsupials and placentals, but much less than the amount in reptile eggs. In contrast to reptiles, the platypus embryo develops inside the egg within the uterus for perhaps four weeks. The incubation period after laying appears to be about 10 days. The eggs are tucked under the broad flat tail, and stuck together and to the fur of the abdomen. After hatching, the young platypuses remain in the burrow for several months, obtaining nutrition by suckling from their mother’s well-developed mammary glands. The young platypuses leave the nest when they have grown fur and are about 30 cm long.

Evolution of Australian biota 191

Marsupials Kangaroos are marsupial mammals. They have internal fertilisation and a very short gestation period in the uterus. At birth, the young climb into the mother’s forward-opening pouch. They attach to a teat and continue development while suckling. When they leave the pouch there is a weaning period before parental care ends. In the red kangaroo the young weigh less than a gram when born, after 33 days in the uterus. They remain suckling in the pouch for 235 days and leave the pouch weighing 4–5 kilograms. During the weaning period, which lasts up to 4 months, the young kangaroo (joey) suckles and eats grass. Kangaroo population numbers are controlled through reproduction. Under good environmental conditions, numbers can increase rapidly, because female kangaroos can be almost continuously pregnant when adult. They can mate again directly after giving birth. If the mother is still suckling her newborn young, the fertilised egg does not develop until the young leaves the pouch. This is known as delayed implantation. At any one time a female kangaroo may have a joey being weaned, as well as a young one being suckled in the pouch and an embryo in the uterus awaiting development. Kangaroos have the amazing ability to produce two kinds of milk at the same time. The milk pro-

duced by the teat for the developing young in the pouch contains much less fat than the milk produced by the teat being used by the joey inside the pouch. When environmental conditions are not good, such as during drought, young joeys do not survive, and any fertilised egg does not implant. Females do not begin reproducing again until conditions improve.

Amphibians The gastric-brooding frog, which has been found only in southern Queensland, has a remarkable reproductive behaviour. It exhibits external fertilisation and internal development. The female releases eggs, which are fertilised during amplexus, as in other amphibians. But instead of leaving the eggs to develop alone and unprotected, the female swallows them. The larvae are incubated in the stomach, which functions like a mammalian uterus. This gastric brooding or incubation appears to last 6 or 7 weeks, during which time the mother does not eat. Digestive juices (pepsin) and acid are not released from the lining of the stomach, and stomach movements stop. Thus the mother is prevented from digesting the eggs and tadpoles. Froglets are born through the mouth of the mother, once they have completed their larval development. The southern gastric-brooding frog has not been seen for some years, and it is feared that it might have become extinct, for reasons that are not clear.

FIGURE 4.58 Older joeys continue to use the pouch for suckling and protection, but also spend time out of the pouch grazing.

FIGURE 4.59 Like other amphibians, the southern gastric-brooding frog exhibits external fertilisation, but incubates the eggs internally in the stomach, which stops digestion until the froglets are ‘born’ through the mother’s mouth.

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Outline the different mechanisms used by these three types of animals to ensure (a) the survival of the embryo, and (b) the survival of the young after birth.

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Name one reproductive adaptation for each of these three types of animals.

Asexual reproduction Asexual reproduction is reproduction by only one parent. There are no gametes produced and there is no fertilisation. The new individuals are produced in a variety of ways, depending on the species. In most cases asexual reproduction results in the production of an immediately self-sufficient new organism from one parent. In others, an abundant food supply is provided for the growth of the new organism. In suitable conditions asexual reproduction is a means by which an organism can rapidly increase its numbers. In asexual reproduction the new offspring are produced by mitosis. They therefore all contain the same genetic information as their parent. We use the word clone to refer to members of a species that are genetically identical.

Some examples of asexual reproduction Binary fission

nucleus divides (mitosis)

cytoplasm divides (cytokinesis)

daughter cells separate FIGURE 4.60 Binary fission in an animal cell.

spore capsule

spores

fungal hyphae

In all single-celled organisms, such as bacteria and protists, each time the cell undergoes cell division or mitosis, two new individuals are produced (Figure 4.60).

Spore formation Fungi reproduce asexually by forming thousands of single-celled spores. If conditions are suitable, each spore will germinate to produce a new fungus (Figure 4.61). Ferns reproduce asexually by producing spores in the spore sacs (sporangia) on the undersurface of their fronds. They are released and dispersed by the wind. If they land in a warm, shady, moist environment, they may germinate after about three months.

FIGURE 4.61 Spore formation in a fungus.

Budding This is another method of asexual reproduction. The parent, by mitosis, forms an outgrowth (a bud) which is a smaller replica of itself. Coral can be formed by budding. The new individual remains attached to the parent, so that eventually a large colony is formed (Figure 4.63).

Vegetative propagation This is the process by which many flowering plants produce new individuals from points on stems or roots called nodes. Grasses, in particular, can spread out to cover large areas in this way (Figure 4.64). Gardeners often propagate plants by asexual methods. Taking cuttings, budding, grafting and tissue culture are all techniques by which new plants or parts of a plant can be reproduced from parts of another or the same plant.

FIGURE 4.62 The sporangia of the bird’s nest fern (Asplenium australasicum), which grows in tropical and temperate Australian rainforests.

Evolution of Australian biota 193

FIGURE 4.63 Coral can be formed by budding. The new individual remains attached to the parent, so that eventually a large colony is formed.

Regeneration Regeneration is the regrowth of parts of an organism which have been removed or lost. This form of reproduction is used in many plant propagation techniques. In animals the power of regeneration varies. In some invertebrates, such as flatworms, whole new organisms can be regenerated from pieces. This is, in fact, an example of asexual reproduction, where more than one organism results from one parent. Many Australian plants have excellent powers of regeneration, particularly after fire or drought. Many trees and shrubs, such as Eucalyptus paniculata and Banksia aemula, have thick bark which enables the trunk and main branches to survive fire and produce new shoots from dormant buds. Mallee eucalypts have large, woody underground swellings called lignotubers that resprout after fire, giving these trees a many-stemmed, shrubby look. Some plants are killed by fire but reproduce from seed that is protected in capsules (e.g. eucalypts, tea-trees), woody cones (e.g. cypresspines) or fruits (e.g. banksias, hakeas).

FIGURE 4.64 Mallee eucalypts can resprout from an underground lignotuber after drought or fire. This lignotuber has been exposed by soil erosion.

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Advantages of asexual reproduction Asexual reproduction can be advantageous when environmental conditions are stable and those conditions are favourable for the organism concerned. Since the offspring produced in asexual reproduction are identical to their parents, these new organisms show no variation. There is little chance of new types evolving. If an organism is successful in its environment it does not matter if there is no variation. This lack of variation may, in fact, be useful. One type of organism can remain unchanged and successfully reproduce asexually for many generations. The growth of coral to form the Great Barrier Reef is a good example of this. We also find this an advantage in horticulture and agriculture

since it enables us to be sure of breeding plants with the characteristics we want. However, what happens to these organisms if the environment does change? Suppose, for example, a new disease or insect pest attacks members of a particular plant species. If it reproduces by asexual methods only, it is possible that none would survive. Offspring produced as a result of sexual reproduction show variation. If the environment changes, there is a chance that some of a species which show variation will still survive. It is through variation that natural selection and evolution can take place. Through natural selection many Australian species have developed specific reproductive adaptations to the Australian environment. This increases their chance of continuity and the successful survival of the species. flowering stem

growing point

scale leaves root

internode node (point of growth of new plant)

part of a rhizome

FIGURE 4.65 Couch grass, which is a common lawn grass in Australia, grows by vegetative propagation from a long above-ground rhizome.

Sphagnum in the Australian Alps The moss Sphagnum cristatum forms vast spongy mounds in the Australian Alps. It is the key species of alpine bogs, which hold huge amounts of water and gradually release it over summer, providing the alpine environment and the forested slopes below with a continuous supply of water in the dry season. Botanists have found that, in Victoria and New South Wales, this moss almost never produces sporebearing capsules; in other words, it almost never reproduces sexually. So how does it manage to colonise whole valleys? Like most plants, this sphagnum grows upright and branches as it grows. It does not need roots, because it takes the nutrients it needs directly into its cells from the water held within the mound. But the stem is very thin and weak, and the extra weight of the new growth gradually squashes the lower

parts of the stem until they die. Eventually the stem dies back to a branching point, and then the original plant becomes two plants. In this way a sphagnum mound can gradually spread outward, eventually colonising a whole valley. Sphagnum has another means of reproducing asexually. Small living fragments—even a few cells— that break off a plant can grow into new plants. Thousands of such fragments can be found floating in ponds and streams, and some will come to rest on a wet, shady bank where they can survive and grow. These methods of reproduction are a great advantage in the alpine environment. They enable the moss to colonise new areas in the short growing time available in alpine areas. They also ensure the survival of the alpine bog community during periods when spores might not have a chance to survive, such as drought.

Evolution of Australian biota 195

living plant

growth

two plants formed

compaction

peat layers (dead plants) (a)

(b)

FIGURE 4.66 (a) As the sphagnum stem grows and branches, it also squashes down and dies at the base, until it eventually becomes two plants. (b) A rare sight on the Australian mainland: Sphagnum cristatum with its distinctive spore capsules.

Describe the reproductive mechanisms used by Sphagnum cristatum that increases its chances of continuity.

Questions 1

a Define ‘meiosis’. b Meiosis is often called ‘reduction division’. Describe how the chromosome number is reduced in this process. Use diagrams to illustrate your answer.

2

Draw up a table summarising the differences between the processes and outcomes of mitosis and meiosis.

3

a Fertilisation and the subsequent development of an individual requires a watery environment. Explain how terrestrial organisms have overcome the problem of fertilisation in a dry environment. b Explain the significance of this in relation to the colonisation of terrestrial environments.

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Prepare large, labelled diagrams showing the structure of a flower. Describe the function of each structure. Distinguish between pollination and fertilisation.

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6

What is the genetic advantage of cross-pollination over self-pollination?

7

Contrast the developmental stages that occur in the reproduction of frogs, birds and placental mammals. In your answer include a discussion of • fertilisation: internal or external? • growth and development: internal or external?; direct or indirect? • how nutritional needs of developing young are met.

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For fertilisation to occur during reproduction in species, three key requirements must be met: i male and female gametes are produced and mature at the same time ii gametes must meet iii a watery environment. Choose an Australian vertebrate or invertebrate, and discuss the reproductive strategies it uses to ensure successful reproduction.

9 Choose a species from the list provided. kangaroo wombat koala possum Tasmanian devil echidna platypus Describe the mechanisms that occur in the species which allow a fertilisation to occur b the embryo to survive and develop c the offspring to survive and develop after birth/hatching.

10 a Distinguish between sexual and asexual reproduction. Include the differences between process and outcome. b Describe each of the following kinds of asexual reproduction, providing at least one example in each case. binary fission spore formation budding vegetative propagation regeneration

F u r ther questions 1

a Decide which of the following types of organisms display internal fertilisation and which show external fertilisation. frog dog bird snake fish crab spider horse jellyfish butterfly

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Draw a flow chart summarising each of the steps involved in the reproduction of a selected flowering plant, beginning with pollination and ending with the ripe fruit.

4

a Identify which of the following are fruits: gumnut peanut tomato potato strawberry avocado zucchini carrot apple walnut pumpkin rosehip passionfruit grapes

b Study the two groups that you have produced in part (a). Describe the relationship between lifestyle and the kind of fertilisation that occurs. 2

Consider the different species of flowering plants listed below. Use an encyclopaedia or field guide to Australian plants to help you group them according to their likely mode of pollination. In each case, give reasons for your choice. grevillea red-flowering gum silver wattle kangaroo paw saw banksia boronia callistemon waratah snow daisy grass-tree wonga vine mistletoe

b Describe the criteria upon which you have made your decision. 5

Consider the number of offspring that result from a single mating of the following different kinds of organisms: frogs, birds and placental mammals. Now consider the independence of the young, the level of care provided by parents and the chances of survival of the offspring. Outline the trends you notice.

Evolution of Australian biota 197

4.4

The future of Australia’s biota OBJECTIVES When you have completed this section you should be able to: ● discuss the importance of palaeobiology in understanding the impact of human activity on our environment ● describe examples of how human activity has affected the Australian environment and native species of plants and animals ● outline some ways in which analysis of the fossil record can help us to understand the factors that are important in determining the distribution of plants and animals ● explain and give examples of how an understanding of the impact of human activity in the past is important in planning conservation strategies ● explain the significance of biodiversity ● discuss current measures used to protect and monitor biodiversity.

Under standing the past activities ● ●

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The knowledge we gain from palaeontology and the study of past environments can help us to understand present-day ecosystems. We can use this knowledge to predict and (if we choose) to determine the future for Australia’s plants and animals. Palaeobiologists build up a picture of the evolution of plants and animals and their environments. They gain knowledge about the longterm changes that have occurred in ecosystems. Australia offers an ideal opportunity to understand these changes—it has been isolated from other terrestrial ecosystems for more than 30 million years and has been mostly geologically stable for more than 300 million years. Research work at Australian universities and at major fossil sites such as Riversleigh and Naracoorte, as well as the many smaller sites such as Murgon and Lightning Ridge, is helping us to appreciate how our unique biota evolved. At Riversleigh the story of 30 million years of evolution of Australia’s terrestrial ecosystems is being uncovered.

One concept that is emerging from this research is that there has been a loss of biodiversity over time. This loss correlates with the contraction of rainforest over Australia as the continent became drier (see Figure 4.15, p. 161). Over the last 200 years we know that the area covered by forest has been reduced still further because of timber harvesting and clearing for agriculture. Most of our rainforest is now found only in small, isolated pockets near the coast of eastern Australia, and much is now in national parks. Are these pockets large enough for long-term sustainability of rainforest ecosystems? Evidence from other parts of the world suggests that the areas currently protected are not big enough. Plants and small animals may survive, but the larger mammals probably will not. The history of the thylacine or Tasmanian tiger, Thylacinus cynocephalus, is an interesting case study. Thylacines were the largest carnivorous mammals in Australia. The fossil record reveals that six species existed in the past. Over the past 25 million years the number of species

declined, and the geographical area in which they were found became smaller. Two hundred years ago, thylacines were already scarce. If the European settlers in Tasmania had known this, would they have tried to conserve them? Understanding the past history of a plant or animal group can help us make decisions about its conservation value. Our knowledge of the plight of the thylacine came too late to prevent us from hunting it to extinction. Analysis of the fossils of plants, particularly pollen, and animals, together with the sediments in which they are found, helps palaeontologists to put together a picture of an ecosystem and its probable climate. Natural ecosystems and the distribution and abundance of organisms across Australia today reflect what has happened in the past. The impact of humans, and in particular their agricultural practices, is having a dramatic impact on the natural ecosystems that remain. Large areas of land are experiencing devastating degradation. The result of this could be that we are presently witnessing a biological catastrophe far greater than the extinctions of the past. Conservation for the future is an important issue. The study of palaeontology shows us what has happened in the past. It is up to us to take this knowledge and use it for the future benefit of Australia’s unique plants and animals.

BIOFACT The Australian Museum in Sydney has two research centres investigating Australian ecosystems: ● The FATE (Future of Australian Terrestrial Ecosystems) project is looking at sustainable resource management for the future. ● CREATE is the Centre of Research into the Evolution of Australian Terrestrial Ecosystems and is researching the evolution of Australian ecosystems over the past 100 million years.

FIGURE 4.67 The last known thylacine died in captivity in Hobart Zoo in 1936.

BIOFACT In New South Wales, 86 out of 130 mammal species (66%) are under threat of extinction.

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Researching Australian fossils: windows to the past Fossil sites in Australia (see map below) represent key stages in the development of Australia’s fauna. They provide links which afford scientists with a basis for documenting evolutionary change to enable them to understand how modern biota have developed.

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19

1

5 6 7 8 10 9 11 12

18

1 Geraldton 2 Broome 3 Gogo 4 Riversleigh 5 Maxwelton 6 Minmi Crossing

13 17 16

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1 Geraldton 2 Broome 3 Gogo 4 Riversleigh 5 Maxwelton 6 Minmi Crossing 7 Hughenden 8 Bluff Downs 9 Muttaburra 10 Winton 11 Roma 12 Murgon 13 Lightning Ridge 14 Strzelecki Ranges 15 Inverloch-San Remo 16 Dinosaur Cove 17 Naracoorte 18 Andamooka 19 Coober Pedy

FIGURE 4.68 Fossil sites in Australia.

Riversleigh: World Heritage fossil site The fossil sites at Riversleigh, in the southern section of Lawn Hill National Park in north-west Queensland, has yielded one of the world’s richest and continuous records of the changes in fauna, habitat and climate at any single locality in the world. There are more than 250 fossil-rich sites at Riversleigh. It has also been recognised as one of the most important fossil sites in the world and was added to the World Heritage List in 1994. Almost half of what is known about the evolution of Australian mammals in the last 30 million years comes from bones found at a single site at Riversleigh. More than 50 new species of mammals were recovered from this site alone. The fossils discovered at Riversleigh represent the following periods in Australia’s prehistory: ● late Oligocene (25 million years ago) ● early Miocene (20–15 million years ago) ● mid to late Miocene (15–10 million years ago) ● Pliocene (5 million years ago) ● Pleistocene (approximately 40 000 years ago) The Riversleigh fossils were preserved in very hard limestone. They include birds, reptiles, mammals, fish and insects. Many of the species discovered there, such as the thingodontans and wynyardiids, are unique and quite different from species living today,

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FIGURE 4.69 The dig at Riversleigh.

while others appear to have been ancestors of species living today, such as kangaroos and koalas. Thingodontans appear to have been mammals somewhere between the carnivorous marsupials and the herbivorous marsupials. Riversleigh has revealed six different species of fossil thylacines, one of which was the largest living mammalian carnivore in Australia.

The Pliocene wetlands of Bluff Downs Bluff Downs is one of the most significant fossil sites of the Pliocene period in Australia. Many animals of the Pliocene grew very large. For example, the Bluff Downs giant python (Liasis sp.) was 8 metres long. It is perhaps the largest snake that ever lived on the Australian continent. Bluff Downs was a wetland environment and the home of a rich diversity of animals. Lake and stream deposits contain the bones of ancestors of modern

species known today. For example, ● ancestral dasyurids (marsupial carnivores) ● flamingos, which are no longer found in Australia ● Koobor jimbarretti (a koala-like mammal).

Murgon: Eocene connection Murgon is located in south-east Queensland. It is the only Australian fossil site that records a diverse vertebrate fauna from the early Eocene period (55 million years ago), approximately 10 million years after the extinction of the dinosaurs. During this period, Australia was still connected to Antarctica and South America. Murgon’s significant fossil records include some of the world’s oldest fossil marsupial remains and one of the oldest placental mammal found in Australia, the condylarth.

rock platform on the Victorian coast, near the town of Inverloch. The site, called Flat Rocks, has yielded a rich collection of dinosaur fossils, but perhaps the most important find so far was very different: it was the jaw of a tiny mammal that lived alongside the dinosaurs. Christened Ausktribosphenos nyktos, this is among the oldest known mammals, living about 115 million years ago during the Cretaceous period. It has features that suggest it was a placental mammal, but until now placentals were thought to have arrived in Australia much later from South-East Asia. One theory is that terrestrial placentals, including Ausktribosphenos, became extinct in Australia— perhaps being displaced by marsupials—and that another group of terrestrial placentals arrived much later to recolonise Australia.

Lightning Ridge: ancient monotremes Lightning Ridge, in western New South Wales, is best known for the magnificent opals that have been mined there since the 19th century. But it is also known for its highly significant fossil site. The early Cretaceous Period (110 million years ago) is when the formation of the Lightning Ridge fossil deposits took place. The most significant fossils found at Lightning Ridge are monotremes—egg-laying mammals. These fossils were the ancestors of the modern platypus and echidnas of Australia and New Guinea. The Cretaceous monotremes were similar in size and shape to the modern platypus.

The Naracoorte megafauna The caves at Naracoorte in south-eastern South Australia are significant to biologists for two reasons: one is the huge colonies of bats that inhabit them, and the other is the largest and best preserved Pleistocene vertebrate fossil assemblage in Australia. In the Victoria Fossil Cave at least 93 vertebrate species have been recovered. Species range in size from giant, extinct mammals, birds and reptiles (iceage megafauna) to modern species such as the Tasmanian thylacine, wallabies, snakes, bats, turtles, lizards, mice, possums, parrots and frogs.

Ancient mammal and giant amphibians: Inverloch–San Remo In 1991, a team working with Dr Tom Rich of Museum Victoria discovered a rich fossil site on a

FIGURE 4.70 Ausktribosphenos jaw.

Another remarkable find in the area was a large amphibian called Koolasuchus cleelandi. Two jaws of this 5-metre labyrinthodont were found during the prospecting of a rock platform near the town of San Remo, not far from Inverloch. Koolasuchus belongs to a group that includes the ancestors of the salamanders. This group became extinct everywhere else in the Jurassic, and seems to have been the last of its kind anywhere on Earth: all other labyrinthodonts died out more than 140 million years earlier, in the late Permian or Jurassic.

The fossil fishes of Gogo At a cattle station called Gogo, in the Kimberley region of Western Australia, superb fossil fishes have been discovered. These fishes, trapped in limestone for more than 370 million years, are providing evidence of the origin and evolution of the first fourlegged animals, the tetrapods. The Gogo fishes inhabited an ancient reef. They include armoured fishes (now extinct) and bony fishes, including 34 species that were previously unknown.

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The need to maintain biodiversity BIOFACT Biological diversity or biodiversity is the variety of all living organisms found on Earth. This variety has three components. 1 At the genetic level there is variation in the genes found within a species. 2 At the species level there is variety in the types of plants, animals and micro-organisms found. 3 At the ecosystem level there is a variety of different environments that support different species.

The variety of life on Earth is continually changing. A study of the Earth’s history and the evolution of living things over time shows that new species arise while others become extinct (see Chapters 3 and 4). Today there is international concern that the rate of loss of biodiversity on the planet is increasing. The main cause of this loss is human activity, particularly the destruction of the natural environment that is causing the rapid extinction of many species. Cities, towns and farms are replacing natural ecosystems. Those natural environments that remain are exposed to pests, introduced species, pollution and climate change. We know that biodiversity is essential for maintaining the functions of natural systems on Earth, such as providing clean water, air and productive soil, and recycling matter. Many (if not most) human activities depend on biodiversity—for example, agriculture, forestry, fisheries and tourism, construction, textile manufacturing, and medicine. The health and strength of our planet, our country and ourselves depend on the maintenance of biodiversity. We need to conserve and protect our biodiversity for future generations. In 1992 in Rio de Janeiro, 157 countries including Australia signed the Convention on Biodiversity. This was drawn up to ensure that countries take action to protect their biodiversity. The three main aims of the convention are: 1 the conservation of biodiversity 2 the sustainable use of its components 3 the fair and equitable use of the benefits arising from the use of genetic resources. Countries which signed the Convention agreed to identify important parts of the biological diversity in their own country that needed to be conserved, and to commence action to do so. To assist with the assessment of priority areas, Australia is playing a leading role in a project known as BioRap. BioRap is a joint project between the World Bank and a number of Australian scientific institutions. It provides a system that enables a country to assess its biodiversity so that plans can be drawn up to conserve vital areas. It is a way of helping to protect and conserve global biodiversity. A number of techniques and data collection processes have been developed to record and classify information about animals, plants, environments, topography and climate. The BioRap tools are being developed, tested and used by scientists in Australian institutions such as the Centre for Resource and Environmental Studies, the Environmental Resources Information Network and the Great Barrier Reef Marine Park Authority. They are also being used to assemble and analyse biodiversity data in Papua New Guinea.

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(a)

(b) FIGURE 4.71 (a) A scientist monitoring a population of the rare and endangered moss Bartramia bogongia on the Bogong High Plains. Global warming, fires, weeds and other problems threaten many Australian alpine species. (b) Bartramia bogongia is found only in the highest alpine areas of south-eastern Australia.

Evolution of Australian biota 203

Biodiversity for food and medicine Food Five thousand plant species have been used as food by humans, but less than 20 now provide most of the plant food eaten by most of the world’s population. Australia’s native species have joined the global food network—for example, our marine fisheries. Australia also has a reservoir of genetic diversity for useful food species, such as the macadamia nut and quandong, which increase the potential for enhancing agricultural productivity. We have 15 of the world’s 16 species of wild soybean—a group of plants that is becoming increasingly important as a human and stock food. In the future, these may be extremely valuable genetic stock, because unlike current commercial varieties, many of these wild plants have genes which allow them to resist leaf rust disease. Native Australian plants have great food potential. ‘Bush’ foods have high nutritional value. Some even have greater amounts of minerals, vitamins proteins, fat and carbohydrates than cultivated plant foods. Acacia seeds, for example, are superior to rice and wheat in energy, protein and fats. The potential of Australian acacias to enhance diets in Africa is currently being investigated. To enable new chemical structures to be discovered which will benefit humans, the conservation of biological diversity is essential.

Medicine Biological resources have long been used for medicinal purposes. A large number of Australian species are the basis of medicinal products. Two species of corkwood (Duboisia) are used to produce hyoscine, a medicine which is used to treat the effects of cancer therapy, motion sickness and stomach disorders. Lymphoid leukemia is treated by the drug tylocrebrine,which is produced from the vine Tylophora.

Many of the drugs used in medicine are derived from plants. Many medicines, in particular antibiotics, are derived from micro-organisms. Aboriginal societies have made use of many native plants as medicines. The bark of a tree found in the Kimberleys is known to Aboriginal people as a powerful painkiller. Research into this bark is currently taking place. Also, the native pepper (Piper novaehollandiae) and blackbean (Castanospermum australe) both offer potential in the treatment of cancer. The search is increasing for new medically active compounds and other Australian biota to contribute to modern medicine and for discovering cures for known diseases. Chemicals produced by animals have also led to discoveries of substances which are useful in medicine. It has been recently discovered that bulldog ants (Myrmecia) have specialised glands which produce antibiotics. The antibiotics reduce disease in their colonies. Scientists believe that this find may have great potential for therapeutic use and as industrial biocides. Prostaglandin E2, a substance which could be important in the treatment of gastric ulcers, was first discovered in the two species of gastric-brooding frogs (Rheobatrachus vitellinus and Rheobatrachus silus), found only in the rainforests of Queensland. These frogs incubate their young in their stomachs. Unfortunately, these frogs have not been seen in the wild for a number of years and are now thought to be extinct. Loss of any species through human neglect is a biological tragedy. Human survival therefore, depends on conserving biodiversity. Biological diversity offers great opportunities for discovering living resources essential to human survival.

FIGURE 4.72 Australian fisheries depend for their sustainability on the maintenance of biodiversity and the protection of ecosystems, especially in breeding and nursery areas for key species such as orange roughy and southern bluefin tuna.

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Since humans use so few plants for food, why should we maintain diversity in our food crops?

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2

Explain how maintaining biodiversity could help in finding new medicines.

Questions 1

Describe the work of palaeobiologists.

4

What is biodiversity and why is it important?

2

Describe the features of the Australian continent that make it an ideal subject for the study of longterm ecological and evolutionary change.

5

Outline the evidence that supports the hypothesis that there has been a loss of biodiversity in Australia over recent geological time.

3

How can knowledge of the history of our Australian ecosystem be beneficial to conservation strategies today?

F u r ther questions 1

Investigate the research that is taking place at one of the following Australian sites: Riversleigh in Queensland, Naracoorte, Murgon or Lightning Ridge in South Australia. a What kind of fossils have been uncovered? b How old are the fossils? c What do the fossils suggest about the kinds of environmental conditions that prevailed at the times that different kinds of organisms existed?

2

Research the effects of humans on Australian ecosystems over the last 200 years. Include a discussion of: • changes in the amount of forest cover, • changes in aquatic environments, and • effects on native flora and fauna.

3

The Worldwide Fund for Nature (WWF) raises money and community awareness about endangered species. The World Heritage List aims to protect natural wilderness areas from exploitation. The Australian Conservation Foundation raises awareness of conservation issues within Australia. Greenpeace takes action to stop the degradation of the environment. Choose one of these groups and research the Australian issues on their agenda. The following may help you:

• Name the plants, animals or areas with which the organisation is concerned. • Outline the problems that have been identified. • Summarise the recommendations made in relation to the problem. • Are there other ways to solve the problem? • How can members of the public make a difference? Search the Internet for information. Use key words to locate different organisations in a search engine. 4

Choose an extinct or endangered animal species for research. Choose from the list provided, or choose another animal in consultation with your teacher. a Describe the organism, its habitat and lifestyle.

Extinct

Endangere d

thylacine

blue whale

rabbit bandicoot

Leadbeater’s possum

gastric-brooding frog

eastern barred bandicoot

King Island emu

green and golden bell frog

b Propose reasons for its extinction or endangered status. c For endangered species outline some conservation strategies that are being or could be implemented to improve the situation.

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Chapter summar y Practical activities

4.1 ●

4.1

4.2



A moving Australian continent



Changing ideas



A timeline for Australia



Australian fossils The great evolution debate Variation in a species





4.3

4.4



Mitosis and meiosis compared





Fertilisation in water and on land Pollination



Living in the past



Biodiversity



Australia was once part of an ancient landmass we call Gondwana. Geological evidence for this includes matching continental margins, the position of mid-ocean ridges, and the spreading zones between continental plates. Biological evidence includes similar fossils found on different continents, and the distribution of extant (still living) organisms on these continents. Australia has been isolated for over 30 million years. During this time its unique flora and fauna evolved, including the now extinct megafauna.

4.2 ●













There is variation between members of any one species. Observable differences include variations in size or colour. At the cellular level there may be biochemical differences. Variation within a species increases the chance of survival of the species when environmental change occurs. Australia’s environment has changed over time. The landscape has changed as a result of volcanic activity and erosion. The climate has become warmer and drier as a result of continental drift and global climate changes. The inland deserts and grasslands of Australia experience significant variations in temperature between day and night. Australia is the world’s driest continent, and water availability is a problem for organisms over a large part of inland and central Australia. Fossil evidence, particularly from sites such as Riversleigh and Naracoorte, reveal changes in distribution and abundance of species, both current and extinct, as rainforests contracted and sclerophyll communities and grasslands spread. Charles Darwin and Alfred Wallace independently proposed a similar theory of evolution by natural selection that accounts for the changes seen in organisms over time. This theory proposes that organisms that survive and reproduce have favourable variations or adaptations. These are passed on to the next generation so that, over time, these characteristics become more common in the population. Charles Darwin’s observations of Australian flora and fauna when he visited in 1836 supported his theory of evolution.

4.3 ●

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The daughter cells produced as a result of the process of mitosis are identical. The daughter cells produced as a result of meiosis show variation; they are genetically different.













Fertilisation is the fusion of one male gamete and one female gamete. It requires a watery medium. In external fertilisation gametes are released directly into the environment. Large numbers are produced to increase the chance of male and female meeting. In internal fertilisation gametes are transferred from the body of one organism to another. This greatly increases the chances of successful fertilisation. External fertilisation occurs mostly in aquatic environments. Internal fertilisation has enabled the colonisation of terrestrial environments where water availability is restricted. Australian flora show reproductive mechanisms for pollination (for example, the grass trigger plant), seed dispersal for example, the waratah) and asexual reproduction (for example, sphagnum moss). Australian fauna show reproductive mechanisms to ensure fertilisation, for example, bower bird courtship displays, and to ensure the survival of the embryo and the young after birth, for example, the pouch of marsupial mammals. The evolution of these reproductive adaptations has increased the chances of continuity of the species in the Australian environment by increasing the chances of fertilisation and survival. Asexual reproduction is advantageous in an unchanging environment, for example, the growth of coral on the Great Barrier Reef.

4.4 ●





A study of past environments is important in predicting the impact of human activity in present environments. Palaeontology, the study of fossils, can help us understand the factors that may determine the distribution of flora and fauna in present and future environments. Biodiversity is the variety of all the living things on Earth. It is important to preserve this variety to ensure the continuity of the ecosystems on Earth. We need to protect plant and animal species and their environments to maintain the Earth’s natural systems and to sustain many human activities.

Evolution of Australian biota 207

EXAM - STYLE QUESTIONS Multiple choice 1

2

3

4

Which of the following represent evidence that Australia was once part of the supercontinent of Gondwana? A geological evidence of more frequent volcanic activity in Australia in the past B similarities between present-day plants and animals on continents currently isolated from one another by the oceans C fossilised evidence of extinct Australian megafauna D changes in the Australian climate from colder to more temperate over the last 65 million years. Geological evidence suggests that South America separated from the Australian–Antarctic land mass around 60 million years ago. Australia is believed to have broken away from Antarctica around 55 million years ago. What does the existence of the American opossum (a marsupial) suggest? A Marsupials evolved independently on different landmasses. B The opossum migrated to South America at some time after marsupial evolution was completed on the Australian continent. C The opossum was brought to the Americas by early nomadic humans. D Ancestral marsupials already existed more than 60 million years ago. Australia has a large proportion of endemic species of plants and animals. What factors contribute to Australia’s richness of unique species? A its long isolation from other landmasses B early human activity such as selective breeding of favourable characteristics C travel to Australia by neighbouring islanders bringing plants and animals with them D change in climate from warmer to cooler over recent geological time. Darwin proposed his theory of natural selection to account for the evolution of species on Earth. Which of the following statements does not represent a key point in this theory? A There is variation within every species. B Organisms in a population that have favourable characteristics have a greater chance of survival and reproduction compared to those individuals without those characteristics.

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C Individuals that develop favourable variations of a particular characteristic are more likely to survive and reproduce compared to other individuals. D Favourable characteristics gradually become more common in a population. 5

Over the last 20 million years, the climate in Australia has become warmer and drier. This has resulted in which of the following changes? A an increase in the number of bushfires B an increase in the area covered by tropical rainforest C an increase in the area covered by grassland D a decrease in the area covered by woody sclerophyll forests.

6

The diversity of plant and animal species on Earth is threatened most by which of the following activities? A the consumption of plants for food B the search for new medications C air and water pollution D destruction of natural environments.

7

How many chromosomes will the body cells of an animal producing gametes with 16 chromosomes usually contain? A 8 chromosomes B 16 chromosomes C 32 chromosomes D 64 chromosomes.

8

Which adaptation in animals increases the chance of successful fertilisation? A laying shelled eggs B direct transfer of male gametes into the female body C production of large numbers of eggs and sperm D internal development of the young.

9

Why is the work of palaeobiologists in building a picture of the long-term changes that have occurred in Australian ecosystems important? A It contributes to our understanding of the evolution of endemic species of plants and animals. B It enables us to predict the impact of human activity in our present-day environment. C It helps us to plan appropriate management strategies to protect endangered species. D all of the above.

10 The Tasmanian tiger or thylacine represents an example of the extinction of a species of Australian mammal. What is the most likely cause of the extinction of the thylacine? A the activity of early Australians in managing areas of land with fire B hunting since European settlement C competition with other species D the natural decline in global biodiversity.

b Describe two ways in which sexual reproduction increases variation within a species. 6

The following map shows Australia as part of the supercontinent Gondwana, some 135 million years ago. 135 million years ago

Short answers 1

Describe how the formation of mid-ocean ridges and spreading zones between continental plates provides evidence that supports the theory of continental drift.

2

a Explain how variation in a population is related to the chances of survival in a changing environment. b Suggest the likely consequence for a species that is unable to adapt to a changing environment. Explain.

3

A quarry near Melbourne is the site of abundant fossil deposits of different kinds of seed-producing plants. Some resemble casuarinas that grow in tropical Australia; others are fossilised eucalypt leaves that are similar to bloodwood eucalypts that are now found only in far eastern Victoria and southern New South Wales. Palaeontological studies date the fossils at around 22 million years old. Describe the changes in environmental conditions in the area over the last 22 million years, as suggested by the evidence at this site.

4

a Identify two reasons why the flower illustrated below is likely to be wind pollinated. b Describe the mechanisms for pollination of a named Australian insect-pollinated flower.

continental boundaries today's continents

South Pole

a Name the other modern-day land masses that were part of Gondwana. b Describe two pieces of geological evidence that support the theory that the supercontinent of Gondwana existed. c Using Glossopteris (a kind of primitive seed fern) as an example, explain the biological evidence that supports this idea. d Describe the changes that have occurred in the Australian climate since its separation from Gondwana. e The Australian emu, African ostrich and South African rhea are examples of different but related flightless birds that inhabit different continents today. What conclusions can be drawn from this information in relation to the evolution of these different groups of birds? 7

Study the evolutionary tree representing examples of extinct and present-day Australian fauna. diprotodon (large wombat-like marsupial; extinct)

ancestral species grass flower

5

The continuation of species relies on successful reproduction between members of the species. a Name two events that must coincide in order that successful reproduction occurs within a species.

northern hairy-nosed wombat (Lasiorhinus krefftii) southern hairy-nosed wombat (Lasiorhinus latifrons)

Evolution of Australian biota 209

a Use Darwin’s theory of natural selection to account for the evolution of the two different species of wombat represented in the diagram. Include a discussion of • natural variation • different environmental pressures • adaptations to different environments • subsequent generations. b Outline two reasons that account for the disappearance of Australia’s megafauna. 8

The diagram shows a cell containing two pairs of chromosomes just before meiosis.

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a Draw the daughter cells produced after meiosis has occurred. b State two differences between the cells you have drawn and the cells produced after mitosis. c Explain how sexual reproduction results in variation in the offspring. 9

Ascidians (seasquirts) are animals commonly found attached to rocks at low tide. Some species can reproduce both sexually and asexually. Through asexual reproduction, large colonies become established on certain rocks. a Identify one advantage of asexual reproduction for seasquirts. b Many Australian plants are capable of reproducing both sexually and asexually. Identify one example of such a plant and describe one condition under which asexual reproduction occurs. c For the seasquirt and also your named plant, explain the advantage of also being able to reproduce sexually.

Chapter 5

MAINTAINING A BALANCE

A multicellular organism is a coordinated collection of organs and systems that are responsible for the uptake and transport of essential nutrients from the environment, removal of waste products produced from cellular activities, and the production of energy. The cells making up a multicellular organism require a relatively constant environment in which to function. If their environment changes too much the health of the cells and ultimately the organism itself will be adversely affected. For example, if a cell’s environment becomes too salty it will shrivel up and fail to function properly. An organism must therefore be able to maintain a relatively stable internal state, regardless of changes to its external or internal environment. The total of all chemical processes occurring within an organism is called its metabolism. Special proteins called enzymes control all chemical reactions that make up an organism’s metabolism. If an organism is to function normally its enzymes must work at optimum efficiency. Enzymes are sensitive to many factors including temperature. Can you remember when you last ran a high temperature? An increase of only a few degrees can have dramatic effect on your enzymes and hence your health. Coordinating systems, such as the nervous and endocrine systems in animals and the hormone system in plants, provide the monitoring, communication and feedback needed to maintain a constant internal environment and respond to changes in the external and internal environment. This is known as homeostasis.

This chapter increases students’ understanding of the applications and uses of biology, the implications for society and the environment, and current issues, research and developments in biology.

5.1

Activity and temperature OBJECTIVES When you have completed this section you should be able to: ● describe the chemical composition and features of enzymes and outline their role in metabolic reactions ● outline the factors that affect enzyme activity ● explain the importance of a stable internal environment ● describe homeostasis in terms of detecting change and counteracting change ● describe the role of the nervous system in terms of detecting and responding to environmental change ● describe how feedback mechanisms operate in maintaining a stable internal temperature ● discuss the wide temperature range within which different life-forms exist, compared with the temperature limits for individual species ● distinguish between endotherms and ectotherms and compare their responses to environmental temperature change ● discuss how plants respond to temperature change.

The role of enzymes activities ● ● ●

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Enzymes Feedback mechanisms Australian ectotherms and endotherms

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The molecules in cells are constantly interacting: being formed, broken down, or exchanged. These chemical reactions—in fact, all the chemical processes occurring within an organism—are called its metabolism. An organism is regulated, and the rate of its chemical activity is maintained, by special large proteins called enzymes. Enzymes are very important to cell functioning. Enzymes can be used over and over again, so only small quantities of them are required in a cell. They are made in the cell when they are needed. Their manufacture is controlled by the nucleus. Different types of cells make different enzymes. The type and amount of enzymes a cell makes depend on the activities and functions that the cell carries out.

Functions and characteristics of enzymes Enzymes are biological catalysts. Catalysts control the rate of a reaction—they speed it up or slow it down—but they are chemically unchanged at the end of the reaction. Enzymes are specific in their action, which means that they affect only one type of reaction. They work by providing a surface or active site where the reaction can take place. The molecules on which an enzyme acts are known as substrates. The substrate molecules bind on to the active site. This binding brings about, or induces, a temporary change in the shape of the enzyme known as induced fit. A chemical reaction occurs and the substrate is changed. In Figure 5.2, the substrate molecule is split. The product or products are then released and the enzyme returns to its original form. Figure 5.3 shows two theories of enzyme action. (a)

substrate

breaking down reactions

building up reactions

FIGURE 5.1 Molecular breakdown and formation reactions in metabolism.

bond broken

active site enzyme

enzyme

enzyme

i. substrate binds to active site

ii. chemical reaction occurs

iii. changed molecules released

(b)

maltase + H2O maltose

+ water

glucose

+

glucose

maltase FIGURE 5.2 (a) Enzyme action. (b) The enzyme maltase catalyses the reaction between maltose and water. It splits the maltose to produce two glucose molecules

S2 S1

S1

S2

P

E

E

E

(a) Lock and key S2 S1

S1

S2

P

E E

E

(b) Induced fit FIGURE 5.3 There are two major theories of enzyme action: (a) the lock and key, and (b) the induced fit. While evidence supports the induced fit theory, the lock and key theory is helpful in understanding how enzymes work.

Maintaining a balance 213

BIOFACT In the 19th century, French physiologist Claude Bernard first recognised the constancy of internal body fluids in mammals. He called this the ‘fixity of the milieu interieur’. In the 20th century, American physiologist Walter Cannon gave this phenomenon the name ‘homeostasis’. Plants and animals regulate their internal environment by controlling the levels of gases, water, salts and waste materials.

TABLE 5.1 Characteristics of enzymes.

Enzymes • are made of proteins • are catalysts, because they control the rate of chemical reaction • remain unchanged at the end of the reaction •are needed in only small amounts • are highly specific: one enzyme catalyses one type of reaction • work best under certain optimum conditions of temperature and pH • may need coenzymes to help functioning

pH is a measure of the acidity of a substance.

The concentration of substrate affects enzyme activity. An increase in substrate concentration will increase the rate of reaction until all enzyme active sites are occupied. This is known as the saturation point; the reaction will then proceed at its maximum rate. Enzymes need certain conditions for maximum efficiency. Most work best at a certain temperature (37˚C in mammals), and the speed of the reaction they are catalysing decreases if the temperature varies from this level. The acidity or alkalinity of the conditions is also important. Acidity is measured on the pH scale, which ranges from 0 to 14. Substances with a pH of 7 are ‘neutral’. Below 7, a substance is ‘acid’, and above 7 a substance is ‘alkaline’. Enzymes work best at a particular level of acidity: any variation above or below the specific level reduces their rate of activity. For example, in a mammalian stomach the conditions are very acidic, with a pH of between 1.5 and 2. The enzymes involved in chemical digestion in the stomach work best in the acidic conditions found there—they stop working when the food moves through to the small intestine where conditions are alkaline, with a pH of 7.5–8.8. This sensitivity to conditions, as with temperature change, is related directly to the protein structure of enzymes. High temperatures and extreme pH will alter or denature them. This is a permanent change, and when it occurs the enzyme no longer catalyses the reaction. Some enzymes need assistants called coenzymes if they are to function properly. These coenzymes consist of other chemicals, such as mineral ions or compounds that come from vitamins.

Homeostasis Homeostasis is the process by which organisms maintain a relatively constant or stable internal environment for body cells. It consists of two stages; detecting changes from the stable state, and counteracting changes from the stable state.

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All enzymes work best under certain (optimum) conditions. Cells need to maintain an optimum environment for their enzymes to work. Multicellular organisms function best, and at optimum metabolic efficiency, if the internal environment provided for their cells is maintained at a constant level. Multicellular organisms regulate their internal environment in order to remain healthy. They monitor all the activities of their cells, their requirements and the wastes they produce, using feedback systems. The organism, as a whole, coordinates the surroundings of its cells to ensure that the composition of the internal body fluid remains within the required limits. The internal environment for cells can never be controlled completely, but the coordinating systems work to maintain a balance within certain limits. This is known as homeostasis.

Responding to change Detecting changes If organisms are to survive and function properly, they need to be able to pick up information from their external and internal environments, interpret this information and react appropriately. Any information that provokes a response is called a stimulus (plural: stimuli). Environments contain many stimuli and organisms have special receptors that detect them.

Any information that provokes a response is called a stimulus. A receptor is an organ or other part which receives a stimulus and transmits it to the organism’s control centre.

vision TABLE 5.2 Stimuli and receptors

hearing

Stimulus

Type of receptor

Light

photoreceptor

Heat, cold

thermoreceptor

Sound, touch, pressure, gravity

mechanoreceptor

Oxygen, carbon dioxide, water, pH, inorganic ions, nitrogenous wastes, glucose

chemoreceptor

Electrical fields, magnetic fields

other specialised receptors

Some examples of stimuli detected in the external environment include light, day length, sound, vibration, temperature, textures and odours. Some examples of stimuli detected in the internal environment of multicellular organisms include the levels of carbon dioxide, oxygen, water, wastes and temperature. Receptors vary from being a patch of sensitive cells to complex sense organs such as the ears and eyes of mammals.

Counteracting changes Receptors detect changes. Organisms may then respond to the change. Where the change affects the organism’s normal (stable) state, the response is homeostatic; that is, one that will counteract the change to ensure that the stable state is maintained. Responses are brought about by effectors. In mammals, effectors may be muscles or glands. Muscles respond by contracting or relaxing, thus bringing about movement. Glands respond by secreting a chemical substance; for example, the salivary glands in the mouth of mammals produce saliva when food is detected. Large multicellular animals use a control centre and rapid communication system within the body to coordinate sensory information with the body’s responses. This system is known as the nervous system. The nervous system works to regulate and maintain an animal’s internal environment and respond to the external environment.

taste

smell

skin

FIGURE 5.4 Some different types of receptors. The yellow regions respond to stimuli. The arrow shows the direction of the nerve impulse.

Plants do not have a nervous system. They coordinate all their responses to environmental stimuli by means of hormones.

Stimulus

Receptor

Control centre

Effector

Response

The human nervous system The human nervous system is made up of the central nervous system, the brain and spinal cord. This system acts as a control centre to

FIGURE 5.5 The stimulus–response pathway.

Maintaining a balance 215

coordinate all the organism’s responses. It receives information, interprets it and initiates a response. The peripheral nervous system is a system of nerves branching throughout the body to and from the receptors and effectors. These act as communication channels and pass messages rapidly to the central nervous system and back. The nervous system works closely with another coordinating system, known as the endocrine system. The endocrine system produces chemicals called hormones, which are made in special glands in response to certain stimuli. The hormones are transported in the blood to the areas where their effects will bring about a response (see p. 249). brain central nervous system spinal cord

spinal nerves

FIGURE 5.6 The human nervous system.

Temperature and life On Earth, living organisms are faced with a large range of air temperatures in different areas—from less than –70˚C at the poles to more than 50˚C in deserts. They must also cope with daily and seasonal temperature changes in their external environment. Temperatures on land vary much more than temperatures in water. As a result, organ216

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Feedback mechanisms To keep the internal temperature constant, the body operates a feedback system of control. In a feedback system, the response alters the stimulus (Figure 5.7). In humans, if receptors detect that the body temperature is raised, mechanisms such as sweating and heat radiation from the skin will occur as a response. This will lower the body temperature, and the

stimulus that originally triggered the receptors will therefore be removed (Figure 5.8). This type of case, where the feedback reduces the effect of the original stimulus, is known as a negative feedback system. Negative feedback systems help to maintain stable conditions and are the major mechanism by which homeostasis is achieved. FALL IN TEMPERATURE

if level is too high it is quickly lowered normal level

normal level

thermostat if level is too low, it is quickly raised

FIGURE 5.7 A feedback system.

switch negative feedback heat

FIGURE 5.8 Components of a negative feedback system that is involved in controlling temperature.

on heater

What is the role of feedback systems in counteracting change?

isms living on land experience a greater daily and seasonal temperature range than do aquatic organisms (see Chapter 1, p. 5). The temperature of the environment is known as the ambient temperature. To survive, organisms must be able to live within the temperature range of their local environment. Most species can tolerate only a narrow temperature range. For example, sugar cane needs a warm (>15˚C) frost-free environment, so it can be grown only in tropical and subtropical regions. Multicellular organisms need to be able to regulate their internal temperature within the range that permits their cells to function properly.

BIOFACT Some bacteria can exist in hot volcanic springs where the water temperature reaches 94˚C. Some seeds, lichens and mosses have been known to survive exposure to temperatures below –200˚C, and many plants can survive temperatures of several hundred degrees in wildfires.

Ectotherms and endotherms Ectotherms Most organisms are ectotherms. Ectotherms have a limited ability to control their body temperature. Their own cellular activities generate little heat. Their body temperature rises and falls with ambient temperature changes. Plants, all invertebrates, fish, amphibians and reptiles are ectotherms.

Ectotherms have a limited ability to control their body temperature. Their body temperature rises and falls with ambient temperature changes.

Endotherms Birds and mammals are endotherms. Their body metabolism generates heat and maintains an internal body temperature that is independent of the external temperature (see Table 5.3). To do this takes energy, so more food is required by endotherms.

Endotherms generate heat from their body metabolism, so their internal body temperature is independent of the external temperature.

Maintaining a balance 217

BIOFACT Endotherms eat more food than ectotherms. This higher food intake results in a increased level of metabolism, which is required to produce heat. Compared to reptiles of the same weight, mammals use at least five times more energy.

TABLE 5.3 Body temperatures of endotherms

Animal

Body temperature

Monotremes

30–32˚C

Marsupials

35–36˚C

Placental mammals

36–38˚C

Birds

38–41˚C

Responses to temperature changes If temperatures are too hot or too cold for an organism, it might die. Aquatic ectotherms usually remain at the temperature of the surrounding water; they do not show any specialised adaptations to regulate their body temperature. Terrestrial ectotherms and endotherms experience a greater range of temperature changes and have receptors to detect these changes. They show a variety of responses to regulate heat gain and loss from their bodies (see Figures 5.11 and 5.14).

Behavioural adaptations BIOFACT One of the great migration mysteries of the 20th century related to the Monarch or Wanderer butterfly, Danaus plexippus. The butterfly occurs throughout most of the United States in summer, but in winter the populations disappear. Those west of the Rocky Mountains migrated to overwintering sites close to the coast, but those east of the Rockies simply disappeared. Volunteers searched high and low across the country, without success. Many theories were advanced, but no-one knew where these Monarchs went. Finally a butterfly expert, F. A. Urquhart, solved the problem. He discovered that the Monarchs joined together in huge aggregations flying south, heading for several overwintering sites—in southern Mexico! The Monarch has now spread by island-hopping across the Pacific to Australia, perhaps helped by ships. It is a common butterfly along the whole of the east coast.

Migration Animals can move to avoid temperature extremes. For example, many birds that spend spring and summer in south-eastern Australia migrate vast distances before the weather turns cold and food becomes scarce (Figure 5.9).

Breeding range

Occurrence during migration Non-breeding range

FIGURE 5.9 The sharp-tailed sandpiper breeds in northern Siberia and migrates south, with some crossing Asia overland and others following the east Asian coastline. The migration takes the birds to non-breeding quarters in the southern hemisphere, with most apparently moving to southern Australia. After the non-breeding period sharp-tailed sandpipers migrate north in order to return to their breeding grounds.

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Hibernation To survive cold conditions, some animals hibernate. They remain in a sheltered spot, their metabolism slows, and in endotherms their body temperature drops. The term aestivation is used for animals that ‘hibernate’ in hot conditions; for example, the Bogong moth migrates to spend the summer months in caves in the Australian Alps (Figure 5.10).

Migrating moths lay eggs

s

egg

Eggs hatch and larvae live in lar soil and emerge va e to feed at night

Summer Spring

pupae

Autumn Winter

Pupae form in soil burrow

s ult ad

Emerging adult moths migrate eastwards at night

Moths are dormant until autumn, when they migrate westwards (a)

(b)

FIGURE 5.10 (a) Huge swarms of Bogong moths (Agrotis infusa) migrate to the Australian Alps in summer, to aestivate in cool caves. (b) The annual life cycle of the moths.

Shelter Animals seek shelter from extreme conditions. They may dig burrows or shelter in caves or rock crevices during the day to escape high temperatures, or at night to escape low temperatures. For ectotherms such as lizards, basking in the sun and sheltering in shade may both be part of the regular daily activity to keep the body temperature reasonably constant. The central netted dragon (Ctenophorus nuchalis) also climbs up into trees or bushes when it is very hot, to seek cooler conditions off the ground (Figure 5.11). Nocturnal activity Brown snakes are found throughout Australia. They are usually active during the day, but when it is very hot they become nocturnal and their active period is at night. Many small desert mammals such as hopping mice, which dig burrows, and desert bandicoots, which make grass-covered holes in the ground, shelter during the day from the heat and are active and feed at night. Controlling exposure Ectotherms such as lizards may alter their posture to expose a larger or smaller surface area to sunlight (Figure 5.11) Endotherms may huddle together or curl themselves up to keep warm and reduce heat loss; for example, penguins and bats huddle in colonies (Figure 5.14), and small mammals curl up and tuck their legs and tails around their bodies.

Maintaining a balance 219

air 43.4ºC

air 28.8ºC

air 28.9ºC

air 29.6ºC

air 35.2ºC 32.2ºC

39.0ºC

36.7ºC 38.9ºC

37.5ºC

Basking–elevated, back to sun, skin dark

6

35.8ºC

39.2ºC

Active on ground– Retreat down burrow some elevated, skin pale

emergence 10

12

43.2ºC

43.5ºC

Elevated–head towards sun, skin pale

43.5ºC

Basking–elevated, back to sun, skin dark

re-emergence

retreat 8

33.1ºC

46.5ºC

56.4ºC

42.0ºC

Time of day 14

retreat 16

18

20

FIGURE 5.11 The central netted dragon (Ctenophorus nuchalis), an Australian desert-adapted lizard, regulates its temperature by changing its behaviour.

Structural and physiological adaptations of endotherms Ectotherms and endotherms have similar behavioural adaptations to hot or cold conditions, but endotherms have more structural and physiological adaptations for maintaining a constant internal body temperature. (The responses of ectotherms are usually behavioural.)

BIOFACT Marine mammals such as whales and seals have a very thick layer of fat beneath the skin, which is called blubber. This layer of fat is the reason that untold numbers of whales have been killed over the centuries. The blubber was cut from the carcass and boiled down to extract the oil, which was used mostly for making lamp fuel and lubricants. Seals were also hunted for their blubber, and for their thin but waterproof skins. The skin of penguins, whales and seals may be only a few degrees warmer than the surrounding water.

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Insulation Fur in mammals and feathers in birds maintain an insulating layer of trapped air that slows down heat exchange with the environment. The thickness of the air layer can be increased in cold conditions by contracting the muscles that lift the fur or feathers away from the skin (Figure 5.15). Some mammals, such as domestic cats, grow a thicker coat of fur in the winter and lose it in the summer. Most animals in cold climates maintain a layer of subcutaneous fat as insulation; for example, the Australian fur seal has a thick layer of blubber. Metabolic activity Endotherms generate heat as a result of their metabolic activity. In cold conditions this keeps the body warm, and heat loss needs to be reduced. In hot conditions heat generated by the body may need to be lost. Physiological adaptations to the cold include increased metabolic activity; for example, shivering increases muscle activity and produces heat. Control of blood flow Endotherms may increase or decrease heat exchange with the environment by controlling the blood flow to their skin and extremities. The amount of blood flowing and the route it takes can be adjusted. This enables the skin temperature to be lowered while maintaining a normal internal body temperature.

Hibernation: cold comfort Many animals respond to a decrease in environmental temperature and a reduction in food supply by hibernating. During hibernation, an organism’s metabolism and energy use are greatly reduced, but physiological control is maintained. An animal saves about 60% of its annual energy requirement by hibernating. When endotherms such as birds and mammals hibernate, their: • metabolic rate decreases, which causes the body temperature to drop close to that of the surrounding environment, but always above freezing (anything below 0˚C can lead to the death of the organism) • heart beat slows down to as few as three beats per minute • breathing rate drops. Hibernating organisms respond to sudden environmental changes. If the external temperature drops suddenly, the organism will arouse spontaneously to ensure essential body functions such as kidney, circulation, muscle and nervous coordination are not impaired at low temperatures. This is achieved by

shivering or movement, which increases circulation, respiration and metabolic rate, breaking down fat reserves and producing heat. This re-establishes homeostasis, and the animal returns to a state of hibernation. For example, the temperature of the little brown bat (Myotis lucifugus) can increase from 10˚C (the hibernating temperature) to 35˚C in 30 minutes. Heat is produced in tissues such as brown fat, shivering skeletal muscle and heart muscle (Figure 5.12). Reptiles such as snakes and lizards are ectotherms, so they need an external source of heat to keep their bodies active. During winter, snakes hibernate to enable them to survive the surrounding cold temperatures. A great deal of preparation needs to take place before animals can enter a state of hibernation. They need to build up their fat reserves and find a suitable place to hibernate during winter. Amphibians (frogs and toads) burrow underground to avoid freezing conditions. Other organisms simply sleep in a sheltered place.

23˚C 18˚C

(b)

(a)

FIGURE 5.12 (a) In a bat coming out of hibernation, the temperature in the region of brown fat is much higher (23˚C) than trunk temperature in the rest of the body. Heat generated is carried around the body to warm other organs. (b) Hibernating little bent-wing bats (Miniopterus australis) in a cave at Camp Mountain, near Brisbane.

1 Explain the advantages of hibernation for animals. 2 Describe three metabolic changes that occur in hibernating birds and mammals.

3 Give two reasons why it is important for animals to find a suitable place to hibernate.

Maintaining a balance 221

FIGURE 5.14 Emperor penguins in Antarctica. By huddling together, penguins reduce the proportion of their body surface that is exposed to the cold air, and so reduce heat loss.

The great coral bleaching 1998 was the Earth’s hottest year on record. It was a ~o effect, including unusually year of a strong El Nin warm ocean waters. Much of the world’s coral reefs were affected by the phenomenon known as ‘coral bleaching’. The coral animals and their symbiotic algae can survive only in a very narrow range of temperatures, and they die if the warm conditions last too long. This devastation of coral in turn affects all the organisms within the coral reef ecosystem. Scientists investigating coral bleaching still do not know if this world-wide destruction of coral reefs is part of natural variability in the ocean and atmosphere systems, or if it can be related to the increase in carbon dioxide from human activities. At temperatures only 1˚C above their range, the symbiotic algae in coral tissues cannot photosynthesise normally. The cells containing the algae are shed from the coral, causing the coral to lose their colour and become ‘bleached’. The coral organisms themselves may be killed as a result. FIGURE 5.13 Bleached coral on the Great Barrier Reef.

Explain why coral bleaching occurs.

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BIOFACT The size and shape of an animal’s body affect its surface area to volume ratio and hence its ability to conserve or lose heat. A large, round body shape has a smaller surface area to volume ratio which reduces heat loss compared to a small thin body. Look at the penguins in Figure 5.14 and compare their body shape and size with most other birds.

blood returning to heart has been warmed

warm blood from heart

FIGURE 5.15 When it is cold, small birds such as this superb parrot reduce the loss of heat to the environment by increasing the thickness of the air layer trapped in their feathers.

Counter-current exchange This is used by some endotherms in cold conditions. Blood vessels leading to and from the extremities of the body (e.g. the legs and tail) are placed close together and chilled blood returning in the veins picks up heat from the arteries going to the extremities. This system is used in the legs of Arctic birds, the fins of seals and the feet of the platypus (Figure 5.16).

vein

artery

Evaporation By controlling the rate of evaporation of water from their bodies, endotherms can help keep themselves cool. For example, dogs pant, birds flutter a membrane in their throat, and humans sweat. Kangaroos often lick their forearms in very hot weather. Their forearms have a good blood supply and very little fur; the moisture evaporates and cools the forearms, thus cooling the blood.

Plant responses to temperature change Plants usually respond to temperature changes by altering their growth rate. Active plant growth can occur within the range 5–45˚C in temperate regions, but in tropical areas growth may cease below 15˚C. Most plants have a growth season and life cycle that follow the seasonal temperature variations of their environment. In response to extreme heat or cold, plants (especially non-woody plants) may die but leave dormant seeds, which often have thick protective seed coats. Alternatively, plants may die back above the ground, leaving roots, rhizomes, bulbs or tubers to survive underground (Figure 5.17). These sprout when favourable conditions return.

cold blood returning from extremities

cooler blood to cold extremities

FIGURE 5.16 Counter-current exchange. This exchange of heat in animals reduces the heat lost from the body and helps maintain the internal temperature.

Maintaining a balance 223

BIOFACT Vernalisation was first studied in wheat. Winter varieties of wheat need exposure to cold before they will flower and produce seeds. They normally need to be sown in autumn for harvesting the following year. It is now common to delay sowing until spring, the seeds being deliberately kept at low temperatures before sowing to ensure that their cold requirements are met.

FIGURE 5.18 The leaves of most eucalypts hang vertically to reduce their exposure to the hot sun.

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FIGURE 5.17 Tuberous roots enable many plants to survive harsh conditions, even though they might die back above the ground.

Temperature may be one factor that controls developmental changes in a plant’s life cycle, from germination through to flowering and seed dispersal. In Australia, too high a temperature during flower formation produces a poor wheat crop, because meiosis in pollen formation in the anthers is very temperature-sensitive. In rice crops in the Murrumbidgee area the reverse is found; temperatures can be too low during flower formation. Vernalisation is the name given to the exposure to cold conditions that some plants require before they can develop flowers. In many parts of Australia, bulbs such as hyacinths and daffodils are more likely to flower if they are chilled in a refrigerator for a few weeks before planting. Seed dispersal in some Australian plants is stimulated by the extreme heat of fire. Banksia, Hakea and some Eucalyptus plants bear fruits with hard woody cases that are not dropped from the parent plant. The heat of a fire stimulates the fruits to open, and the seeds are released. Some plants, such as Christmas bells and kangaroo-paws, flower more profusely when the plant has been exposed to the heat and smoke of a bush fire. Frost during periods of new growth may damage plants, but many plants have leaves that are frost-tolerant. For example, after frost the leaves of camellias appear semi-transparent, but on thawing return to normal. Reflective leaf surfaces which reduce the amount of radiation that is absorbed can help keep a plant cool in hot conditions. Leaves may be light or silvery coloured or have waxy or shiny surfaces. In some plants, the orientation of the leaves results in the smallest possible surface area being exposed to sunlight and heat. For example, the leaves of most Eucalyptus trees hang vertically (Figure 5.18). Transpiration from leaves has a cooling effect on plants. Plants in hot dry climates, however, usually conserve water by reducing transpiration in the hottest part of the day.

Questions 1

Explain what is meant by the term ‘metabolism’.

2

a What is an enzyme? b Outline the role of enzymes in organisms. c Use diagrams to help you explain what is meant by the specificity of an enzyme. d Name three other characteristics of enzymes.

3

a Define homeostasis. b Explain why homeostasis is important in organisms. c Describe the two stages of homeostasis.

4

a Define a stimulus. b Give examples of different kinds of stimuli.

5

how the responses help the organism regulate body temperature. 6

a Describe some ways in which plants respond to heat and cold. b Compare some adaptations to extreme heat shown by some species of Australian flora.

7

a What is meant by a ‘feedback system’ of control? b Use the example of body temperature to explain how feedback mechanisms maintain a stable internal environment.

8

Complete the following table, which summarises some responses to different environmental temperatures in humans.

a Explain how the body temperature is controlled in i ectotherms ii endotherms. b List five species that are ectothermic and five that are endothermic. Include some Australian examples. c Use one example of an ectotherm and one example of an endotherm from your list in (b) to compare an organism’s responses to change in environmental temperature. In each case explain

Stimulus

Response

Decrease in environmental temperature (cold weather) Increase in environmental temperature (hot weather)

F u r ther questions Study the following graph of enzyme activity.

rate of enzyme-catalysed reaction

1

3.0

a Describe what happens to enzyme activity as the temperature increases from 10°C to 40°C. b What is the optimum temperature for the enzyme? c What happens to the enzyme above 40°C? 2

Some chemical substances inhibit enzyme activity. This may be temporary and reversible, or permanent and irreversible. Research examples of each type of inhibition. Suggest two ways by which an enzyme could be irreversibly inhibited from working.

3

a Ectothermic organisms are sometimes referred to as ‘cold-blooded’. Why is this term incorrect? b Describe the relationship between the body temperature of an ectotherm and its environment

2.0

1.0

10

20 30 40 temperature ˚C

50

Maintaining a balance 225

4

a Outline the benefits to an organism in raising its body temperature above that of its environment. b What kinds of organisms are able to do this?

5

a Find out how sweating in humans works to cool the body. b Most mammals do not have sweat glands distributed over most of their skin surface. i Explain why not. ii What alternative mechanism is used for cooling in these mammals? Describe how the mechanism works to cool the body.

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6

Draw a concept map to summarise the way in which body temperature is regulated in mammals. Include each of the following: hypothalamus detectors feedback mechanism

7

optimum temperature external environment internal environment

Tulips are plants adapted to very cold climates. They are not native to Australia. Find out how nurseries artificially control their plant stocks to produce flowers that are not adapted to our harsh climate.

5.2

Water for transport OBJECTIVES When you have completed this section you should be able to: ● identify the form(s) in which materials are carried in the bloodstream of mammals, including oxygen, carbon dioxide, nitrogenous wastes, water, lipids, salts, products of digestion ● explain why oxygen is needed by cells and carbon dioxide must be removed ● explain how the features of haemoglobin assist it in carrying out its role ● describe the structure of arteries, veins and capillaries and relate their structure to their function ● summarise changes that occur in the composition of blood circulating around the body and identify where these changes in composition occur ● describe the pathways and mechanisms for water transport in plants ● outline current theories that account for the movement of organic materials into and through the phloem.

The mammalian circulator y system Mammals have a closed circulatory system consisting of a pump (the heart) which sends a fluid (the blood) through a network of tubes (blood vessels). These vessels transport materials rapidly throughout the body, to and from the cells and the external environment. Interstitial or body fluid drains into the lymphatic system and the lymph vessels return the fluid, known as lymph, to the blood (see Chapter 2, p. 91). The circulatory system has several other functions (Table 5.4).

activities ● ● ● ● ●

Changing pH Investigating blood cells Measuring blood gases Blood products Conducting tissues—xylem and phloem

Composition of the blood If a sample of mammalian blood is taken and spun in a centrifuge, it will separate into two parts: the plasma and the cellular matter (Figure 5.20). Plasma makes up about 55% of the volume of blood. In whole blood, red and white blood cells and small particles called platelets are suspended in this plasma.

Maintaining a balance 227

TABLE 5.4 Functions of the mammalian circulatory system.

Function

Description

Transport

The major function of the circulatory system is to transport water, gases, nutrients and wastes.

Blood clotting

This complex mechanism repairs damage to blood vessels and seals wounds to prevent loss of blood.

Defence against disease

White blood cells help to fight infection in the body. Antibodies provide immunity against further attack (see Chapter 7, p. xxx).

Temperature

The flow of blood distributes heat around the body. Control of the amount of blood regulation passing close to the skin helps control heat loss from the body (see p. xxx).

extracellular (15 L)

interstitial fluid

intracellular (25 L)

other cells

blood plasma red blood cells blood = 5 L FIGURE 5.19 The amount of fluid in human tissues.

Plasma is a sticky, straw-coloured, slightly salty liquid. It is made up of about 90% water plus various other substances carried in solution.

Plasma Plasma is a sticky, straw-coloured, slightly salty liquid. It is made up of about 90% water plus various other substances carried in solution. It contains salts carried as ions in solution, as well as large plasma proteins. These salts and proteins play a role in maintaining the pH of the blood. (Normal human blood has a pH of 7.4.) There are different types of plasma proteins, including antibodies, clotting factors and lipid transporters. Many substances are transported in the plasma, and their amount in the plasma changes as the blood circulates. These include waste materials such as urea and carbon dioxide (as hydrogen carbonate ions), products of digestion such as amino acids and sugars, and hormones.

Red blood cells

Red blood cells (erythrocytes) contain the pigment haemoglobin. Their function is to transport respiratory gases, particularly oxygen, around the body.

Red blood cells (erythrocytes) are disc-shaped and biconcave, and are thinner at the centre than at the edges. They contain the pigment haemoglobin. Their function is to transport respiratory gases, particularly oxygen, around the body. In humans, the red blood cells have no nuclei. They remain in the blood for about 3 months and are then destroyed in the liver or the spleen. Every second, about 1 million old red blood cells are replaced by new ones. One millilitre of blood contains about 5 or 6 million red blood cells.

FIGURE 5.20 Blood can be separated into its two main components in a centrifuge. The clear yellow part is the plasma (55% by volume). The dark red part is the cell fraction (45% by volume), consisting of red blood cells, white blood cells and platelets.

FIGURE 5.21 Red blood cells are 7–8 µm in diameter and 1–2 µm thick.

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White blood cells White blood cells (leucocytes) are of several different types. They all contain a nucleus. There are between 4000 and 12 000 in 1 mL of human blood. Although less numerous than red cells, most are much larger. Two important types of white cell are phagocytes and lymphocytes. Phagocytes can actively move about by flowing their cytoplasm and can move from the blood into the tissue fluid. They surround and ingest bacteria, foreign bodies and dead cells, and collect at areas of infection or injury (Figure 5.23). Lymphocytes act specifically against foreign material. They make antibodies which help the body’s defence against disease (see Chapter 7, pp. 361–363).

Platelets Blood platelets are fragments of cells made in the bone marrow. They are very small—about 3 µm in diameter—and play an important role in helping the blood to clot.

FIGURE 5.22 White blood cells (arrows) among red blood cells.

Lymph and interstitial fluid

streptococcus

Lymph and body or interstitial fluid is blood without red blood cells, platelets and the large plasma proteins. In the lymph vessels a large number of lymphocytes made in the lymph glands are added to the lymph.

Transporting substances in the blood Table 5.5 summarises how the blood carries various substances to every cell in the body.

FIGURE 5.23 A phagocyte engulfing a Streptococcus bacterium.

TABLE 5.5 Substances transported in the blood.

Substance

F ro m

To

F o rm

C a rr i e d b y

Oxygen

lungs

body cells

oxyhaemoglobin

red blood cells

Carbon dioxide

body cells

lungs

mainly hydrogen carbonate ions

red blood cells and plasma

Waste nitrogenous material

liver and body cells

kidneys

mostly as urea

plasma

Water

digestive system and body cells

body cells

water molecules

plasma

Salts

digestive system and body cells

body cells

as ions in the plasma

plasma

Other products of digestion

digestive system and liver

body cells

as separate molecules, e.g. glucose, amino acids

plasma

Maintaining a balance 229

Each haemoglobin molecule contains four active sites where oxygen molecules can be attached. There are 200–300 million molecules of haeomoglobin in every blood cell.

BIOFACT Haemoglobin contains iron. A lack of iron in your diet makes you anaemic (tired and pale) because your body is not able to supply its cells with enough oxygen and, consequently, energy. Carbon monoxide is a colourless, odourless gas that can be present in car exhaust fumes. It binds to haemoglobin at the same site as oxygen, and cannot be removed once attached. Inhaling too much carbon monoxide can be fatal, because it starves the cells of oxygen.

film of water

Oxygen transport Mammalian cells need a lot of energy, and they must have a continual supply of oxygen for respiration. Oxygen from the air diffuses into the blood in the lungs and is transported in the circulatory system to all body cells. Oxygen diffuses across the respiratory surfaces of the lungs into the blood because it is in a higher concentration in the air than in the blood (Figure 5.24). Oxygen is not very soluble in water (see Chapter 1, p. 5). Blood is a watery liquid; 100 mL could carry just 0.2 mL of oxygen if it relied only on oxygen being dissolved in the plasma. In mammals the red blood contains haemoglobin. Each haemoglobin molecule contains four active sites where oxygen molecules can be attached. Haemoglobin molecules are found in red blood cells, and their presence increases the oxygencarrying capacity of the blood 100 times, to about 20 mL of oxygen in 100 mL of blood. This ability to transport large quantities of oxygen to the tissues gives mammals a considerable adaptive advantage. In the lungs, when the oxygen concentration is high, oxygen combines with haemoglobin in the red blood cells to form oxyhaemoglobin. Each haemoglobin molecule combines with four oxygen molecules. in lungs haemoglobin + oxygen → oxyhaemoglobin Hb + 4O2 → Hb(O2)4 Oxygenated blood is bright red. It is transported to the tissues where oxygen levels are low. At this lower concentration the reverse reaction occurs, and the oxygen released diffuses into the body cells.

lung alveolus

in tissues oxyhaemoglobin → haemoglobin + oxygen Hb(O2)4 → Hb + 4O2

O2 interstitial fluid capillary wall cell

blood plasma

alveolus wall cell

red blood cell

Carbon dioxide Carbon dioxide occurs in high concentrations in the body tissues. It diffuses into the circulatory system where it may be carried in the blood in any of three ways. About 70% of this carbon dioxide combines with water to form hydrogen carbonate ions in the red blood cells. These are then carried in the plasma. Carbon dioxide

CO2

FIGURE 5.24 An oxygen molecule diffusing into a mammal’s lungs must pass across the following layers: • a surface film of water on the alveolus • cell membranes of an alveolus cell • cell membranes of a capillary cell wall • cell membranes of a red blood cell wall.

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+ water → carbonic acid → hydrogen ions + hydrogen carbonate ions

+ H2O →

H2CO3



H+

+

HCO3–

About 23% of the carbon dioxide combines with haemoglobin to form carbaminohaemoglobin. (This does not prevent the reaction between oxygen and haemoglobin.) The remaining 7% is dissolved directly in the plasma. At the respiratory surfaces, where carbon dioxide levels are low, the reverse processes occur and carbon dioxide diffuses out from the blood and across the respiratory surface to the external environment. In animals, gaseous exchange is assisted by the active movement or ventilation of air or water past the respiratory surface. In mammals this is called breathing.

Measuring blood gases When your blood is well oxygenated, your skin looks pink; but if you are seriously under-oxygenated your skin will look bluish (a condition called cyanosis). It isn’t possible to tell exactly how oxygenated you are by the colour of your skin. In hospitals it is routine to use a pulse oximeter to monitor the oxygen saturation of the blood in patients undergoing any procedure that requires anaesthesia or sedation, or those whose breathing or circulation is abnormal. This involves a device like a peg that sits on the finger and measures the transmission of light through the tissues. An alarm rings if the oxygen saturation falls below a certain level, usually 90%. If a patient shows signs of dangerously low oxygen levels or high levels of carbon dioxide, blood is taken from an artery for laboratory measurement of these gases. Symptoms of low oxygen: • cyanosis • visual hallucinations. Symptoms of high carbon dioxide: • drowsiness

• bounding pulse • headache • tremors.

Blood donation and transfusion The collection, storage and transfusion of blood is strictly regulated by health authorities. Donors are screened for general health, past medical history and risk of viral infections such as HIV (AIDS). Usually, 470 mL of blood is taken. It is collected in a plastic bag containing an anti-clotting agent, and stored for a maximum of 35 days. Before use, the blood is classified according to the antigens ABO and rhesus, and antibodies to hepatitis B and C. Compatibility testing is carried out before a blood transfusion to make sure there are no antibodies in the recipient’s blood that react with the donor’s red cell antigens. Whole blood is used only to replace massive rapid blood loss, and in exchange transfusions. The various components of the blood can be separated from whole blood so that precise replacement of the patient’s requirements is possible. The components include red blood cells, platelets, granulocytes, clotting factors VIII or XI, and plasma.

Artificial blood

FIGURE 5.25 A pulse oximeter monitors blood oxygen saturation and pulse rate during surgery.

1 Explain why monitoring blood gases in hospital patients is important.

The loss of large volumes of blood is treated by the administration of fluids, such as normal saline or dextrose, into a vein. Normal saline is a solution of sodium chloride with the same concentration as blood and other tissues (0.9%). Dextrose is 4% glucose in a 0.18% saline solution. These solutions are used as a substitute for plasma to maintain the blood pressure—they do not supply oxygen to the tissues. Recently, blood substitutes have been developed, and two of these—modified haemoglobin and perfluorochemicals—are being trialled for human use. These blood substitutes are chemically treated to sterilise them, do not need to be cross-matched to the patient (thus saving valuable time in an emergency) and can be stored for more than a year. The Australian Red Cross is an humanitarian organisation that provides many community services, including the collection and distribution of blood and blood products. Blood is donated free and stored at a Red Cross Blood Bank. Visit www.giveblood.redcross.org.au to find out more about blood donation.

2 Outline the problems associated with blood transfusions and the administration of fluids such as saline to patients who have lost a large amount of blood.

Maintaining a balance 231

The structure of blood vessels There are three types of circulatory vessels: arteries, capillaries and veins.

Blood flows through a system of tubes or vessels. These are not simply inert pipes: they are under the influence of the nervous system and can control the flow and distribution of blood to the tissues. connective tissue ARTERY

muscle wall epithelial layer

epithelial layer CAPILLARY

elastic layer connective tissue muscle wall VEIN epithelial layer FIGURE 5.26 The structures of arteries, veins and capillaries.

elastic layer

FIGURE 5.27 A cross-section of a capillary carrying red blood cells. Capillaries are so small that only one red blood cell can pass through at a time.

There are three types of circulatory vessels: arteries, capillaries and veins. Blood flowing to the body cells from the heart travels through arteries. These become progressively smaller until they form a network of capillaries surrounding the cells of the body tissues. Veins leave the capillary network and return blood to the heart. 232

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Arteries Arteries are thick-walled, elastic and muscular. Elastic fibres in the walls allow the vessels to expand and recoil with each heart beat. These pulse waves maintain the pressure on the blood, sending it in spurts towards the body tissues. You can feel a pulse where an artery runs just below the skin, especially on the wrist and neck. The fibres in the walls of large arteries are more elastic than those in smaller arteries (arterioles). They have more muscle fibres, which (by relaxing and contracting) can control the diameter of the vessel and thus the flow rate of the blood.

Arteries are thick-walled, elastic and muscular.

Capillaries Capillary walls are an extension of the inner layer of arteries and veins. They are only one cell thick and the diameter—between 7 and 10 µm—is so narrow that red blood cells must pass through in single file and may be bent as they do so. The capillaries surround the tissue cells, and no cell is very far from a capillary. They provide a very large surface area over which the exchange of materials between the blood and the body cells occurs.

Capillaries provide a very large surface area over which the exchange of materials between the blood and the body cells occurs.

Veins The capillaries unite to form small venules, then larger veins which return the blood to the heart. The walls of veins are thinner than those of arteries, with less muscle and a wider diameter tube. The blood flows at a much lower pressure after passing through the capillary network. It is kept moving by one-way valves within the veins and by muscles pressing on the veins (Figure 5.28).

Veins return blood to the heart. They are less muscular than arteries and have a wider diameter.

tendon

valve closes and prevents backflow of blood

Lymph vessels

FIGURE 5.28 Movement of blood in veins.

Within the tissues is a separate network of lymph capillaries which unite to form larger vessels similar to veins. Lymph vessels have no muscle in the vessel walls but do have a system of one-way valves (Figure 5.29). Many run between body muscles, whose contractions help keep the lymph moving. At various points along the vessels are lymph nodes or glands. The lymph vessels in the villi of the small intestine are known as lacteals. Fatty acids, glycerol and small fat droplets are absorbed into the cells lining the small intestine after lipid digestion and reformed as chylomicrons which enter the lymph.

The heart The heart is a muscular pump which keeps the blood circulating around the body. The mammalian circulatory system is known as a double

FIGURE 5.29 Lymphatic valves.

Maintaining a balance 233

The mammalian heart consists of four chambers: the right and left atria (singular atrium) and the right and left ventricles.

BIOFACT The heart has a ‘pacemaker’ region of special muscle cells in the right atrium. These contract spontaneously and send nerve impulses through the two atria, causing them to contract together. Contraction of the two ventricles follows. The heart rate is then controlled by the body’s nervous system which acts on the pacemaker to slow down or speed up the rate.

circulation system since, on every circuit of the body, blood passes through the heart twice. The route taken is body–heart–lungs–heart– body. The mammalian heart consists of four chambers: the right and left atria (singular atrium) and the right and left ventricles (Figure 5.30). The heart is divided into two halves by the septum, and blood in the right and left sides does not mix. The upper chambers or atria receive blood from the veins. When the lower chambers or ventricles contract, they send the blood at high pressure around the body. The one-way flow of blood through the heart is controlled by valves at two points. The atrioventricular valves are like flags anchored by fibrous cords between the atria and ventricles. The semilunar valves are cup-shaped and are found at the openings of the arteries. aorta pulmonary artery

systemic vein (superior vena cava) valves

coronary artery

BIOFACT A well-conditioned athlete may have a heart rate of around 40 beats per minute, or even less. The heart pumps the same amount of blood through the system at a greatly reduced rate because: ● its capacity (size) increases with exercise; and ● its strength increases, so it pumps more efficiently.

carotid artery pulmonary artery lung

right atrium right ventricle systemic vein

liver

left atrium left ventricle aorta

intestine kidney

legs FIGURE 5.31 Blood circulation in humans. Note that it is usual for an animal to be drawn as if it is on its back, facing you. This is why the right side is on the left of the drawing.

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right atrium

left atrium

coronary artery

left ventricle right ventricle

valve right ventricle systemic vein (inferior vena cava)

pulmonary artery pulmonary veins left atrium valve thick wall of left ventricle aorta

FIGURE 5.30 The human heart.

head and arms

lung

right atrium

aorta

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The human heart normally beats between 60 and 80 times a minute at rest. This can increase to well over 100 times a minute during exercise. Blood enters the heart from the body via the systemic caval veins into the right atrium. When the atrium contracts it pumps blood to the right ventricle. Contraction of the ventricle closes the tricuspid valve so blood cannot flow backwards, and the blood is pushed out via the pulmonary arteries to the lungs. Here gaseous exchange occurs and the blood picks up oxygen. The now oxygenated blood returns to the heart by the pulmonary vein and enters the left atrium. It passes to the left ventricle and on contraction leaves the heart by the aorta to circulate to all parts of the body.

The pulmonary circuit The pulmonary or lung circuit provides the flow of blood from the heart to the lungs and then back to the heart (Figure 5.31). Blood flow here is under lower pressure than in the systemic circuit, and the rate of flow is faster. Little body fluid is formed in this circuit. The blood has just returned from the body and contains high carbon dioxide concentrations and low oxygen levels. It loses its carbon dioxide in the lungs and collects oxygen, and returns to the heart as bright red oxygenated blood.

The systemic circuit The systemic circuit or body provides the flow of blood from the heart to the rest of the body, except the lungs, and its return. Blood flows under high pressure from the contraction of the large left ventricle but is gradually slowed down by the many large blood vessels in the circuit. The blood pressure forces some fluid out of the blood to become part of the body fluid. The oxygenated blood gives up its oxygen as it reaches the tissues. Any ions or nutrients required by cells also leave the blood, and waste products of metabolism such as urea and carbon dioxide enter the blood. The blood returns to the heart, now dark red in colour, in the veins. Blood flowing through the kidneys loses its urea and has its water and salt composition balanced (see p. 247). Blood flowing to the intestines collects the products of digestion. These are carried first to the liver, where the level of many circulating substances is controlled.

Carbon dioxide is produced as a result of respiration. When it dissolves in water, it forms carbonic acid.

BIOFACT At rest, humans breathe in and out at a rate of 10 to 14 times a minute. The normal pH of the blood is 7.4. Although we can consciously hold our breath and deliberately vary the rate and depth of our breathing, we mostly rely on the automatic regulation of breathing by the control centre in the brain.

BIOFACT

The role of the liver After food is absorbed it is carried in the blood to the liver, where its fate depends on the needs of the cells. The liver is important for the control of the level of many circulating substances and for the breakdown of unwanted compounds. For example, the liver controls the level of glucose circulating in the body. As the demand for glucose for respiration increases, more is released into the bloodstream. If we have enough glucose, the excess may be stored in the liver as glycogen. Excess amino acids usually cannot be stored. They are converted to urea (which is removed from the body) and an acid which can be used. This process is called deamination. The liver has numerous other functions: ● It produces bile which is stored in the gall bladder and released into the small intestine during digestion. Bile is sent from the liver to the small intestine when lipid is present. ● It regulates blood cholesterol levels and stores some of the excess lipids we eat. It also stores vitamins A, B and D, as well as potassium, copper and iron. ● It breaks down many unwanted or potentially poisonous substances, including alcohol. ● It breaks down old red blood cells and regulates some hormones.

Blood and lymph are kept in circulation by: ● the heart muscle, which pumps the blood around the body ● the contraction and relaxation of muscles and elastic fibres in the walls of the blood vessels, particularly the arteries ● one-way valves in the veins and lymph vessels which prevent the backflow of blood ● the contraction of body muscles which press on the blood vessels and help keep the blood moving.

Circulation of lymph The fluid that bathes the body cells is returned to the blood circulation via the lymphatic system (Figure 5.32). This is a one-way system that begins in very small vessels in the tissues. These join up to form larger and larger vessels. At two points at the base of the neck on the right and left sides of the body, the lymph is emptied into two large veins and taken to the heart. FIGURE 5.32 The lymphatic system.

Maintaining a balance 235

Gases in our body Response rate of breathing increases Effector rib muscles and diaphragm Control system breathing control system in brain

Negative feedback CO2 levels reduced

Receptor receptors in aorta and carotid arteries Stimulus increase in CO2 in blood

Living cells need oxygen for respiration (see Chapter 1, pp. 15–16) and carbon dioxide is produced as a result of respiration. When it dissolves in water, it forms carbonic acid. In body fluids, including the blood, the presence of acid lowers the pH. To maintain the normal pH balance carbon dioxide must be removed quickly. In mammals, any change in blood pH is monitored by receptors in the medulla of the brain and the walls of the aorta and carotid arteries in the neck. Nerves send messages to the breathing control centre in the medulla and, in response, the rate and depth of breathing are altered. Too much carbon dioxide (i.e. too low a pH of the blood) causes the rate and depth of breathing to increase. Low levels of carbon dioxide lead to a slowing of the rate and depth of breathing (Figure 5.33).

FIGURE 5.33 Feedback mechanism for carbon dioxide regulation.

Transpor t mechanisms in plants The xylem of flowering plants consists of xylem vessels, tracheids, fibres and parenchyma.

Plants have two transport systems: the xylem and the phloem. Xylem transports water and mineral ions upwards from the roots to the leaves. Phloem transports organic materials, particularly sugars, up and down the stem to other parts of the plant. (see Chapter 2, p. 88). In young plants the xylem and phloem are within the vascular bundles, separated by the actively dividing cambium cells. In the stem, cambium divides and the cells produced differentiate to form xylem to the inside and phloem to the outside.

pit tracheid

The structure of xylem

FIGURE 5.34 A tracheid.

The xylem of flowering plants consists of xylem vessels, tracheids, fibres and parenchyma. The vessels and tracheids transport water and mineral ions (xylem sap) for the plant. They differentiate to form long tubes. Vessels may be up to several metres in length. As they develop, lignin is deposited in their cell walls, in spiral, ring or net patterns, strengthening them and making them impermeable to water. Pits develop in the walls to enable water and solutes to pass through. Pits are not holes but round or oval areas where the cell wall is very thin. Tracheids form tubes of long, thin, overlapping cells (Figure 5.34). Water passes from one to another through the pits. Xylem vessels are larger and the end walls of the cells making up the vessel break down to form a continuous tube (Figure 2.66, p. 87).

vessel member

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cell wall

plasma membrane

plasmodesma

(a) (b)

FIGURE 5.35 The three pathways for water transport through xylem. (a) Water moves from cell to cell by osmosis, leaving one cell across the membrane and cell wall and crossing the cell wall and membrane of the next cell. (b) Water moves in the cell cytoplasm or symplast from one cell to the next through tiny channels or plasmodesmata in the cell walls (see also p. 53). (c) Water moves along the cellulose cell walls or apoplast, not through the cells.

(c)

symplast

apoplast

In the final stage of development the cytoplasm disappears and the cell dies, leaving a system of strong tubes to transport water and provide a framework of support for the plant. The tubes are often surrounded by the xylem fibres which provide additional strength and support. Water flows more rapidly through xylem vessels than through tracheids because the tubes are larger and, being continuous, offer less resistance to movement.

Entry of water into a plant Water enters plants through the root hairs and has to travel across the cortex of the root to the xylem. There are three possible routes, and water may take one or more of them on its pathway (Figure 5.35).

Transport of materials in xylem Water can rise in the xylem at the rate of 15 metres per hour. This rise is against gravity and is not brought about by any pumping mechanism in the plant. Instead, the passive upwards movement depends on transpiration and the physical properties of water itself (Figure 5.36). Evaporation of water from leaf cells through the stomates initiates the pull of the transpiration stream (see p. 87). Water is drawn up the xylem tubes to replace this loss (see Figure 5.37a). Movement through narrow tubes by capillarity, the attraction (cohesive force) between water molecules themselves, and the adhesive force between water and the cellulose cell walls transmit the pull along the length of the xylem tubes.

water travels further up narrow tubes

Root pressure Internal fluid pressure in the roots can sometimes cause water to rise up the stem in the xylem (see Figure 5.37b). At night mineral ions may be actively taken in through the roots, but transpiration is low. Pressure builds up and water is pushed up the stem. It can be seen in some plants as drops on the ends of the leaves early in the morning. This is known as guttation. It does not, however, play a major role in the transport of water in the xylem.

FIGURE 5.36 The very strong surface tension of water enables it to rise higher in narrower tubes. This effect is called capillarity.

Maintaining a balance 237

active transport passive diffusion

Transpiration

evaporation from leaves

capillarity cohesive forces

Root pressure

internal fluid pressure active uptake of salts osmotic uptake of water

osmotic uptake of water (a)

(b)

FIGURE 5.37 (a) Movement of fluid through xylem vessels is largely due to transpiration. (b) In some plants, under some circumstances, internal fluid pressure in the roots (root pressure) causes fluid to rise up through xylem vessels.

The branching network of xylem vessels ensures water is transported to all parts of the plant. The xylem vessels can be thought of as water pipes. The pits in the sides of the walls, however, mean that they are leaky pipes. As the water is pulled upwards, some leaks out through the pits, either across into another vessel or out into the surrounding tissues. Water flows upwards faster, and with less leakage, in larger xylem vessels. In the smaller vessels, particularly the fine branching veins, the rate of flow is less and more water is lost by leakage.

The structure of phloem

FIGURE 5.38 Longitudinal section of phloem showing sieve cells.

The phloem of flowering plants consists of phloem fibres, phloem parenchyma, sieve cells and companion cells. The sieve cells or sieve elements are elongated cells joined end-to-end to form a series of connecting tubes (Figure 5.38). Where a sieve cell joins to the next sieve cell there is no cell wall, but a specialised membrane with pores in it known as the sieve plate. A mature sieve cell has only a thin layer of cytoplasm pressed against the cell wall, containing few organelles. The large internal space is for the conduction of organic materials or phloem sap. The companion cells are linked to the adjacent sieve cells through the cell walls by many fine connecting tubes or plasmodesmata. Companion cells are thought to take on many metabolic functions for the sieve element.

Transport of materials in phloem Organic materials including sugars, amino acids and hormones are transported by the phloem. This movement is called translocation (Figure 5.39). It enables a plant to distribute resources wherever they are 238

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plasma membrane

cell wall (apoplast)

plasmodesma

mesophyll cells

water active uptake by uptake osmosis of sugars

sieve-tube cell companion cell

phloem parenchyma cell bundle sheath cell MESOPHYLL VASCULAR BUNDLE symplast FIGURE 5.40 The two possible ways in which nutrients are loaded into the phloem: symplastic loading through plasmodesmata, and apoplastic loading through the cell walls.

needed, but especially to the growing points and reproductive structures, including developing fruits and seeds. Flow of materials in the phloem is an active process that requires energy. It occurs by a mechanism known as the source–path– sink system and is driven by a pressure gradient generated osmotically.

active uptake of sugars from phloem bud

stem

active uptake of sugars from phloem root

FIGURE 5.39 Movement of fluid through phloem.

Phloem loading at the source

Once loaded at the source, the materials flow towards a sink. A sink is a region of the plant where sugars and other nutrients are actively being removed from the phloem. It might be in the roots, stem, flowers, or storage areas of the plant. As the sugars are actively taken out from the phloem, water flows out with them. This reduces the pressure in the sieve cells at the sink region. This pressure difference between the source and sink ends of the phloem tubes drives the phloem sap flow (Figure 5.41). The direction followed by the phloem sap depends on where the sink areas of the plant are operating. Having reached a sink area and been unloaded, the transported nutrients are either used in metabolism or stored.

xylem vessel

phloem sieve tube

iration stream transp

Phloem unloading at the sink

The phloem of flowering plants consists of phloem fibres, phloem parenchyma, sieve cells and companion cells.

pressure flow

In the leaves, sucrose is made in the process of photosynthesis. Amino acids may be synthesised in the leaf using nitrogen brought up in the xylem. Both of these products plus other mineral nutrients from the xylem are ‘loaded’ into the phloem in the leaves. There are two theories as to how this may occur (symplastic loading and apoplastic loading), although it is likely that both methods are used by plants (Figure 5.40). In symplastic loading, sugars and other nutrients move in the cytoplasm from the mesophyll cells to the sieve elements through plasmodesmata. This theory requires there to be many plasmodesmata between leaf cells. In some plants many have been found, but not in all. In apoplastic loading, sugars and other nutrients move along a pathway through the cell walls until they reach the sieve element. They then cross the cell membrane to enter the phloem tube. Materials would pass into the sieve element by active transport. As the sugars enter the phloem, the concentration of phloem sap increases and the osmotic pressure at this ‘source’ end of the phloem tube is high.

source cell sink cell

movement of sucrose movement of water

FIGURE 5.41 Pressure flow in a sieve tube.

Maintaining a balance 239

The formation of wood

cortex vascular cambium primary xylem primary phloem

In plants that form wood, the vascular bundles unite by division of the cambium to form a continuous ring. New xylem and phloem cells produced after the ring has formed are known as secondary xylem and phloem (Figure 5.42). Each year new xylem cells are produced as the old ones become blocked. The parenchyma cells deposit compounds such as oils, tannins, resins and gums into the xylem tubes through the pits. This process is known as tylosis. The tubes stop transporting water, and the deposits harden and become known as heartwood. Actively conducting secondary xylem is known as sapwood. Growth rings can be seen in the wood of many shrubs and trees. These represent the xylem that was produced in one growing season. The age of trees can be estimated by counting the growth rings (Figure 5.43).

primary phloem secondary phloem primary xylem secondary xylem cortex vascular cambium cork cambium FIGURE 5.42 Secondary growth in woody plants arises from secondary meristematic tissues, the vascular cambium and cork cambium.

(a)

(b) FIGURE 5.43 It is the particular patterns of xylem vessels, tracheids, fibres and rays in different types of tree that give wood its characteristic appearances. These differences affect both its appearance and its strength. (a) In some trees, for example Castanea, early wood (spring) has wider thin-walled tracheids than late wood (late summer). (b) In other species such as Tilia, xylem tissue laid down throughout the growing season is uniform.

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Questions 1

Complete the table below, summarising the composition of the blood.

Blood component

Description Role

Plasma Red blood cells

6

An oximeter is a device used to measure the oxygen saturation of the blood. Explain why it is important to monitor blood oxygen saturation during surgery.

7

Summarise the pathways and processes involved in the upward movement of water in plants.

8

Outline how the theories of symplastic loading and apoplastic loading are thought to account for the movement of materials into the phloem at the leaves.

9

Draw up a table comparing the xylem and phloem in the transport system of plants. Your table should include ● the kinds of materials transported ● the direction of flow ● whether the conducting tissue is living or nonliving ● a brief description of the structure of the tissues ● whether the process of transporting materials requires energy or not.

White blood cells Platelets

2

For each of the following substances carried in the bloodstream, identify the a form in which it is carried b the blood component that transports it. oxygen carbon dioxide water salts nitrogenous wastes products of digestion

3

a Why do cells require oxygen? b Explain why carbon dioxide must be removed from cells.

4

Red blood cells contain the pigment haemoglobin. Explain how haemoglobin assists red blood cells in their specialised function.

5

Complete the following table, comparing the structure and function of blood vessels.

Vessel

Diagram

How structure is re l a t e d t o f u n c t i o n

Artery Vein Capillary

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F u r ther questions 1

Red blood cells contain no nuclei. They survive in the blood for about three months. Using your knowledge of cell structure and function, explain the connection between these two pieces of information.

2

The diagram illustrates the structure of the mammalian heart. h a

b

6

a Outline the complications sometimes associated with blood transfusions. b Modified haemoglobin that has been extracted from red blood cells is sometimes used as an alternative to blood transfusions. This kind of treatment is especially useful in treating patients in emergency situations. Outline the advantages of this.

7

Research and write a magazine-style article about blood donation in NSW which answers the following questions: ● What products are extracted from donated blood? ● How are these products used? ● Name the diseases for which donated blood is checked. ● Who can be a blood donor? ● What is the role of the Australian Red Cross Blood Bank?

8

Investigate a disease of the circulatory system. Choose from the list below, or consult with your teacher before deciding on a different disease. high blood pressure atherosclerosis angina coronary heart disease cardiomyopathy For the disease you have chosen: ● describe how the condition affects the heart and the rest of the body ● outline the symptoms suffered by a patient. ● summarise the possible causes ● describe the treatment, including dietary recommendations. The National Heart Foundation’s website (heartfoundation.com.au) may be a useful place to begin your research.

9

a What is ringbarking? b Explain why ringbarking kills trees. c Suggest a way of treating a tree that has been ringbarked, so that it has a chance of surviving.

c

g

d

f e

Identify parts (a) to (h). Use arrows to show the path taken by the blood as it travels through the heart. 3

Haemoglobin is the oxygen-carrying pigment present in the blood of mammals. a Describe the chemical structure of haemoglobin. b Describe how the shape of red blood cells enhances their oxygen-carrying capacity.

4

Describe how the chemical composition of blood changes as it is transported around the body. In your answer, name the tissues in which the changes take place.

5

Foetal haemoglobin is different from the haemoglobin present in the bloodstream after birth. a Find out how the structure of foetal haemoglobin differs from the structure of adult haemoglobin. b Haemoglobin is an efficient oxygen-carrying molecule. Why is there a difference between foetal and adult haemoglobin?

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5.3 Regulation of substances OBJECTIVES When you have completed this section you should be able to: ● outline the role of water in cells and explain why the concentration of water in cells should be maintained within a narrow range for optimal function ● compare the role of the kidneys in fish and mammals ● explain why diffusion and osmosis cannot adequately remove nitrogenous wastes ● explain the role of passive and active transport in the mammalian kidney ● explain how filtration and reabsorption in the nephron contribute to stable body fluid composition ● summarise the role of aldosterone and antidiuretic hormone in regulating water and salt balance ● define enantiostasis and explain its importance in regulating water and salt balance in estuarine organisms ● outline some adaptations that minimise water loss in Australian plants.

Water in cells Water is essential for life. It makes up between 70 and 90% of living organisms. It is the solvent in which most substances dissolve and is the transport medium for distributing them. Water is the solvent for all the metabolic reactions in living cells. It takes part directly in many of them and is formed as a product in many others, including respiration. Living cells work best in an isotonic environment—one in which the solute concentration is the same both inside and outside the cell. They are very sensitive to changes in solute concentration and may lose or take in large amounts of water by osmosis if the concentration in their external environment changes too much. Should this happen, cells usually die. Living organisms try to ensure the water balance is maintained in their cells and the concentration of solutes is held constant so that the cells can function properly (Figure 5.44).

activities ● ● ● ● ● ●

Dissection of a kidney Renal dialysis Replacing hormones Water conservation in animals Water conservation in plants Salt regulation in plants

Maintaining a balance 243

Water is the solvent for all the metabolic reactions in living cells. It takes part directly in many of them and is formed as a product in many others, including respiration.

In large multicellular terrestrial mammals such as humans, the interstitial fluid that bathes their cells is kept isotonic to the internal solute concentration of the cells. HYPERTONIC SOLUTION H2O

BIOFACT The three water compartments of the body are: ● cellular fluid—approximately 65% of body fluids ● interstitial fluid and lymph— approximately 28% of body fluids ● plasma—approximately 7% of body fluids

ISOTONIC SOLUTION H 2O

HYPOTONIC SOLUTION

H 2O

H2O

ANIMAL CELL

shrivelled

normal

H2O

H2O

lysed H2O

H2O

PLANT CELL

FIGURE 5.44 Water balance in cells.

plasmolysed

flaccid

turgid

The removal of wastes Proteins are made up of amino acids. When amino acids are broken down, ammonia is formed. Ammonia is poisonous to cells and must be removed immediately or converted to a less toxic substance such as urea or uric acid. The main waste products of humans are urea and carbon dioxide.

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As a result of all the metabolic processes that occur in cells, waste products are constantly being formed. If they were allowed to accumulate in cells and tissues they would slow down metabolism and poison the cells. Ammonia, the nitrogenous waste product from protein metabolism in cells, is highly toxic and needs to be removed as quickly as possible or converted to a less harmful form. Different animals excrete different waste products. There is a correlation between the type of wastes produced and the animal’s environment. Aquatic animals, fish and invertebrates mostly excrete ammonia. Ammonia is toxic, but can be released continuously and directly into the water and is quickly dispersed. On land, however, animals need to conserve water. By converting ammonia into less toxic forms, they can hold it for longer in the body and excrete it periodically.

The role of the kidney The fish kidney The primary role of the kidneys is osmoregulation—the regulation of the water and salt concentrations in the body. In fish, excretion of nitrogenous waste products (ammonia as NH4+) occurs across the gills. The kidneys adjust the levels of water and mineral ions in the fish’s body in order to maintain a constant concentration of internal fluid for the cells (Figure 5.45).

The primary role of the kidneys is osmoregulation—the regulation of the water and salt concentrations in the body.

wa

wa

te

r

r

(a)

te

Bony fish living in fresh water maintain a higher concentration of solutes in their body than the concentration in the water outside (that is, they are hypertonic to their surroundings). Water therefore tends to diffuse into the body and so fish need to continually get rid of the excess. Their kidneys produce copious amounts of very dilute urine in an almost continuous stream in order to achieve this. As fresh water has a lower concentration of ions than the fish do, the kidneys actively reabsorb salts to prevent their loss.

rarely drink water

t un e mo urin a ge te lar dilu of

constantly drink water and salts

. ncl ts i ium sal mon am

Proteins are made from amino acids. They are made, used and broken down in cell metabolism. Mammals are unable to store amino acids, so any excess becomes nitrogenous waste to be removed. These excess amino acids are transported to the liver where they are broken down by a process called deamination. This involves removing the part containing nitrogen (the amino group, –NH2) to form urea. The remainder is converted to a carbohydrate which may be stored (as glycogen) or used immediately. Urea is transported by the blood to the kidneys and excreted in the urine.

(b)

r

Deamination

ter

The kidneys of mammals regulate the internal salt and water concentrations of the body, and excrete urea, the nitrogenous waste produced by mammals.

wa

The mammalian kidney

s unt mo ll a urine a sm of

wate

Saltwater fish Bony saltwater fish have the opposite problem. Their internal body fluids are less concentrated than the surrounding water. To avoid water loss from their body, marine fish keep drinking salt water. They absorb the water and salts. The water is retained and the salts actively excreted, some via the gills and some via the kidneys. Saltwater bony fish excrete very little urine. Marine cartilaginous fish (sharks and rays) have their tissues isotonic with the sea water so there is no net movement of water in or out. In this way they avoid the osmoregulation problems of bony fish.

sa am lts in mo cl. niu m

Freshwater fish

FIGURE 5.45 (a) The internal environment of freshwater fishes is more concentrated than their external environment. They tend to lose salts and gain water. (b) Marine fishes have an internal environment that is more dilute than their external environment and so tend to gain salts and lose water.

The kidneys of mammals regulate the internal salt and water concentrations of the body, and excrete urea.

Maintaining a balance 245

Renal dialysis Renal dialysis is an artificial process in which wastes in the blood are removed by diffusion across a partially permeable membrane. Dialysis helps individuals whose kidney function is so impaired that products of metabolism, including urea, creatinine and uric acid, build up in the body instead of being eliminated. High concentrations of these substances can cause symptoms such as tiredness, weakness, loss of appetite and vomiting. The level of creatinine in the blood is often used as a measure of the degree of kidney failure. If both kidneys stop functioning due to disease, patients experience end-stage renal disease (ESRD) or total kidney failure—an immediate life-threatening condition. The number of Australians needing treatment for kidney disease increased by 26.5% in just four years, from 1993 to 1997, and the number of new cases of ESRD each year has more than doubled in the past 15 years. The two most common causes of renal disease are glomerulonephritis (inflammation of the kidney filters) and diabetes mellitus. Other causes of kidney failure are kidney stones, polycystic kidney disease, severe hypertension, and drugs. People who have lost their kidneys (for example, due to injury or cancer) or who have suffered severe kidney damage are kept alive by dialysis. There are two forms of dialysis: haemodialysis and peritoneal dialysis.

Haemodialysis Haemodialysis was first used to treat human patients in 1945. It removes excess waste products and water from the patient’s blood, which is drawn from a vein and passed into a dialysis solution. The blood moves through plastic tubing to the dialyser, which is a bundle of hollow fibres made from partially permeable membrane. The membrane allows wastes to pass through but stops red and white blood cells, platelets or proteins from doing so. This is similar to the filtration stage of normal kidney function (Figure 5.45). Here diffusion takes place from blood to the dialysis solution and vice versa. The dialysis solution has similar ion components to blood, but without any waste products. After the diffusion process, the cleaned blood is returned to the body. To prevent the blood from clotting during dialysis, the anti-clotting agent heparin is added to the blood as it moves through the dialyser. Haemodialysis can be used only for 4 or 5 hours at a time, about three times a week, because it is dangerous to use high quantities of heparin, because 1 List three examples of occasions when renal dialysis may be required.

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blood cells may be damaged as they pass through the plastic tubes, and because there is an increased risk of infection.

artery

to dialyser

superficial vein bubble trap

from dialyser

dialyser membrane

fresh dialysing solution

constant temperature bath

used dialysing solution

FIGURE 5.46 In haemodialysis, blood is drawn from a vein and passed through plastic tubing to a dialyser. Wastes are filtered out by passing the blood through a partially permeable membrane.

Peritoneal dialysis Unlike haemodialysis, peritoneal dialysis is undertaken inside the body. A dialysis solution is introduced into the peritoneal (abdominal) cavity through a catheter. The natural membrane lining of the peritoneal cavity is a partially permeable membrane, so waste products and excess water from the body can pass through the membrane into the dialysis solution. The solution is drained from the abdomen into a disposable collection bag. This process is carried out daily, using four lots of dialysis solution totalling about 2 litres in volume. Each lot of solution is allowed to take up wastes for about 4 hours.

dialysis solution inflow

dialysis solution outflow

catheter

FIGURE 5.47 In peritoneal dialysis, a catheter is inserted into the peritoneal (abdominal) cavity to introduce fresh dialysis solution, and then to drain it when wastes have been removed from the blood.

2 Briefly describe the difference between haemodialysis and peritoneal dialysis.

Structure of mammalian kidneys Kidneys are compact, bean-shaped structures (Figure 5.48). They produce urine, a yellowish fluid composed of materials removed from the blood as it passes through them. Urine leaves the kidneys via the ureters and is stored in a muscular bag, the bladder. The bladder expands as it fills with urine. At a certain point this expansion stimulates nerve endings in the bladder walls which send a message to the brain. The brain sends a message to the sphincter muscles surrounding the base of the bladder, which relaxes so that urine can pass through the urethra. This is usually under the conscious control of the mammal. glomeruli in cortex

Internal structure of the kidney Mammals have two kidneys. Each kidney is made up of about one million small filtering units called nephrons. It is in these structures that urine is produced. The structure of a nephron is shown in Figure 5.49. The starting point is a Bowman’s capsule, a small cup-shaped structure situated in the cortex. This leads to a narrow, convoluted tube which makes a loop in the medulla back up to the cortex and then joins a collecting duct or tubule. The collecting duct transports urine to the pelvis of the kidney, which leads to the ureter. The nephrons are surrounded by a dense network of capillaries.

cortex medulla renal artery renal vein pelvis ureter pyramid

FIGURE 5.48 The internal structure of a human kidney.

The formation of urine The kidneys continuously process an enormous volume of blood to form a small volume of urine. There are three processes in the formation of urine: filtration, reabsorption and secretion.

Filtration Blood is brought to the kidneys by the renal artery. This divides into smaller vessels which, when they reach a Bowman’s capsule, form a network of capillaries called a glomerulus (see Figures 5.49, 5.50). The blood pressure is so high in the glomerulus that some of the liquid from the blood is forced through the walls of the blood vessels into the Bowman’s capsule. This liquid consists of blood plasma, but contains no blood cells or large plasma proteins. Small soluble molecules pass through by a process of passive filtration. This liquid is known as the glomerular filtrate. The filtrate contains some substances that the body can re-use and some that are wastes. They are all forced into the first (proximal) part of the nephron tubules. Along the length of the tubule the composition of the filtrate is carefully adjusted until it contains only unwanted substances. It is then called urine. Filtration is a non-selective process. Filtrate passing into the Bowman’s capsule contains blood plasma that includes: • water • nitrogenous wastes such as urea • food materials such as glucose, amino acids, vitamins and minerals • other ions such as bicarbonate • other ingested substances such as penicillin and aspirin • hormones.

branch from renal artery blood returns to renal vein glomerulus filtration occurs here

Bowman's capsule

distal tube salts reabsorbed

proximal tube

glucose and water absorbed

collecting duct

nephron tubule loop of Henle water reabsorbed

to pelvis of kidney

FIGURE 5.49 A nephron.

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Reabsorption

branch of renal artery pyramid

collecting tube

medu lla

cortex

glomerulus and Bowman's capsule

pelvis of kidney FIGURE 5.50 The production of urine: a section through the cortex and medulla of a kidney.

BIOFACT Human kidneys filter about 204 litres of fluid from the blood each day. From this, about 1 litre of urine is produced and the rest is reabsorbed. In humans, approximately 1300 mL of blood enters the kidneys each minute and 1299 mL leaves. The difference is the amount of urine we produce—about 1 mL per minute.

Surrounding each nephron tubule is a large capillary network. As the filtrate travels down the tubule, materials the body can re-use are reabsorbed into the blood. These materials are the nutrients and other essentials—glucose, amino acids, some vitamins, minerals, bicarbonate and water. Reabsorption is an active process that requires energy. It occurs in the proximal and distal parts of the tubule and the loop of Henle.

Secretion Secretion is a selective process by which the body actively transports substances from the blood into the nephron. This occurs in both the proximal and distal parts of the tubule.

Regulation of body fluid composition The nephron is a regulatory unit; it selectively reabsorbs materials required to maintain homeostasis. The readjustments occur as substances are moved in either direction—reabsorption back into the blood or secretion back into the nephron. This regulation helps to maintain the constant composition of the blood and interstitial fluid. In the proximal tubule, most of the bicarbonate ions are reabsorbed and there may be some secretion of hydrogen ions. This helps maintain the constant pH of the blood and body fluids. Drugs such as aspirin and penicillin, and poisons identified by the liver, are actively secreted into the tubule. Nutrients such as glucose and amino acids are actively transported from the tubule back into the blood. Regulation of salts also occurs. Sodium ions (Na+) are actively – transported back into the blood. Chloride ions (Cl ) follow passively. As the salt moves out, water passes by osmosis back into the blood. Potassium ions are also reabsorbed here. In the loop of Henle, in the descending part, the walls are permeable to water but not to salt. Water passes across by osmosis. In the ascending part, the walls are permeable to salt but not to water. Salt passes out passively across a thin-walled section and then actively across a thickwalled portion. The salt passing out makes the interstitial fluid of the medulla area of the kidney quite concentrated. In the distal tubule, selective reabsorption and secretion again occur to adjust the pH of the blood and the level of salts, particularly sodium and potassium. The walls of the collecting ducts are permeable to water but not salt. Water passes out by osmosis, and the final filtrate or urine is formed (Figure 5.51).

Active and passive transpor t Diffusion and osmosis are passive forms of transport across cell membranes. They do not require the expenditure of energy—only active transport requires energy. The materials are very often transported against the concentration gradient (see Chapter 2, p. 62).

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proximal tube Bowman's capsule

nutrients NaCl H2O K+ HCO3–

glomerulus low salt concentration

drugs poisons

water

distal tube H2O

NaCl

K+

HCO3–

H+

collecting duct salt ascending limb

descending limb salt high salt concentration

loop of Henle

water

passive transport active transport

FIGURE 5.51 The function of a nephron and collecting duct.

In the kidneys both forms of transport are used in regulating the body fluid composition. Passive transport occurs in filtration and in the osmosis of water back into the blood. Active transport occurs in the secretion of substances into the nephron, the active transport of nutrients back into the blood, and the selective reabsorption of salts required by the body.

Active transport across membranes Active transport uses energy to pump or carry materials across the membrane which otherwise might have been blocked by the diffusion gradient or their own properties. For example, some molecules cannot cross the membrane if they are too large, if they cannot dissolve in the membrane, or if they carry electric charges or fatty sections that bind them to the membrane. In active transport, specific carrier proteins in the membrane may bind with the substance and carry it through the membrane. Endocytosis is a another type of active transport across a membrane. It involves the formation of a pouch that carries the matter through the membrane. There are three types of endocytosis (see Chapter 2, p. 64).

Active transport uses energy to pump or carry materials across the membrane which otherwise might have been blocked by the diffusion gradient or their own properties.

The endocrine system and hormones The endocrine system consists of ductless glands in the body which secrete hormones (Figure 5.52). Hormones are chemical messengers that travel in the blood. They reach all parts of the body through the blood, but only certain target cells in organs respond to each hormone. They bring about changes in the metabolic activity of the body (Table 5.6). Hormones are kept at a fairly constant level in the blood by feedback systems.

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TABLE 5.6 The roles of hormones.

Role

Description

Control of the internal environment

Hormones maintain homeostasis by regulating the amounts and type of many body chemicals.

Emergencies

Hormones enable the body to cope with stress—physical or emotional.

Growth

Hormones ensure growth and development take place in a smooth, controlled way.

Reproduction

Hormones are involved in reproduction from gamete formation to maintenance of the placenta, birth and nourishment of the newborn.

Hormones are chemical messengers that travel in the blood. They bring about changes in the metabolic activity of the body

pituitary makes growth hormone and several hormones which control the other endocrine glands thyroid makes thyroid hormone pancreas makes insulin adrenals (one on each kidney) make adrenalin ovaries testes

make sex hormones

FIGURE 5.52 The locations and functions of the endocrine glands.

• diuresis is loss of urine • diuretics are substances that increase the volume of urine • alcohol, tea and coffee have a diuretic effect • antidiuretics are drugs that reduce the amount of urine • ADH increases water retention • aldosterone increases salt retention

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Hormonal regulation of water and salt levels When the glomerular filtrate passes through the nephron tubule, the amount of water and salts that is reabsorbed matches the body’s needs. For example, if you drink a lot of liquid, the excess water is excreted as large amounts of dilute urine. But if the body contains little excess water or is losing water rapidly as sweat on a hot day, then the kidneys reabsorb the maximum possible amount of water from the filtrate. The result is that only a small amount of concentrated urine is formed. The amount of water and salts lost in the urine is controlled by hormones. Two hormones—antidiuretic hormone (ADH) and aldosterone— help to regulate salt and water concentrations, as well as blood pressure and volume. The blood which leaves the kidney via the renal vein has had its nitrogenous wastes removed and its water and salt composition balanced. The function of the kidney is therefore twofold: it acts as an excretory organ and removes nitrogenous waste; and it has a homeostatic function, helping to maintain a constant internal composition of the body fluids.

Antidiuretic hormone (ADH) Reabsorption of water is controlled by a hormone in the blood called antidiuretic hormone (ADH). ADH is made in the hypothalamus but stored in and released from the pituitary gland in the brain. Receptors in the hypothalamus monitor the concentration of the blood. If there has been water loss from the body, such as from excessive sweating during exercise, so that the blood is more concentrated than normal, ADH is released into the blood and circulates to the kidneys (Figure 5.53). ADH increases the permeability of the walls of the distal tubules and collecting ducts to water. In the collecting ducts the final concentration of urine is determined, according to how much water is reabsorbed. The collecting ducts run through the kidney medulla, where the interstitial fluids are usually quite concentrated, with high salt levels. In the presence of ADH, water passes freely by osmosis out of the ducts back into the body. As the blood returns to normal concentration by negative feedback, less ADH is secreted. If blood concentration is low—for example, after a lot of water has been drunk—very little ADH is released. The permeability of the collecting duct walls is decreased, less water is reabsorbed, and more is passed out with the urine.

RESPONSE 1 thirst drinking increases water in blood more ADH produced

receptors in hypothalamus hypothalamus

negative feedback – less ADH produced

stimulus – loss of water, concentration falls

water ADH

kidney – water reabsorbed, water balance restored

ADH secreted by pituitary RESPONSE 2

collecting duct

FIGURE 5.53 The role of antidiuretic hormone (ADH).

We have seen that the amount of salt lost in the urine is adjusted according to intake and the body’s needs. The amount of salt in the tissues is important in determining the amount of water lost by the kidneys. Increased salt results in water retention. Water retention, controlled by ADH, reduces the concentration but not the total amount of salt. The amount is regulated by another hormone, aldosterone.

Aldosterone Aldosterone is produced by the adrenal glands, which are situated above the kidneys. If there is an increased blood volume and blood pressure (resulting from high salt concentrations, which causes water to be retained), the output of aldosterone is reduced. Less salt and water is reabsorbed by the nephron tubules and increased amounts of salt and water are lost in the urine. If the body needs salt, the opposite occurs—water is not retained, the adrenals release more aldosterone, and salt is reabsorbed from the tubule.

BIOFACT Lack of aldosterone can result in low sodium levels, high potassium levels and high acid levels in the blood. These are all potentially dangerous conditions, but they can be reversed with hormone replacement therapy using a synthetic hormone called fludrocortisone.

Enantiostasis: survival in an estuar y Why is an estuary special? An estuary is formed where a river meets the sea. In this environment, fresh water draining from the land mixes with saline water from the sea. On the flood tide the sea invades the estuary. On the ebb tide the fresh water invades, the water is shallower, and areas of land, such as mudflats, may be exposed. Estuaries are rich, productive ecosystems. They are nutrient traps: water from the sea and from the river is slowed and sediment Maintaining a balance 251

FIGURE 5.54 Mangroves in the estuary of the Bohle River, near Townsville. Mangroves fringe estuarine areas around much of Australia. They shed 1 kg of leaf litter for every square metre of mangrove tree. This material is broken down by bacteria and fungi. It has been estimated that one teaspoonful of North Queensland mud from a mangrove swamp contains 10 000 million bacteria. Mangroves support complex food webs including many invertebrates such as crabs, copepods, worms and molluscs, and large numbers of fish and birds.

Enantiostasis is the maintenance of metabolic and physiological functions in response to variations in the environment.

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settles. The sediment and decaying organic matter form a rich soup that supports a vast community of organisms. The water is calmer than the sea and much shallower. There is plenty of light for photosynthesis. The sun also heats the shallower water, providing warmer conditions than in the ocean. Many species of fish and invertebrates use estuaries as quiet breeding and nursery areas for their young. Estuaries are also home to some unusual plants, including mangroves and seagrasses (Figures 5.54, 5.55). But for living organisms there is the problem of coping with changes in salinity. There is a salinity gradient in an estuary, with high salinity at the ocean end and low salinity at the other end. There is the daily cycle of tides bringing salt water in and out. There are major periodic changes in salinity, such as at times of huge (king) tides, a flow of flood waters from upstream, or during drought when the amount of water flowing in the river decreases. Bays, sounds, inlets, coastal swamps, marshes and tidal mud flats are other environments where living organisms experience great changes in salinity. Enantiostasis is the maintenance of metabolic and physiological functions in response to variations in the environment. In estuaries this ability is fully tested, and many organisms have a wide tolerance to changes in salinity. Others have a narrower salinity tolerance, and are found only in certain areas—major environmental changes will kill them. Some animals can avoid the changes. Fast-swimming organisms such as fish can move away. Most molluscs can close their shells and wait until the tide returns. Many bottom-dwellers burrow or dig deep into the mud or sand. But plants cannot move. They must tolerate the high salt environment and find ways to cope with the salt entering their tissues. Many Australian estuaries are threatened by human activity, particularly along the eastern coast. In the past, the ecological significance of such areas was not recognised and many of these areas were considered wasteland that needed clearing, dredging or filling in. Only in recent years has their importance to the fishing industry been recognised.

FIGURE 5.55 Seagrasses are flowering plants that grow in estuarine conditions, forming large underwater meadows. They are feeding grounds for many fish and invertebrates, and the nursery for their young. Larger animals, such as turtles and dugongs, inhabit seagrass meadows, and in sheltered bays on the south coast of Australia, pygmy right whales swim over the seagrass. Seagrass meadows are found in bays and inlets all around Australia. In New South Wales they can be found in Botany Bay, Jervis Bay and Lake Macquarie.

Many high-value commercial species, including prawns, oysters and barramundi, spend all or part of their life in estuarine habitats. If these habitats are destroyed, the industries that rely on these resources will be lost. A national inventory has been made of Australian estuaries and enclosed marine waters, and many have been declared as marine and estuarine protected areas, or marine parks.

Maintaining salt concentrations in plants Halophytes are plants adapted to living in salty environments. They are able to tolerate higher levels of salt than other plants, or have special mechanisms to control their levels of salt. Three different mechanisms are salt exclusion, salt excretion and salt accumulation. Salt excluders prevent the entry of salt into their root systems by filtration. This is a passive process that does not use energy and relies on the transpiration stream. It can be very successful. The grey mangrove Avicennia marina can exclude 95% of the salt via the filtration system in its roots and lower stems. Other mangroves that rely on this system are the red mangrove Rhizophora stylosa and the orange mangrove Bruguiera gymnorrhiza. Salt excreters have special salt glands, usually in the leaves. Salt is concentrated here and then actively secreted from the plant. The salt can often be seen, and tasted, as salt crystals on the leaves. Rain washes the salt off. Mangroves such as the grey mangrove Avicennia marina and the river mangrove Aegiceras corniculatum have salt glands, as do saltbushes (genus Atriplex) (Figure 5.56). (a) FIGURE 5.56 (a) Atriplex is a genus of the saltbush family that produces a covering of bladder-like hairs into which salt is excreted at extraordinarily high concentrations (S, stalk; B, bladder). (b) Older leaves have a silvery, glistening appearance due to salt crystals and collapsed bladders.

(b)

Maintaining a balance 253

Salt accumulators concentrate or accumulate salt in a part of the plant, usually the bark or older leaves, which is then shed. The milky mangrove (Exoecaria agallocha) sheds old leaves full of salt. The succulent samphire plant (Sarcocornia quinqueflora), found on salt marshes, accumulates salt in swollen leaf bases which then drop from the plant.

Minimising water loss in plants Xerophytes are plants adapted to arid or dry conditions. Many Australian terrestrial plants show a variety of adaptations to conserve water and minimise water loss. Water storage is shown by the bottle tree (Brachychiton rupestris) and the boab tree (Adamsonia gregorii) that store water in their soft, fibrous trunks. Succulent plants such as the noonflowers or pigfaces (e.g. karkalla (Carpobrotus rossii) and rounded noonflower Disphyma crassifolia), and saltbushes (genus Atriplex), store water in fleshy leaves and stems. Water collection or the ability to obtain as much water as possible is exemplified by the extensive root systems of many xerophytes. These include deep tap root systems to reach deep underground water, and wide, shallow roots to soak up surface moisture. Desert plants are often widely spaced because of root competition below ground. One such species is the mulga (Acacia aneura). Its branches are also arranged so that any rain falling is channelled directly down to the roots. Structural adaptations include features that help minimise water loss from transpiration, such as: ● leaves with a thick or waxy cuticle; for example, the leaves of eucalypts and mangroves ● small leaves with a reduced surface area; for example, the needle-like leaves of Hakea plants, or the replacement of leaves with photosynthetic stems such as in the she-oaks (Casuarina and Allocasuarina) or the flattened leaf stems (phyllodes) of some wattle (Acacia) species (Figure 4.21 p. 165) ● reflective leaf surfaces; these may be pale, shiny, hairy or crystalline (Figure 5.57b) ● hairy leaves that reduce airflow across the leaf thus reducing evaporation; often the undersurface of the leaves or growing buds are covered in hairs (Figure 5.57a) ● stomates sunk into pits or grooves and/or a reduced number of stomates; for example, Hakea and Eucalyptus leaves ● thick bark (as in the mulga) or extra thickening of cell walls to prevent wilting; many Australian xerophytes have leaves that do not wilt ● rolled up leaves to minimise water loss; for example the porcupine grass or spinifex Triodia.

FIGURE 5.57 Two adaptions that reduce water loss in harsh Australian environments: (a) hairy leaves and flowers (Ptilotus species) and (b) a reflective crystalline surface (a liverwort, Riccia crystallina).

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(a)

(b)

junction of leaf edges stomatal grooves

(a) leaf open

(b) leaf rolled

Physiological adaptations alter a plant’s metabolic activities. Examples include: ● leaves hanging vertically that change orientation during the day to ensure that only the edges, not the full surface of the leaves, are exposed to the sun. This reduces both heat absorption and water loss (see Figure 5.18, p. 224). ● the closure of stomates during the hottest part of the day. This is usually associated with plants, such as succulents, that use a different photosynthetic pathway (CAM or C4 photosynthesis) and open their stomates at night to take in carbon dioxide. ● dormancy periods when leaves, or all above-ground parts, die off during hot, dry conditions. Mallee eucalypts for example, die back and regenerate when conditions are favourable from swollen underground lignotubers (Figure 4.64, p. 194) ● tough, hard seeds that can survive long dry periods plus accelerated life cycles may be shown by plants in response to a short wet season in arid environments. Ephemeral plants such as Sturt’s desert pea (Clianthus formosus) germinate, grow, flower and produce many seeds within six to eight weeks after heavy rains. ● tolerance to drying out or dessication. The leaves of the resurrection plant (Borya constricta) can be dry and shrivelled for four to five months, but become green and swollen again after rain.

FIGURE 5.58 Cross-section of a Triodia leaf (a) open in normal conditions, and (b) inrolled during dry conditions.

FIGURE 5.59 Borya constricta, the resurrection plant.

Maintaining a balance 255

Questions 1

a Explain the significance of water in the metabolism of cells. b Why is a constant internal concentration of water important for the proper functioning of cells?

2

Why must metabolic wastes be removed from cells, and from the body?

3

a Describe how the nephron functions to remove metabolic wastes from the bloodstream. In your answer, discuss the site and process of each of the following: filtration, secretion and reabsorption. b State whether or not each of the three processes in (a) is active or passive. Explain your choice in each case. c Outline the differences between the blood entering the kidney and the blood leaving it.

4

Discuss the similarities and differences in the functions of human kidneys and fish kidneys.

5

Summarise the roles of the hormones a ADH b aldosterone in regulating water balance and salt concentrations in the body.

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6

a Define ‘enantiostasis’. b Describe the kinds of environmental variations to which estuarine plants and animals are subjected. c Outline the ways in which some estuarine organisms function to overcome the problems associated with their particular environment.

7

Water conservation is critical for many species of plants living in areas of Australia that have little rainfall and high rates of evaporation. Draw up separate lists of adaptations of Australian plants that help them to a increase their water uptake b reduce their water loss.

8

a What are halophytes? b Describe how the adaptations of some halophytes assist them in surviving in their particular habitats.

F u r ther questions 1

The poisonous gas carbon monoxide is present in tobacco smoke and car exhaust fumes. Why is carbon monoxide poisonous? How would you treat a person who has been overcome by fumes containing carbon monoxide?

2

a List all of the ways in which your body can lose water. b Describe some ways in which your body is able to reduce water loss on hot summer days.

3

During a rough tackle, a footballer suffers a powerful blow to the back. Later he notices blood in his urine. Which tissue in the kidney is likely to have been damaged? Explain why you think so.

4

A young boy buys a freshwater goldfish as a birthday present for his father. He slips the goldfish into his father’s fish tank, which contains marine

fish. The next day the goldfish becomes sluggish and dies. Use your understanding of osmosis and water balance in cells to explain what happened. 5

Mangroves are estuarine plants that are subject to extreme environmental conditions including fluctuating salinity; their roots are permanently inundated with water. a Most plants whose roots become waterlogged die. Explain why this is so. b Describe the features of mangroves that make them well-suited to their harsh environment. Explain how these features allow them to survive in i extremely saline conditions ii waterlogged soils

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Chapter summar y Practical activities 5.1

5.2



Enzymes





Feedback mechanisms Australian ectotherms and endotherms



Changing pH



Investigating blood cells Measuring blood gases Blood products Conducting tissues—xylem and phloem



● ●

5.3



Dissection of a kidney



Renal dialysis



Replacing hormones Water conservation in animals Salt regulation in plants Water conservation in plants







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5.1 • Enzymes are proteins that affect the rate of metabolic reactions. Most enzymes are specific; that is, each one catalyses a particular reaction. Each enzyme works best within particular pH and temperature ranges. • pH is a way of describing the acidity of a substance. • The maintenance of a constant internal temperature enables animals to operate at optimal metabolic efficiency. • Homeostasis is the process by which organisms maintain a relatively stable internal environment. • Homeostasis consists of two stages: detecting and counteracting changes from the stable state. • The nervous system in animals has receptors in sense organs to detect stimuli, and effectors in muscles and glands to respond to environmental changes. • There is a broad range of temperatures over which life is found, but individual species are active in a limited temperature range. • Ectotherms and endotherms react differently to changes in ambient temperature. Ectotherms (for example, invertebrates, fish and reptiles) have limited ability to control their body temperature, and changes of temperature in their environment can greatly affect their metabolism. Endotherms (for example, mammals and birds) can maintain a constant internal temperature. Land animals show structural, physiological and behavioural adaptations to temperature changes. • Plants on land alter their growth rate according to the temperature, and leaves may hang vertically or have reflective surfaces to reduce heat gain from radiation. 5.2 • Plants and animals transport dissolved nutrients and gases in a fluid medium. • In mammals, the circulatory system transports blood and lymph around the body. Blood and lymph are fluids that supply cells with the substances they need and remove unwanted materials such as carbon dioxide and nitrogenous waste. Most substances are carried in the plasma. Oxygen is carried by haemoglobin in red blood cells as oxyhaemoglobin. Some 67% of the carbon dioxide is carried in the plasma as bicarbonate ions, 24% is carried on haemoglobin, and the remaining 9% is dissolved in the plasma. • The adaptive advantage of haemoglobin is that it greatly increases the oxygen-carrying capacity of the blood, enabling large quantities of oxygen to be transported rapidly to cells. • Transfusions of blood or blood products can be life-saving. The Australian Red Cross is an humanitarian organisation that provides many international and local services, including the distribution of blood and blood products. • Arteries are thick-walled, elastic and muscular tubes that withstand and transmit the pressures produced by the pumping heart. They transport blood from the heart to the rest of the body. Veins have thinner walls and are less muscular than arteries. They return blood to the heart. Capillaries form a network of tiny vessels surrounding the cells of body tissues, where substances are exchanged between the blood and the cells.

• Blood changes in chemical composition as it moves around the body. In the lungs oxygen is collected and carbon dioxide is released. Around the body cells oxygen is removed and carbon dioxide picked up, nutrients are lost and nitrogenous waste picked up. Nutrients that are the products of digestion are picked up from the intestines. In the kidneys nitrogenous waste is lost and the water and salt content of the blood is regulated. • Oxygen is required by living cells for respiration. Carbon dioxide must be removed because it combines with water to form carbonic acid, which lowers the pH of the blood (normally 7.4). Body cells will not function if the pH varies. • In plants, water is transported passively upwards by the ‘pull’ of the transpiration stream through narrow xylem tubes. Translocation of materials both up and down a plant in the phloem is an active process that uses a source– path–sink system. 5.3 • The concentration of water in cells is maintained within a narrow range to ensure the optimal functioning of cells. • The removal of wastes is essential for the continued metabolic activity of cells, because their build-up would be poisonous.. • The kidney is the organ of excretion in many animals. It regulates the internal water and salt levels in fish and mammals, and in mammals it also excretes nitrogenous waste. • The processes of osmosis and diffusion rely on small molecules moving along a diffusion gradient. These processes are inadequate for removing dissolved nitrogenous wastes in large multicellular organisms. • Passive transport, such as diffusion, is movement requiring no energy. Active transport requires energy to transport materials. In cells this movement is across membranes. In the mammalian kidney passive transport occurs in filtration and the osmosis of water back into the blood. Active transport occurs in the secretion of substances into the nephron, and in the movement of nutrients and selective reabsorption of salts back into the blood. • The processes of filtration and reabsorption work together to regulate the body fluid composition. Filtration passively separates out small soluble products from the blood, and reabsorption actively takes back those substances to adjust the body fluid composition to its normal levels. • The hormones aldosterone and antidiuretic hormone (ADH) regulate the amount of water and salts reabsorbed back into the body from the nephron tubules. • Enantiostasis is the maintenance of metabolic and physiological functions in response to variations in the environment. For example, organisms that live in estuaries need to be able to cope with varying concentrations of salt in their environment. • Plants need an adequate supply of water for survival. Many terrestrial Australian plants show adaptations to minimise water loss, such as extensive root systems, hard leaves with a thick or waxy cuticle, and reduced leaves.

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EXAM - STYLE QUESTIONS Multiple choice 1 Which statement best describes enzymes taking part in a chemical reaction in a cell? A Enzymes need to be replaced at the end of each reaction. B Enzymes affect the rate of the chemical reaction. C Enzymes are made up of complex lipid molecules. D Enzymes work best at 37°C.

6 Which statement best describes the transport systems in large, multicellular land plants? A They move organic materials produced in photosynthesis via the phloem. B They transport water and dissolved minerals upwards from the roots through the xylem. C They are made up of both living and non-living tissues. D all of the above statements.

2 Which is the best representation of the stimulus–response pathway? A receptor → stimulus → control centre → response → effector B stimulus→ receptor → control centre → effector → response C control centre → stimulus → receptor → effector → response D effector → control centre → receptor → stimulus → response

7 What are the main changes in the composition of blood as it circulates around the body from the heart? A Carbon dioxide and oxygen decrease, and urea increases. B Sodium chloride and ammonia decrease, and oxygen increases. C Urea and carbon dioxide decrease, and glucose increases. D Oxygen and glucose decrease, and carbon dioxide increases.

3 Which of the following is an example of an adaptation to high ambient temperatures by an Australian mammal? A the huddling behaviour of bats to reduce overall heat loss B the migration of birds to avoid temperature extremes C the burrowing behaviour of desert bandicoots during the day D the nocturnal habit of lizards. 4 Feedback mechanisms are important in the homeostatic control of the body’s internal conditions. In a feedback system of control, which of the following is true? A The stimulus alters the original response. B The response alters the original stimulus. C The response reduces the effect of the original stimulus. D The response increases the effect of the original stimulus. 5 A key role of the circulatory system of mammals is to transport oxygen to the body cells. How is oxygen carried in the blood of mammals? A as dissolved oxygen in the blood plasma B in the white blood cells C through an extensive network of veins D in the form of oxyhaemoglobin

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8 An athlete who has just completed a 20 km run will have lost an excessive volume of water through sweating. To regulate water and salt balance, which of the following occurs? A Dilute urine will be produced. B An increased volume of urine will be produced. C ADH will be released into the bloodstream, increasing the reabsorption of water from the collecting ducts in the kidney. D Vasopressin will be released into the bloodstream, decreasing the reabsorption of water from the collecting ducts in the kidney. 9 Mangroves are plants that grow in conditions of varying salinity. The adaptations of mangroves to harsh estuarine conditions represents an example of which of the following? A homeostasis B narrow environmental tolerance limits C halophytosis D enantiostasis. 10 Which of the following statements best describes the type of adaptation to reduce water loss shown by plants growing in areas of water scarcity? A the ability to close stomates during very hot conditions B leaves with a thick, waxy cuticle C hairs on leaves D all of the above.

Short answer 1

Enzymes are the biological catalysts of many chemical reactions that occur in cells. Most enzymes are highly specific. a Explain what is meant by the term ‘catalyst’. b What is meant by the ‘specificity’ of an enzyme? c Name one other characteristic of enzymes.

2

a To what environmental factor are hibernating animals responding? b Describe two advantages gained by animals as a result of hibernation.

3

6

Trypsin is a digestive enzyme secreted by the pancreas of vertebrate animals. A biologist studying enzyme activity isolated trypsin from a mammal and two different species of fish. She tested the enzyme activity at different temperatures. Her experimental results are illustrated in the graph.

Enzyme activity (trypsin)

Study these diagrams of two blood vessels. trout

perch

dog

inner layer

muscle and elastic fibres

0

B

a Which diagram represents an artery? Give two reasons for your answer. b Describe a feature of capillaries that enables them to provide materials to cells efficiently. 4

5

A biologist investigating the transport of materials in plants introduced radioactive carbon dioxide (14CO2) into a closed chamber containing a leafy shrub. The shrub was subjected to a light source and allowed to photosynthesise. After 2 days the distribution of 14C was assessed. a Name the vascular tissue in which you would expect the 14C to appear. Explain why you think so. b There are two current theories that attempt to explain how sucrose formed in photosynthesis is ‘loaded’ into conducting tissue at the leaves. Name and outline one of these theories. Choose one native Australian plant adapted to arid conditions that you have studied in this course. a Describe the features of the plant that suit it to conditions of water scarcity. b Explain how the features you have described assist the plant in minimising water loss.

15

20 25 30 Body temperature (°C)

35

40

a What is meant by the optimum temperature of an enzyme? b What is the optimum temperature for trypsin in the i trout ii perch iii dog? c Describe what happens to the activity of trypsin after 10°C in the trout, 28°C in the perch and 34°C in the dog. d Fish are ectotherms. Suggest a reason for the difference in optimal temperature of the enzyme between the trout and the perch.

non-elastic fibres A

10

7

Consider the stimulus–response pathway that enables the body’s nervous system to detect and respond to changes in temperature. stimulus ↓ receptor ↓ control centre ↓ effector ↓ response a Use the stimulus–response pathway above to help you explain how the body detects and responds to a decrease in environmental temperature. Name the stimulus, receptor, effector and response.

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b Choose an example of an Australian endotherm you have studied. Describe one behavioural response displayed by the organism in relation to i increased external temperature ii decreased external temperature. 8

When whole blood is allowed to settle in a test tube, two fractions separate. A clear liquid fraction, 55%

B cell fraction, 45%

a What is the name given to the fluid fraction A? b Name one substance carried in this fluid. c Name the two types of cells in the cell fraction B. d State one function for each of the two cell types you named in (c).

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9

a Outline one reason why water is important for living cells. b Why is the removal of metabolic wastes important for ongoing cell function? c Aquatic animals usually excrete the metabolic waste ammonia directly into the watery environment in which they live. i What special problem do land animals have that aquatic animals do not, in terms of metabolic waste disposal? ii Complete the table summarising metabolic waste disposal in different terrestrial organisms.

O rganism F o rm in which nitrogenous wastes a re excreted

Advantage of excretion in t h i s f o rm

Mammals Insects

d Describe one difference and one similarity in the function of human kidneys and fish kidneys.

Chapter 6

BLUEPRINT OF LIFE

No organism lives forever. So for a species to continue to be successful it must be able to pass on its characteristics to a new generation. Each generation passes on the chemical information contained in its chromosomes—its DNA. What is it about our DNA that not only makes us human but also gives us specific characteristics such as blue eyes or brown hair? Early scientists did not know exactly what or where the inheritable information was. The discoveries of the structure of DNA and the behaviour of chromosomes during cell division have provided us with knowledge of the genetic code. As our understanding of the code increases, so does our ability to manipulate it. Modern biotechnology uses various techniques such as cloning and the production of transgenic species. These techniques are increasingly used to alter the genetic information and therefore the characteristics transferred from one generation to the next. Much of this research is directed towards increased yield and longevity of food products. The segregation and independent assortment of genetic information, combined with fertilisation in sexual reproduction, produces variation that acts as the raw material for natural selection. When environments change, some variations may be selected, leading to species becoming better suited to their environments. Mutations—changes in the DNA information on chromosomes—are another source of variation. If a mutation occurs during the reproductive process then the new information will be passed on to the next generation. Most mutations, however, are either lethal or harmful. Identifying mutations, their causes and effects is an important area of scientific research. With increased knowledge we may be able to prevent harmful mutations and counteract their effects.

This chapter increases students’ understanding of the history, nature and practice of biology, and the applications and uses of biology, implications of biology for society and the environment and current issues, research and developments in biology.

6.1 The evidence for evolution OBJECTIVES When you have completed this section you should be able to: ● explain how changes in physical and chemical conditions of the environment have affected the evolution of plants and animals ● describe evidence supporting the theory of evolution, including palaeontology, biogeography, comparative embryology, comparative anatomy and biochemistry ● outline the key steps in Darwin’s theory of evolution by natural selection ● explain how the theory of natural selection accounts for the evolution of species ● distinguish between convergent and divergent evolution.

activities ● ● ● ● ●

Modelling natural selection Change in a species Vertebrate forelimbs Technology and change Theories of evolution

The Earth is an ever-changing place. It has been changing since its formation about 4600 million years ago, and it will continue to change until the Sun becomes a nova and the Earth is returned to the interstellar dust from which it was formed. Change occurs at every possible scale, and at every possible rate. For example, the type and abundance of gases in the atmosphere are slowly changing, as they have for thousands of millions of years; continents continue to move and become drier, colder, wetter or warmer; seas, forests and deserts come and go; and water, wind, earthquakes and volcanoes reshape the land. How do these changes affect species, and how do species cope with change?

Environmental change Evolution is the process of change that occurs in living organisms over many generations. It is the result of natural selection by the environment of favourable variations in the organisms.

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Evolution is the change in living organisms over many generations. Changes in the environment of living organisms can lead to the evolution of plant and animal species. These environmental changes in condition may be physical, such as temperature changes, or chemical, such as changes in water salinity. Change may also be triggered by competition—for example, competition for resources such as food and water, or competition to reproduce.

A plague of rabbits Rabbits and myxomatosis Since their introduction to Australia in 1859, European rabbits have become an ecological and economic pest. In 1950 the myxoma virus was introduced to attempt to control their numbers. The virus is carried by fleas or mosquitoes and causes the disease myxomatosis in rabbits. In the first few years after its release, 99% of infected rabbits died. By 1953 the death rate from the disease was 95%. But 10 years later the death rate from a new outbreak of the infection had fallen to 50%, which is the level that still occurs today. Why did this happen? Firstly, some rabbits—perhaps less than one in a thousand—survived the first infections because they had a greater resistance to the disease than other rabbits. If one rabbit in a thousand survived, its offspring could inherit this resistance, so the numbers in the population of rabbits resistant to myxomatosis would increase. This is just what has happened. Secondly, the virus itself has decreased in virulence. That is, strains have developed which no longer kill rabbits. It had been hoped to exterminate rabbits from Australia by this method, but in most areas the number of rabbits eventually increased again.

Rabbits and calicivirus A second method to control rabbits has been introduced into Australia—calicivirus, or rabbit haemorrhaging disease, which affects only rabbits. Calicivirus is spread by direct contact between rabbits, and causes death within 24 hours of infection. The virus was accidentally released from an island in South Australia where it was being tested in 1995. It spread quickly through South Australia and western New South Wales to all other states.

The official release was in 1996, and by 1999 major reductions in rabbit abundance were reported in almost all regions. In arid areas up to 90% of rabbits were killed, while in wetter regions about 65% were killed. There has been significant regeneration of native trees and shrubs, particularly in dry areas. Each season rabbits are caught and inoculated with the virus and released, to spread the disease to the new generation of rabbits. In September 2000 came the first news of rabbits caught in Victoria that were carrying antibodies that made them immune to calicivirus. By 2002 the proportion of rabbits killed by calicivirus remained at 90% in dry areas but was down to 40% in wetter areas. The reasons for this difference are currently being investigated. In 2003 it was announced that a new, cheaper way of spreading the disease each season to newborn rabbits was being introduced. The calicivirus is applied to carrots and these are spread around rabbit warren areas. The virus remains viable long enough for rabbits to eat the carrots and become infected. Check out the Australian Academy of Science’s articles and useful sites on this topic at: www.science.org.au/nova/001/001sit.htm

The future Scientists are currently developing a genetically modified strain of the myxoma virus that will make infected rabbits sterile. This work is being conducted by the Cooperative Research Centre for Biological Control of Pest Animals. The Centre’s strategy and latest results can be seen at: www.pestanimal.crc.org.au

FIGURE 6.1 The European rabbit ran wild in every corner of Australia until the introduction of myxomatosis.

1

Predict what might eventually happen to the relationship between the rabbit and the calicivirus.

2

Predict the likely success of a method using modified myxoma virus as a contraceptive.

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Environmental change: two case studies Physical change: peppered moths in England Separate peppered moth populations are usually either pale or dark in colour. In unpolluted forests, pale moths are well camouflaged on the pale, lichencovered tree trunks. Dark moths are more conspicuous. The birds that prey on these moths eat more of the conspicuous dark moths because they are more visible, so the pale moths survive and reproduce, and therefore dominate the population. In polluted forests near industrial cities, where the tree trunks are blackened, the dark moths have an advantage in colour. (The light-coloured moths are more likely to be seen by birds on the tree trunks, so they are more likely to be eaten.) After many

(a)

years of selection, dark moths have had a higher survival rate in the polluted environment, and so have become dominant.

Chemical change: mosquitoes and DDT When DDT was first used to kill mosquitoes, low concentrations were extremely effective. In subsequent sprayings, low concentrations were ineffective and stronger doses were needed. The few mosquitoes which survived the first spraying had a natural resistance to DDT and this was passed on to their offspring. The population changed from mainly susceptible to mainly resistant due to selection by the DDT in the environment.

(b)

FIGURE 6.2 Peppered moths. Light and dark moths on (a) burnt bark and (b) lichen-encrusted bark.

Evidence for evolution Palaeontology Palaeontology—the study of fossils—provides evidence that living organisms have changed over time (see Chapter 3, p. 116).

Transitional forms Transitional forms are examples of organisms that indicate the development of one group of organisms from another or from a common ancestor.

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Transitional forms are examples of organisms that indicate the development of one group of organisms from another or from a common ancestor. They help us understand how evolutionary change may have come about. The fossil record suggests that the modern groups of vertebrates appeared in the following order:

jawless fish bony fish amphibians reptiles birds mammals

500 400 360 300 190 150

million million million million million million

years years years years years years

TABLE 6.1 Features that the crossopterygian shared with fish and amphibians.

ago ago ago ago ago ago

Features shared with fish amphibians

The theory that they developed from each other or from common ancestors is also supported by fossil evidence. Many intermediate types have been found which show the transition from one major group to another.

scales

lobe-fins (useful on land)

fins

lungs

gills

Example 1 Fish that could absorb oxygen from air appeared 400 million years ago at the end of the Devonian age. It is thought that amphibians developed from fish along this line of descent. The crossopterygian fish (Figure 6.3) had bones in its fins, which suggest it could ‘walk’ (drag itself) on land. FIGURE 6.3 The crossopterygian fish had bones in its fins, which suggested it could ‘walk’ on land.

bones in fins

Example 2 Mammal-like reptiles, which had a jawbone between that of reptiles and mammals, appeared in the late Permian (Figure 6.4). There is also evidence that these mammal-like reptiles (called therapsids) were warm-blooded—another feature shared with some dinosaurs and mammals. The therapsids provide evidence that the mammals developed from reptiles. The fact that present-day monotreme mammals (platypuses and echidnas) lay eggs, as reptiles do, also supports this theory.

TABLE 6.2 Features that Archaeopteryx shared with reptiles and birds.

Features shared with re p t i l e s b i rd s long tail

wish-bone

claws

feathers

no keel solid bones teeth (a)

(b)

(c)

FIGURE 6.4 The jawbones of (a) a reptile, (b) a therapsid and (c) a mammal.

TABLE 6.3 Features that seed ferns shared with ferns and gymnosperms.

Example 3 A small flying dinosaur with feathers (Archaeopteryx) appeared in the late Jurassic. This dinosaur had feathers, and shared other features with both birds and reptiles. The existence of Archaeopteryx supports the theory that birds developed from reptiles (see Figure 6.5).

Features shared with f e rn s g y m n o s p e rm s leaves

life history

Example 4 The fossil record suggests that modern groups of true land plants appeared in the following order:

general appearance

naked seeds

seed ferns, lycopods, horsetails, ferns gymnosperms angiosperms

400 million years ago 300 million years ago 185 million years ago

habit of growth stems

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wish-bone teeth solid-bones

long tail

FIGURE 6.5 The features of Archaeopteryx suggest that birds developed from reptiles.

feathers claws

While the origin of the angiosperms is not well documented, the seed ferns appear to be ancestors of present-day gymnosperms. Seed ferns are now extinct, but fossil specimens show that they had a fernlike appearance, and they were at first classified as ferns. However, naked seeds can be seen attached to the leaves, and all the evidence suggests that their life history was similar to that of gymnosperms.

Biogeography Biogeography is the study of the distribution of living things.

FIGURE 6.6 A fossil seed fern, showing the seeds on the margins of the leaves.

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Biogeography is the study of the distribution of living things. Particular types of plants and animals are found in certain continents and not others. For example, the animals and plants of Asia and Australia are very different. This phenomenon was first described by Alfred Wallace, who suggested a line (now called Wallace’s Line) to separate the distribution of these organisms. Other scientists, such as Richard Lydekker, later suggested other separating lines (Figure 6.7). It is believed Australia’s unique mammals and angiosperms result from periods of evolution in isolation. By looking at the pattern of distribution of an organism today, plus its fossil distribution in the past, we are able to reconstruct its evolutionary history.

Philippines Lydekker's Line Borneo Indonesia

New Guinea

Wallace's Line

Australia FIGURE 6.7 Wallace’s Line and Lydekker’s Line represent two ideas about the boundary between the faunas of Asia and Australia.

The trans-Pacific distribution of waratahs Waratahs are flowering plants belonging to the family Proteaceae. They all have similar flowers which are pollinated by birds. There are three genera of waratahs, with a distribution spanning the southern Pacific Ocean. Embothrium (Chilean firebush)—includes one species; grows in forests of South America, in countries such as Chile and Argentina. Oreocallis—includes four species; distributed on both sides of the Pacific; two species occur in New Guinea and north-eastern Australia; the other two occur in Peru and Ecuador. Telopea—includes four species; confined to eastern Australia; three species occur on the mainland (including the New South Wales waratah, Figure 6.8), and one in Tasmania. The present-day distributions of these closely related species—in the eastern parts of Australia and New Guinea and the western parts of South America— suggest that the two regions may have been connected in the past, and were separated by the opening of the Pacific Ocean.

(a) FIGURE 6.8 (a) Waratahs provide evidence that Australia was once connected to South America. (b) The distribution of the three genera of waratahs.

Oreocallis Telopea Embothrium (b)

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Comparative embryology All chordates possess pharyngeal (throat) gill pouches at some stage of their development. Only in fish and amphibian larvae do these develop into gills.

FIGURE 6.9 The basic vertebrate pattern of six pairs of aortic arches is modified during later development in different vertebrates. The adult fish has a pattern of circulation similar to the basic pattern. In frogs and humans, several aortic arches have been lost with the loss of functional gills and the development of lungs for gas exchange. Dotted lines show vessels that have been lost during development.

Comparative embryology is the study of embryos of different organisms, looking for similarities and differences between them. The similarity between the embryos of different vertebrate species suggests a common ancestry. All chordates possess pharyngeal (throat) gill pouches at some stage of their development. Only in fish and amphibian larvae do these develop into gills. Why do human embryos possess gill pouches? We know that life on Earth began in an aquatic environment, and it is likely that we inherited them from an aquatic ancestor.

Shark • single ventricle • single circuit gills

body

body

pump

;;;;

Frog • single ventricle • double circuit lungs pump

to head

gill opening

to body

; ;;

body

lungs

pump pump

to head

aortic arches

Human • double ventricle • double circuit

to head

;;; ;;;

to body (aorta) to lungs

to lungs to body V heart heart heart A from body from body from lungs from body from lungs

Comparative anatomy Comparative anatomy is the study of the differences and similarities in structure between different organisms. The structures they have in common are evidence of similar inherited characteristics from common ancestors. Modern-day vertebrates are easily grouped into classes because they possess quite distinct features. However, many underlying similarities suggest they are more closely related to each other than appearances might suggest. The structures they have in common are evidence of similar genes which have been inherited from common ancestors. Example 1—the pentadactyl limb Most land vertebrates show a similar basic pattern in the bones of their arms and legs (Figure 6.11). This pattern is called the pentadactyl (five-digit) limb. It is believed they inherited this from their aquatic ancestors, the lobe-finned fish. FIGURE 6.10 The embryos of four vertebrates in three comparative stages of development are remarkably similar. (a) pig, (b) cow, (c) rabbit and (d) human. (These drawings by the zoologist Ernst Haeckel were published in the nineteenth century.

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Example 2—xylem Structural similarities between some plant groups suggest that they shared a common ancestor. Ferns, conifers and flowering plants all have vascular tissue, including conducting vessels (xylem) which transport water throughout the plant. In a leaf they are found in the ‘veins’. This suggests that these groups of plants had a common ancestor.

Most land vertebrates show a similar basic pattern in the bones of their arms and legs. This pattern is called the pentadactyl (five-digit) limb.

reptile

bird

amphibian

lobe fin bat

whale

Similar structures in organisms that have a common origin, regardless of their function, are known as homologous features.

human

FIGURE 6.11 The pentadactyl limb is evidence that present-day vertebrates are closely related. They are thought to have inherited this structure from an ancient aquatic ancestor.

Biochemistry Studies of a wide range of animals have found that many possess similar molecules, which is further evidence of common ancestry. The degree of similarity reflects genetic closeness. Some of the molecules studied are haemoglobin, RNA and hormones. When studying proteins, scientists look at the amino acid sequences as a clue to relationships. These studies show that humans appear to be most closely related to the chimpanzee, then the gorilla, gibbon, monkeys and other mammals. Other studies are based on the compatibility of blood when it is mixed. Closely related animals have a small antigen–antibody reaction. These studies help clarify the groups into which animals should be classified. Today the DNA of organisms can be compared directly using DNA sequencing methods. The nucleotide sequences making up genes and chromosomes can be analysed and compared. The greater the similarity the closer the relationship between the organisms concerned. Blueprint of life 271

DNA hybridisation

BIOFACT Hybridisation studies of the DNA of primates, supported by studies of haemoglobin, have shown that humans are more closely related to chimpanzees than to gorillas. Orangutans are a ‘sister species’ to all three other primates—they were the first to diverge from a common ancestor. The current view of the evolution of primates is as follows: Ramapithecus Orangutans Gorillas Humans Chimpanzees According to many anthropologists, this means that the family of ‘apes’ includes humans: we are not only desended from apes—we are apes!

DNA hybridisation can be used to identify similarities in DNA structure. The following method, known as chemical hybridisation, is used to compare DNA molecules from different species. 1 Two strands of DNA are separated using heat. 2 The single strands formed are mixed with single strands from another species. 3 The two different strands will join to form a hybrid molecule. However, not all pairs of bases will match. This is because there are differences in the sequence of bases for each species. 4 The degree of pairing depends on the similarity of the sequences in the DNA from the two different kinds of organisms. A high degree of pairing will occur if the two sequences are very similar. A low degree of pairing will occur if the two sequences are very different. Species which have diverged recently from a common ancestor will be expected to show a high degree of hybridisation because their DNA sequences will be very similar. However, species which have diverged from a common ancestor a very long time ago would show less hybridisation.

DNA from species 1

heat hybrid DNA strand

DNA from species 2 heat

BIOFACT DNA sequencing techniques now allow scientists to analyse and compare genes, chromosomes and even whole genomes of different organisms. The major method of sequencing was developed by Frederick Sanger in 1974. He received the Nobel Prize in Chemistry in 1980 for this work. Using the Sanger sequencing method, sections of DNA 500 to 800 bases long can be ‘read’. Advances in the 1980s and 1990s made it possible to automate Sanger’s method and speed up the process. DNA databases are now established and work continues to map the genomes of living organisms. The Human Genome Project released a draft of the human genome sequence in 2000 and completed the project in 2003.

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low complementarity high complementarity heat applied to separate DNA molecules into two strands

base pairing causes strands of DNA to align with complementary DNA

FIGURE 6.12 DNA hybridisation. This technique can be used to identify similarities in DNA structure. Note that the closeness of the two hybrid DNA strands can also be estimated by measuring the temperature at which they separate. The more bonds formed, the higher the temperature needed to separate the strands.

Other evidence for evolution The age of the Earth Scientists believe there has been life on Earth for over 3500 million years. During that time the continents on Earth have changed location and shape and environmental conditions have changed. Geologists divide the Earth’s history into geological eras and periods. The evolution of living organisms, such as the horse, can be traced over this geological time scale (see Table 3.2, p. 120).

Domesticated animals and cultivated plants Selective breeding by animal and plant breeders creates changes in species. From the variations in each generation, humans select the features they prefer in plants and animals, and eliminate others. For example, the cultivated carrot has a much larger root than the wild carrot that still occurs in natural environments.

Evolution by natural selection In Chapter 4, the theory of Charles Darwin about how change occurred in living organisms was described (see p. 157). Evolution occurs by a process of natural selection, in the following steps:

FIGURE 6.13 Domestic dog breeds, such as the Afghan hound, have been developed to combine practical features, such as the ability to hunt by sight, with features pleasing to humans, such as height, tail shape and fur colour.

1 In any population there are differences (variations) between individuals; all the members of one species are not identical. 2 In any generation there are offspring that do not reach maturity and reproduce; the characteristics of these organisms are removed from the population. 3 Those organisms that survive and reproduce are well adapted to that environment; they have favourable variations (survival of the fittest). 4 Favourable variations are passed on to offspring; they become more and more common in the population.

Convergent evolution Natural selection over many generations can result in similar adaptations in species that live in similar environments. This process is called convergent evolution. For example, seals and dolphins live in the open oceans. They have limbs modified as flippers, they are strong swimmers, and they hold their breath and stay under water longer than most other mammals. They also have a thick layer of fat under the skin to help conserve their body heat in cold water, and they eat fish. But despite these similarities, they belong to different orders of mammals— seals are no more closely related to dolphins than they are to humans. Their similarities are the result of evolutionary convergence through natural selection in a similar marine environment. Similarly striking examples of convergence can be found in any environment. For example, desert organisms, flying animals and animals living underground often show remarkable similarities.

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(a)

FIGURE 6.14 (a) The Australian marsupial sugar glider and (b) the American flying squirrel have many features in common, although they are not closely related. They are an example of convergent evolution.

(b)

Divergent evolution Divergent evolution (also called adaptive radiation) is the process that begins with one species and produces organisms that look different from each other because they have evolved from isolated populations in different environments. Charles Darwin described 14 species of finches on the Galapagos and Cocos Islands. They all had greyish-brown to black feathers and had similar calls, nests, eggs and courtship displays. However, their habitat and diets were different; they had different body sizes and their beaks varied in size and shape (see Table 6.4). Had the 14 species been created separately, or could they have evolved from a common ancestor? Darwin believed they had evolved from a common ancestor. On each island, the forms that had survived were those most suited to the food resources available (Figure 6.15). It was examples such as this that led him to suggest the process of natural selection to explain how such differences could have come about.

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woodpecker finch

mangrove swamp finch

2. Tree finches

3. Warbler finch

vegetarian finch large insect-eater

medium insect-eater 4. Cocos Island finch

small insect-eater

FIGURE 6.15 The origin of Darwin’s finches.

1. Ground finches (cactus-eating and seed-eating)

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TABLE 6.4 Darwin’s finches.

Type of finch

Examples

Number of species

Diet

Habitat

Ground finches

6

seeds

arid coastal

Tree finches

6

insects*

moist forests

Warbler type

1

small insects

bushland (arid or humid)

Cocos Island

1

small insects

tropical forests

* one species vegetarian

Questions 1

Outline, using a named example, how changes in the physical or chemical conditions in the environment can lead to changes in a species.

4

Use Figure 6.10 to explain how the embryology of organisms provides important information in our understanding of evolution.

2

Explain how transitional forms of organisms in the fossil record support the theory of evolution. Outline two animal examples and one plant example.

5

Study the forelimbs of the vertebrates shown in Figure 6.11. Explain what your observations suggest about the relationship between the different organisms.

3

a Explain what is meant by biogeography. b Outline how the biogeography of the waratah lends support to the theory of evolution suggested by Alfred Wallace. c Is the evolution of the waratah an example of divergent or convergent evolution? Explain your answer.

6

a Explain how biochemistry is used to analyse evolutionary relationships. b Outline how biochemical studies suggest a relationship between humans and other mammals.

7

Explain the difference between convergent evolution and divergent evolution. Use examples to illustrate your answer.

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F u r ther questions 1

Study the following table, which illustrates the percentage difference in the nucleotides in the DNA of humans and other primates. a Identify the species most closely related to humans. Identify the species most distantly related to humans. b Use the information to draw an evolutionary tree that reflects the relationships shown in the table.

Species tested against human DNA

2

Discuss the effectiveness of the myxoma virus and calicivirus in controlling rabbit populations in Australia. Include a the reasons for release of the virus b the short-term and long-term effectiveness of the myxoma virus c i the short-term effect of calicivirus—gather recent data to quantify your results ii the predicted long-term effectiveness of calicivirus (give reasons for your answer here). d potential problems associated with the use of these forms of biological control.

4

Suggest which of the following areas of study provide the most reliable evidence for evolutionary pathways. Explain why you think so.

P e rcentage d i ff e rence

Human

0.0

Chimpanzee

2.4

Gibbon

5.3

Green monkey

9.5

Capuchin monkey

3

15.8

palaeontology, biogeography, comparative embryology, comparative anatomy, biochemistry

Explain why a strain of virus that causes sterility in rabbits is more likely to have long-term effectiveness compared to viruses that cause illness and death, such as the myxoma virus and calicivirus.

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6.2

Mendel and the inheritance of characteristics OBJECTIVES When you have completed this section you should be able to: ● outline the experiments and procedures used by Mendel in his study of heredity in pea plants ● describe and interpret monohybrid crosses involving simple dominance ● explain the difference between homozygous and heterozygous genotypes ● use examples to distinguish between allele and gene ● distinguish between dominant and recessive genes and explain how these relate to the phenotype of an organism ● understand the significance of Mendel’s experiments and explain why his work was not recognised for so long.

Variation: environment or inheritance? activities ● ● ●

Family trees Genetics problems Hybridisation

Humans change their environment to make it as suitable as possible for themselves and their activities. In doing so we usually ignore the impact of our actions on other organisms. By changing the environment we change the selecting agents operating on other populations. This may change the direction of evolution for those organisms and for ourselves.

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The theory of evolution is supported by scientific evidence from many sources, but the theory by itself does not describe exactly how characteristics are passed on from one generation to the next. There are two causes of variations in populations: environment and inheritance. Environmental variation occurs because of the conditions that an organism experiences during its life-time. An animal may be small and thin because it has been unable to find food to grow larger. A tree may have bent branches because of the prevailing wind direction. Both of these are examples of environmental rather than inherited features shown by an organism. Environmental variation can change during the life of an organism. For example, an organism can grow larger if food resources become abundant, and a tree can become straight if prevailing winds become gentler. Inheritance is fixed for life; it is the genetic information an organism obtains from its parents. Every population shares a collection of genes by interbreeding. There are different genes for each feature, and every

individual possesses a unique combination of these genes. For example, in humans there are several genes for height, eye colour and hair colour, and even one for tongue-rolling (Figure 6.16). A person who is tall might also be blue-eyed or brown-eyed with dark or blonde hair. Your combination of features was produced by chance events during meiosis and fertilisation. How is the inheritance of these different features controlled? We know that genes in the chromosomes carry this information, and that it is passed on to the next generation in reproduction. If we want to find out more precisely how inheritance works we need to carry out controlled breeding experiments. Humans do not make the best subjects for studying inheritance for a number of reasons. For example, we can’t ask two people with features we are interested in to mate and produce offspring just so that we can see the results! Humans also have a slow reproductive rate. We tend to produce offspring only one at a time, at a maximum rate of about one a year and with one generation about every 25 years. We also have a high chromosome number and very many different characteristics. Our understanding of the way inheritance works has come mainly from studying other living things. An organism that scientists have found to be extremely useful is a fruit fly, Drosophila melanogaster. It occurs in varieties with clearly visible differences, and it has only four pairs of chromosomes. It is easily kept in the laboratory, where every 10 days a new generation of several hundred flies can be bred. Other commonly studied organisms include the house mouse (Mus musculus) and many of our agricultural livestock and plants, such as sheep, cattle, corn and wheat.

FIGURE 6.16 Can you roll your tongue? This ability is an inherited characteristic.

a

b

c d

e ii Y

X

iii ru h

ru h

Y X iv

ii iii iv

FIGURE 6.17 Drosophila. The cells of the fruit fly on the left have an extra segment of chromosome 3, which has become attached to the Y chromosome. As a result, this fly has (a) misshapen eyes, (b) dark-patterned thorax, (c) imperfect cross-veins, (d) broad wings, and (e) incurved hind legs. The normal fly is shown on the right.

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Mendel’s experiments Genetics is the study of heredity and variation. Mendel is often known as the ‘founder of genetics’.

The first studies of inheritance were carried out by Gregor Mendel (1822–1884), using the garden pea (Pisum sativum). Mendel was an Augustinian monk who lived in a monastery at Brunn in Austria (now Brno, Czechoslovakia). In 1866 he published the results of his work in a paper called ‘Experiments in plant hybridisation’. It was totally ignored: no-one in the scientific world at the time recognised its significance. It was not until the beginning of the 20th century that his work was rediscovered, acknowledged and acclaimed. round seed

yellow seed

smooth pod (inflated)

green pod

violet flower

tall stem

FIGURE 6.18 The seven characteristics of the garden pea studied by Mendel.

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terminal flowers

wrinkled seed

green seed

constricted pod (tight)

yellow pod

white flower

short stem

axillary flowers

Why did Mendel succeed? Let us look at some of Mendel’s breeding experiments: the way he studied heredity. We now recognise that it was the experimental techniques he used which led to his success. The garden pea shows some easily observed alternative forms and Mendel chose seven pairs of these to study (Figure 6.18). Before beginning an experiment he bred each variety for 2 years to make sure that the character (his term for ‘characteristic’) was consistent. (Today we recognise the importance of using ‘pure-breeding’ lines in inheritance experiments.) Mendel then deliberately bred (crossed) one variety with another and observed what happened in the next generation. In order to make sure self-pollination did not occur, Mendel removed the stamens from one of his breeding pairs and then pollinated it by hand with pollen from the stamens of the other of the pair. Mendel did this not once, but many times. He kept careful records and counted the results he obtained. He also studied one character at a time, so that the effects of each could not be confused.

anthers removed ripe anthers releasing pollen

brush transferring pollen

male parent

Outcomes of a monohybrid cross A breeding experiment that looks at the inheritance of only one characteristic is known as a monohybrid cross. The offspring of a cross are known as the F1 or first filial generation. Mendel cross-bred tall and short plants, and all the offspring were tall. parents: F1:

female parent

FIGURE 6.19 A method of hand-pollinating a pea-flower.

A breeding experiment that looks at the inheritance of only one characteristic is known as a monohybrid cross.

tall plants × short plants all tall

He then took these tall offspring and allowed them to self-fertilise, and obtained the following results for the second filial generation (F2). F1: F2:

tall × tall both tall and short plants, but more tall than short ones

In the F2 generation, most of the plants were tall but some were short. By carefully counting the numbers of offspring Mendel found the ratio of tall to short was approximately 3 : 1. This is known as the monohybrid ratio. He repeated these experiments testing all the characteristics he had used in garden peas. They all gave similar results (Table 6.5). Blueprint of life 281

Mendel’s results TABLE 6.5 Results of Mendel’s experiments.

C ro s s

F1

F2 from self-fertilisation of F1

Ratio

1 round × wrinkled seeds

all round

253 plants bearing: 5475 round : 1850 wrinkled seeds

2.96 : 1

2 yellow × green cotyledons

all yellow

258 plants bearing: 6922 yellow : 2001 green seeds

3.46 : 1

3 inflated × tight pods

all inflated

1181 plants 882 with inflated : 229 with tight pods

3.85 : 1

4 green × yellow pods

all green

580 plants 428 with green : 182 with yellow pods

2.35 : 1

5 violet × white flowers

all violet

929 plants 705 with violet : 224 with white flowers

3.15 : 1

6 axial × terminal flowers

all axial

858 plants 651 with axial : 207 with terminal flowers

3.14 : 1

7 tall × short stems

all tall

1064 plants 787 with tall : 277 with short stems

2.84 : 1

Mendel’s explanations BIOFACT Mendel also carried out experiments involving two characteristics. This is known as a dihybrid cross. For example, he crossed pure-breeding round green peas with pure-breeding wrinkled yellow peas. In the first generation (F1) all the peas were round and green. He then allowed the F1 to self-fertilise to produce the F2 generation. In this F2 generation he found that the offspring occurred in a predictable ratio of 9 round green : 3 wrinkled green : 3 round yellow : 1 wrinkled yellow. The 9 : 3 : 3 : 1 result is known as the dihybrid ratio.

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Dominant and recessive genes Most people in Mendel’s time thought that inheritance occurred by blending: that the offspring of sexual reproduction showed a blend of the characteristics of each parent. One of Mendel’s first conclusions from his breeding experiments was that this is not so. The offspring resulting from the tall crossed with the short plant were not intermediate in height; they were just as tall as the tall parent. This form he called dominant. The other parental form, not expressed in the F1, he called recessive. But this recessive form appeared, unchanged, in the F2 generation. Mendel knew nothing of chromosomes and how they behaved in meiosis. We can use our present-day knowledge to explain Mendel’s results, but Mendel’s conclusions themselves are all the more remarkable because he reached them only by analysing his results. He deduced that the units of inheritance controlling a characteristic must occur in pairs. These pairs separated independently at gamete formation; the offspring received one from each parent and the way that they combined at fertilisation was random. The F1 which appeared tall must also have inherited a unit for shortness which was not expressed. Mendel called these units of inheritance ‘factors’. Today we call them genes.

Each individual has a pair of genes for each characteristic. The members of the gene pair are called alleles. When we refer to all the genes of an organism we talk about its genotype. Organisms that contain identical genes in their gene pairs are said to be homozygous. Pure-breeding lines are always homozygous for a given characteristic. As a result of crossing two organisms by pure breeding for a given characteristic, the F1 generation will contain two different alleles making up their gene pair. They are said to be heterozygous. Heterozygous individuals are hybrids; that is, they are the offspring of two different parents. In the example of a monohybrid cross on page 281, the F1 (Tt) plants looked tall; their phenotype (or appearance) showed tallness, but genetically they were heterozygous: they contained one allele for tallness and one for shortness.

• The genotype is the genetic composition of an organism. • The phenotype is the appearance of an organism. • Homozygous means having two identical forms of the gene for a particular characteristic, e.g. TT. • Heterozygous means having two different forms of the gene for a particular characteristic, e.g. Tt. • Alleles are genes for the same characteristic.

Genetic diagrams

parents: gametes: F1:

TT × tt all T all t Tt

The F1 (Tt) plants are tall, but in terms of their inheritance they contain one factor for tallness and one for shortness. In the F1 cross the Tt plants will form gametes, but only one of the factors will enter each gamete. This means that the F1 plants will produce gametes of two sorts, containing either T or t. These gametes will combine at random. This is a chance event, but we can predict the possible outcomes. The possibilities of their combining can be represented by drawing up a Punnett square (Figure 6.20). Any offspring containing T will appear tall. From the grid we can expect three of these tall plants to every one short plant. Tt T or t TT Tt Tt

  

F1: gametes: F2:

×

gametes of parent Tt gametes of parent Tt

We can look at Mendel’s explanations again using genetic diagrams. It is usual to give the characteristic we are studying a letter of the alphabet: a capital letter for the dominant form, and a small (lower-case) letter for the recessive form. Remember that each individual contains a pair of factors (genes) for each characteristic. For the monohybrid cross of tall with short plants, we will use the letter T to represent tallness. Mendel’s original tall pea plant would be TT, and the short plant would be tt. The result of the cross would be as follows:

T

t

T

TT

Tt

t

Tt

tt

FIGURE 6.20 The Punnett square is named after Reginald Crundall Punnett, the geneticist who invented it. A typical Punnett square is shown above.

Tt T or t tt 1 short

3 tall

Probability You may notice from Table 6.5 that Mendel’s results in the F2 generation were not exactly in a 3 : 1 ratio, but usually very close. The 3 : 1 ratio is the probability ratio based on the random chance of the gametes uniting. Probability deals with predicting the chance of an event occurring. In the F2 there are three chances out of four that a plant will show the dominant character, and one chance out of four that it will show the recessive. Blueprint of life 283

In the tall × short experiment the F1 parents had inherited both T and t. When the gametes were forming, each gamete received either T or t. We can say the probability of a gamete containing T is 0.5; that is, there is a 50% chance of it containing T. Similarly the probability of a gamete containing t is 0.5, or 50%. Look again at the Punnett square showing the chances of each type of gamete uniting:

12 / 12 /

T

14 /

12 /

t

14 /

T

12 /

t

TT

14 /

Tt

Tt

14 /

tt

Chance of TT is one in four = 1/4 Chance of tt is one in four = 1/4 Chance of Tt is two out of four = 1/2

Overall the probability ratio or chance of obtaining TT : Tt : tt genotypes = 1 : 2 : 1. Therefore the probable ratio of tall to short phenotypes is 3 : 1. With probabilities we are predicting a result. The actual result may vary from the expected result due to chance. The larger the sample, the closer will be the observed result to the expected result.

The rediscovery of Mendel

BIOFACT Mendel’s second law, the Law of Independent Assortment, states that each pair of factors sorts out independently of other pairs at gamete formation. This means that either factor of a pair can combine with either factor of another pair. The law applies to characteristics on different chromosomes.

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Mendel published the results of his work on inheritance in scientific journals in 1866. At that time little was known about the cell, and the processes of meiosis and mitosis were unknown. Darwin’s theory on the origin of species had already been published, in 1859. But Darwin could not account for the origin of the variations he observed in species. Yet Mendel’s work, and its importance for the theory of natural selection, went unnoticed by the scientific community for more than 30 years. It was not until 1900, when three scientists independently produced similar data, that Mendel’s scientific contribution was recognised. In the 20th century, biologists, with the knowledge of the mechanisms of inheritance and the work done by Darwin, have been able to explain the processes of natural selection—the selection of genes in a population through the survival and reproduction of organisms that are ‘most fit’ to survive in their environment (see Section 6.4).

Mendel’s Laws From his work, Mendel summarised his work to explain the inheritance of characteristics in two laws. Mendel’s first law, the Law of Segregation, states that factors for the same characteristic occur in pairs in an individual. These pairs separate at gamete formation, so that a gamete contains only one of each factor.

Dominant

Recessive

dark hair

light hair

frizzled hair

straight hair

early grey hair

not grey early

freckles

no freckles

white forelock

no white forelock

obese figure

lean figure

BIOFACT The pattern of inheritance of a particular characteristic may sometimes be followed by keeping a family pedigree. A pedigree is a chart of family members across the generations. Humans, racehorses, dogs, cats and other domestic animals may have pedigrees recording the history of inheritance of traits or characteristics in their family.

FIGURE 6.21 Some dominant and recessive genes in humans.

Blueprint of life 285

Questions 6

Contrast the genotype and the phenotype of an organism.

7

a Use Table 6.5 to describe Mendel’s monohybrid cross of violet and white flowers. b Explain how the results of such a cross indicate the dominance and recessiveness of each characteristic.

Consider a monohybrid cross between two purebreeding strains of plants, one with axial flowers and the other with terminal flowers. Using the notation A = axial and a = terminal, write down the genotypes for the a parent plants b F1 offspring c gametes produced by the F1.

8

Explain the following terms: a dominant b recessive c allele d homozygous e heterozygous.

Gregor Mendel carried out his experiments with pea plants during the mid 1800s. However, his outstanding contribution to our current understanding of the mechanisms of heredity were not recognised until the beginning of the 20th century. Explain why this was so.

9

Summarise Mendel’s Law of Segregation and Law of Independent Assortment. Use the hypothetical genotype AaBb of an individual to illustrate how these two laws apply.

6

The following pedigree illustrates the inheritance of Huntington’s disease in a family. Huntington’s disease is a degenerative disease of the nervous system resulting in loss of coordination, mental breakdown and eventually death. It is a dominant characteristic and generally does not develop until at least 35 years of age. In this pedigree all individuals in generation II are over 50 years of age.

1

Explain the difference between environmental variation and inherited variation in populations. Give examples of each.

2

Describe the methods used by Gregor Mendel to ensure consistency and accuracy in his experiments with pea plants.

3

4

5

Contrast monohybrid and dihybrid crosses.

F u r ther questions 1

Explain why fruit-flies, mice, rabbits, corn and wheat are far better subjects for the study of inherited characteristics than humans are.

2

Describe five examples of variations in the phenotype of humans which are the result of environmental influence.

3

Explain why two children of the same parents are not usually identical.

4

The ability to roll the tongue is a dominant characteristic. Is it possible for two non-rollers to have a child who can roll the tongue? Explain your answer, using appropriate notation and giving probabilities.

5

286

In Drosophila, normal wing is dominant to short wing. Let N = normal wing and n = short wing. Use a Punnett square to show the expected ratio of genotypes and phenotypes in each of the following crosses: a short × short b short × heterozygous normal c heterozygous normal × heterozygous normal.

Heinemann Biology

I

affected male 1

2

affected female

II 1

2

3

4

III 1

2

3

4

5

IV 1

2

3

6

7

a Use appropriate notion to assign genotypes to the parents in generation I. How can you be certain of their genotypes? b What is the probability of individual i III–1 ii IV–3 developing the disease? Explain your answer in each case. c Suggest why no-one in generations III and IV shows the disease. The curly wolf (Lupus spirale) is famous for its long, spiral tail. However, straight-tailed curly wolves are occasionally born. The animal breeder at the Twirly Hill Zoo needs to know if a particular spiral-tailed curly wolf is carrying the gene for a straight tail or

not; in other words, is it homozygous (AA) or heterozygous (Aa)? The usual test cross to investigate this is to mate the wolf with a straighttailed curly wolf (aa). Explain how the results of such a cross indicate the genotype of the curly wolf in question. 8

Lack of pigment (albinism) is recessive to normal pigmentation in all species. An albino man marries a normal woman and they have one albino child. a Draw a family pedigree representing this information. Assign genotypes to each family member in the pedigree, using appropriate notation. b What is the probability of any other children being albino?

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6.3

Chromosome structure— the key to inheritance OBJECTIVES When you have completed this section you should be able to: ● describe the work of Sutton and Boveri in establishing the significance of chromosomes ● describe the chemical composition of chromosomes and genes ● identify the components of DNA ● outline the behaviour of chromosomes during meiosis ● explain how the structure and movement of chromosomes during meiosis are related to the inheritance of genes ● explain how gamete formation and sexual reproduction contribute to variation in offspring ● explain how sex-linked genes and codominant genes are inherited and why they do not follow simple Mendelian ratios ● outline Thomas Hunt Morgan’s work in the understanding of sex linkage ● use examples to explain the appearance of particular phenotypes resulting from homozygous and heterozygous genotypes in codominance ● explain how environmental factors can affect the expression of genes.

The impor tance of chromosomes activities ● ● ●

288

A model of meiosis Codominance and sex linkage The effect of environment on phenotype

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In 1902 a young American geneticist, Walter Sutton, proposed a chromosomal theory of inheritance. He suggested that Mendel’s inheritance factors (genes) are carried on chromosomes. Sutton formulated his theory after observing meiosis in grasshoppers. He noted the following features: 1 During meiosis, the chromosomes in each grasshopper cell lined up in pairs, and each pair of chromosomes was the same size and shape. 2 Homologous pairs of chromosomes segregate during meiosis so that each gamete receives one chromosome from each pair.

3 After fertilisation, the resulting zygote had a full set of homologous chromosomes. About the same time, Theodor Boveri, a German zoologist and cytologist, also demonstrated that there was a connection between chromosomes and heredity. Together, Sutton and Boveri are considered to be the founders of the chromosomal theory of inheritance (called the Sutton–Boveri theory). Their research into the activities of chromosomes provided the first conclusive evidence that chromosomes carry the units of inheritance and occur in distinct pairs. Sutton and Boveri also realised that organisms have many more hereditary characteristics than they have chromosomes. Therefore, they reasoned that each chromosome must carry hundreds of inheritance factors. (The term ‘gene’ was introduced by German biologist Wilhelm Johannsen in 1909.) Many scientists were not convinced by Sutton and Boveri’s theory. But the American biologist Thomas Hunt Morgan later supported their findings, and confirmed the chromosome theory of heredity.

• Discrete factors (genes) control inheritance. • Genes are found on the chromosomes in the nucleus of cells. • Genes are made of DNA. • A single gene is a specific sequence of DNA bases.

The chemistry of chromosomes and genes The structure of DNA Each chromosome is made up of about 60% protein and about 40% DNA. The DNA is coiled tightly around a protein core. DNA is a huge molecule; it is a nucleic acid, properly called deoxyribonucleic acid. If all the DNA from one cell were stretched end to end, it would measure about 2 metres long. DNA is a double-stranded helical molecule. It is made of a series of subunits called nucleotides. One nucleotide contains a sugar, a phosphate and a base. The sugar is called deoxyribose (this means ribose that has lost an oxygen atom). Each sugar has a phosphate and a base attached. So part of a DNA molecule can be represented as shown: sugar—base phosphate

There are four different bases in DNA—adenine, guanine, thymine and cytosine. They are usually written as A, G, T and C.

P |

S— |

—S— |

P

P

|

| —S— |

S— | P |

phosphate

|

P

P

| |

| —S— |

P

P

|

| —S— |

| P |

S— sugar—base

There are four different bases in DNA—adenine, guanine, thymine and cytosine. They are usually written as A, G, T and C. If the DNA molecule did not have a spiral twist, it would resemble a ladder. The sides of the ladder are the sugar–phosphate groups, and the steps of the ladder are the bases (Figure 6.22). The bases on the two sides are bound together; adenine joins to thymine and guanine joins to cytosine. No other pairing is possible because the bases are complementary; their chemical structure makes any other pairing impossible. There are thus four nucleotides in DNA, shown in Figure 6.23. These nucleotides are linked together. Thus, we can represent a section of DNA as in Figure 6.24.

P | —S— | P | —S— |

S— sugar—base

P |

S—

S—

P FIGURE 6.22 The double-stranded helical DNA molecule.

Blueprint of life 289

TABLE 6.6 Components of nucleotides.

A

G ro u p

S t ructure

Symbol

G C Sugar (deoxyribose)

C

O

C

C C

T

Phosphate

O O

FIGURE 6.23 The four nucleotides in DNA.

C

Bases:

P O NH3

Adenine (A)

N

N N

N N

G C

purine

N OH

Guanine (G)

C

O

N

CH3

N

Thymine (T)

NH3

G A

O

T T

N

A

pyrimidine

NH3

A T G

C

FIGURE 6.24 A section of DNA.

N

Cytosine (C) O

N

Information is stored in the sequencing of bases (A, G, T and C) along the DNA molecule. A gene is a particular sequence of bases. Different genes have different sequences and are of different lengths along a chromosome. Genes that occur on the same chromosome are said to be linked. This is because they are usually inherited together. Genes that occur on the same chromosome are said to be linked. This is because they are usually inherited together.

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Formation of gametes In meiosis, haploid gametes are formed. They contain half the normal (diploid) number of chromosomes, because the chromosome pairs separate. (See pp. 171–175.) During meiosis, chromosome material is exchanged between chromosomes. This is called crossing over. Both of these observable events during meiosis result in the production of unique gametes, different from each other and different from their parent.

Crossing over The genetic information can be jumbled even more in the first stages of meiosis if crossing over occurs. In crossing over, adjacent chromatids twist around each other, split where they touch, and join up with different pieces (Figure 6.25). In this way linked genes are separated and rejoined to form new combinations. This is a further way by which variation is produced in sexual reproduction.

chiasma

homologous chromosomes line up together; chromatids break where they are twisted

chromatid ends join to ‘wrong’ pieces

homologous chromosomes move apart

separated chromosomes carry new gene combinations

FIGURE 6.25 The process of crossing over.

Variation as a result of sexual reproduction All gametes vary genetically as a result of meiosis. In sexual reproduction, two gametes are brought together and, in fertilisation, fuse to form a diploid zygote. This new individual is a unique combination of genes, inherited from both parents. Fertilisation in sexual reproduction increases variation in a species, because (1) it is sheer chance that determines which gametes will be involved in fertilisation, and (2) the chance of the same type of egg and the same type of sperm again being produced and uniting is remote.

Variations of Mendel’s ratios Sometimes in a monohybrid breeding experiment it appears that blending has occurred in the F1. This phenomenon is known as incomplete dominance. The phenotype of the heterozygote appears to be intermediate between the phenotypes of the homozygous parents.

Codominance Sometimes in a monohybrid breeding experiment it might seem that the dominant and recessive inheritance does not apply. This is only because not all genes have dominant and recessive alleles. If both alleles of homozygous parents are expressed in the heterozygous genotype, a third phenotype is seen. Let us look at an example. In the case of snapdragons, Antirrhinum majus, a red-flowered plant crossed with a whiteflowered plant gives F1 offspring which all have pink flowers. P F1

In a monohybrid cross, codominant alleles show three phenotypes: one for each of the two homozygous genotypes and one for the heterozygous genotype.

red × white all pink

Blueprint of life 291

However if the pink F1 plants are crossed, the F2 generation contains plants with red flowers, pink flowers and white flowers in a ratio of 1 : 2 : 1. pink × pink F1 F2

red

pink

1

:

2

white :

1

These results show us that Mendel’s inheritance rules still apply. Because both alleles are expressed in the heterozygote, they are both represented symbolically by capital letters. We can call the allele for red flowers AR, and the allele for white flowers AW. A red plant is ARAR and a white plant is AWAW. parents

red ARAR

gametes F1

all AR

F1 cross

pink ARAW

gametes

white AWAW

× all AW pink ARAW ×

AR 1/2 AW

pink ARAW ×

1 2 /

AR 1/2 AW

ARAR ARAW ARAW AWAW

{

F2

1 2 /

×

1

:

2

:

1

Draw up a Punnett square of the possible gamete combinations to check this. A well-known example in humans is blood, which can be group A, group B, group AB or group O. O is the recessive allele, and both A and B are dominant to O. However, A and B are codominant; when a homozygous blood group A parent and a homozygous blood group B parent have a child, that child’s blood group will be AB.

Sex linkage Sex determination Sex is a genetically determined characteristic. Humans have 46 chromosomes which occur in 23 pairs. In fact, we have 22 pairs plus two sex chromosomes. In females both of these chromosomes look alike and are called X chromosomes. In males the sex chromosomes are unalike and are called X and Y. So a female is XX and a male is XY. Sex linkage Males have received their single X chromosome from their mother. The Y chromosome from their father carries very few genes. One gene it does carry causes masculinity by activating the testes. This results in the production of male hormones which cause sexual activity and the secondary sexual characteristics such as facial hair and a deeper voice. X chromosomes carry a variety of genes which affect many functions, but not many are related to sex. Characteristics determined by genes on the X chromosome are said to be sex-linked because they occur more commonly in one sex than the other. In humans, sex-linked traits are usually recessive phenotypes which are more common in males. For example, red–green colour blindness, a condition in which people are unable to distinguish reds from greens,

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Boy or girl? When the sex cells (gametes) form by meiosis, only one sex chromosome from the pair will be found in the eggs or sperm. All haploid eggs from a female contain an X chromosome. Sperm from a male, however, may have inherited either an X or a Y. There are therefore two types of male sperm: one will have received an X chromosome, the other will have received a Y chromosome. What will happen when an X-carrying sperm fertilises an egg and when a Y-carrying sperm fertilises an egg?

There is a 1 in 2 chance that fertilisation will produce a boy or a girl. Note: In humans, the male is the sex that produces two different kinds of gametes and determines the sex of the offspring. In other organisms this may be different. In birds, moths and butterflies, for example, the male has two X chromosomes and the female only one. (There may or may not be a Y chromosome.) In bees, unfertilised haploid eggs become males and fertilised diploid eggs become females.

XX

ovum-producing cell

sperm-producing cell

X Y

meiosis meiosis X X

Y sperms

X

ova

all ova will contain one X chromosome female

half the sperms will contain an X chromosome and half will carry a Y chromosome male

FIGURE 6.26 Sex determination in humans. Half of the sperms contain an X chromosome, and half carry a Y chromosome. The chances of a boy or a girl being produced from the fertilisation of an ovum is therefore 1 in 2.

occurs in 8% of males but less than 1% of females. Normally, dominant genes on the X chromosome produce pigments in the eye. These pigments are sensitive to colour. Colour-blind people lack one of the pigments (most often the red-sensitive one) so they cannot distinguish this colour. A colour-blind person has a recessive allele (c) which results in no pigment. Since the Y chromosome carries no allele for this trait, a single recessive on the X is effective in males. In females the other X, with the dominant allele (C), will produce the pigment. TABLE 6.7 An example of sex linkages.

Male

Female

Phenotype

XCY

XCXC

normal colour vision

XCXc (carrier)

normal colour vision

c

XY

c c

XX

red-green colour blindness

Note: The gene for colour-sensitive pigment is on the X chromosome.

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Morgan and the understanding of sex linkage In the early 1900s, Thomas Hunt Morgan, an American geneticist and zoologist, was trying to replicate Mendel’s work with pea plants using an animal, the fruit fly Drosophila. In one experiment he mated a whiteeyed male with a red-eyed female (W = red, w = white): Experiment 1: establishing dominance white-eyed male × parents XwY gametes Xw or Y offspring XWY F1 generation

red-eyed female XWXW all XW XWXw

all F1 have red eyes All the F1 generation had red eyes, so that red eye colour was dominant and white eye colour was recessive. When Morgan crossbred the F1 generation, the F2 results were as follows: red-eyed females red-eyed males white-eyed females white-eyed males

2459 1011 0 782

Experiment 2: F1 cross parents gametes offspring F2 generation

red-eyed male XWY XW or Y XWY or XwY

F2 males 50 : 50 red : white

×

red-eyed female XWXw W X or Xw XWXW or XWXw F2 females all red (none white-eyed)

Not only did the expected ratio of 3 : 1 red-eyed to white-eyed not appear, but all the white-eyed flies were males. Morgan then conducted a further cross. Today this is called a test cross. He mated the original white-eyed male parent with an F1 redeyed female. The results were as follows: red-eyed females 129 red-eyed males 132 white-eyed females 88 white-eyed males 86 Experiment 3: test cross original white-eyed male × parents XwY gametes Xw or Y offspring XWY F1 generation or XwY males 50 : 50 red : white

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F1 female XWXW or XWXw XW or Xw XWXw or XwXw females 50 : 50 red : white

Note that these are the expected results. Morgan actually found the number of white-eyed flies in these experiments was lower than predicted by Mendel’s ratio. The reason is that white-eyed flies are more likely to die before reaching sexual maturity. White-eyed females did appear. What could be the reason for there being none in the F2 generation? Morgan hypothesised that the gene for eye colour in Drosophila must be carried on the X chromosome and that the Y chromosome has no gene for eye colour. Females have two X chromosomes, so the F2 red-eyed females would all be heterozygous for eye colour, having inherited one dominant red allele from their mother and one recessive white allele from their father. The test cross produced white-eyed females that had inherited an allele for white eye colour from both parents. Further tests showed Morgan’s hypothesis to be correct, and geneticists became aware that genes could be sex-linked. Morgan’s work was also important because it gave further support to Sutton’s hypothesis that genes were on the chromosomes. In 1933 he was awarded a Nobel Prize for his lifetime’s work.

Environmental effects The environment includes all the surrounding forces that act on an organism or its cells. Sometimes, although a particular gene has been inherited, the environment does not allow its particular characteristic to be fully revealed or expressed. The environment may affect the expression of a gene in an individual. Identical inherited characteristics do not always result in identical organisms because of the effect of the environment. For example, two people with the same genetic inheritance for tallness might grow to different heights because of differences in their nutrition or their health. The complete expression of tallness in Mendel’s pea plants was only possible under optimum conditions of warmth, light, nutrients and water. Identical twins result from one fertilised ovum and have identical genotypes. Studies on identical twins who are brought up in different environments provide clear evidence of the importance of environment on genetic expression (Table 6.8).

Identical inherited characteristics do not always result in identical organisms because of the effect of the environment.

BIOFACT The colour of hydrangea flowers depends on the soil acidity (pH). In acid soils of pH 5 or less the flowers are blue. Best plant growth occurs in pH range 5.5 to 6.5, and the flowers are then mauve. Flowers are pink in neutral soils (pH 6.5 to 7). Whiteflowering varieties always stay white. The colour of hydrangea flowers can be changed by applying a ‘blueing’ tonic that contains aluminium and iron, or lime (calcium carbonate) to the soil in the months before flowering.

TABLE 6.8 Effects of environment on genetic expression: average differences in selected physical characteristics between pairs of twins.

D i ff e rence in

50 pairs of identical twins re a red together

50 pairs of non-identical twins re a red together

19 pairs of identical twins re a red apart

Height

1.7

4.4

1.8

Weight

4.1

10.0

9.9

IQ

3.1

8.5

6.0

Note: The smaller the number, the smaller the difference between twins.

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Questions 1

Outline the contribution made by Walter Sutton and Theodor Boveri to the study of heredity.

2

Describe the relationship between chromosomes and genes. Use a diagram to illustrate your answer.

3

Define ‘nucleotide’, and outline its relationship to DNA. Use a diagram to explain your answer.

4

The following diagram represents part of a single strand of a DNA molecule.

5

Explain how crossing over contributes to variation during the production of gametes. Use diagrams to illustrate your answer.

6

a Define ‘sex linkage’. b Describe a specific example of the work by Thomas Hunt Morgan that led him to hypothesise about the existence of sex linkage.

7

a Define ‘codominance’. b The MN human blood group is another example of codominance. If a MM homozygous male has a child with an NN homozygous female, what will be the blood group of their child? Use a diagram to illustrate your answer.

A T C

Draw the other strand of the DNA molecule, using the correct complementary base sequence.

F u r ther questions 1

If a heterozygous blood group A person has children with a heterozygous blood group B, what genotypes may be shown by the offspring? Use a diagram to illustrate your answer.

2

The pedigree below shows the inheritance of a type of rickets that is resistant to vitamin D therapy. The disease is a sex-linked, recessive trait.

= male = female = disease

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a Assign genotypes to the original parents. b What is the genotype of the twin who married? Is it possible that she will have a son who is not affected or a daughter who is affected? Explain. 3

Find out what is meant by Y-linked inheritance. Describe the inheritance patterns for Y-linked inheritance, using specific examples.

6.4

The mechanism of inheritance OBJECTIVES When you have completed this section you should be able to: ● describe how DNA is replicated and outline the significance of this process ● use a model to explain how DNA is involved in the production of polypeptides ● explain the relationship between proteins and polypeptides ● define mutations and explain how mutations in DNA can lead to the generation of new alleles ● discuss the evidence for the mutagenic nature of radiation ● identify some mutagens ● explain how the sources of variation lend support to the theory of evolution by natural selection ● explain the difference between the idea of punctuated equilibrium and Darwin’s concept of a gradual evolution.

DNA replication A replica is a copy of something. During mitosis, chromosomes are replicated or copied. DNA replication begins with the separation of its two strands—starting at one end, the bonds break between each base pair, so that the two DNA strands unzip. They form what is known as the replication fork. Binding proteins prevent the strands from rejoining. A complementary copy of each exposed strand is constructed from new sugar–phosphate–base units. This process is catalysed by the enzyme DNA polymerase. Only complementary bases can lock onto the free bases of the original DNA. As the parent molecule unzips it forms the template for the synthesis of the complementary strand. One of the two new strands is built as a continuous strand. The other strand is built by linking DNA fragments together.

activities ● ● ● ● ●

A model for polypeptide synthesis One gene—one protein Mutations The development of resistance Solving the structure of DNA

The significance of the ability of DNA to replicate is that identical copies of the genes can be made. Blueprint of life 297

Unravelling the structure of DNA Until the late 19th century it was thought that inherited characteristics came from only one parent. But experiments on inheritance in plants showed that characteristics came from both parents, and scientist began to search for the source of this inheritance. At that time scientists were convinced that characteristics were passed on by proteins. The first breakthrough came through the work of an English doctor, Frederick Griffiths, in the 1920s. He worked with bacteria and deduced that another substance in the cells must be involved. This substance came to be called the ‘transforming principle’. Griffiths’ work came to the attention of Canadian geneticist Oswald Avery, working at the Rockefeller Institute in New York. When the ‘transforming principle’ was isolated by other workers, Avery and his coworkers analysed its chemical properties and deduced that it was the molecule DNA. In a paper published in 1944 they suggested that this molecule could be the means by which genes are passed from one generation to the next. But for many scientists DNA was not complex enough to be able to carry the information needed to pass on genetic information. They knew it consisted of phosphate, deoxyribose, and the bases adenine, cytosine, guanine, and thymine, but until the 1950’s no-one knew how these fitted together. Its structure had to be much more complex than they thought. FIGURE 6.27 In the early 1950s in England, two young scientists, Rosalind Watson and Crick’s original model of Franklin and Maurice Wilkins, decided to make a crystal of the DNA DNA. molecule to study its structure. They succeeded in getting DNA to crystallise, and by using X-ray crystallography they obtained an X-ray diffraction pattern. This pattern gave the first clue to the shape of the DNA molecule, which led to an understanding of how DNA functions. In 1953, James Watson and Francis Crick, also working in England, put together a model of DNA. They had enough information to make an accurate model after they examined Franklin and Wilkins’ X-ray pattern and the results of American scientist Erwin Chargaff’s research during the 1940s. Chargaff had noticed a pattern in the amounts of the four bases (adenine, guanine, cytosine and thymine) in DNA. Watson and Crick’s model showed two chains, twisted into spirals (the ‘double helix’), of alternating sugar and phosphate units linked by pairs of the four bases. Adenine is always paired with thymine, and cytosine with guanine. This is represented as: A–T C–G These four bases form the basic structure of the DNA of all organisms, although different species have a different number and arrangement of the bases. In 1962 Watson, Crick, and Wilkins received the Nobel Prize in Chemistry for their discoveries. Sadly, Rosalind Franklin had died, and never received the recognition she deserved for her part in the unravelling of the structure of DNA. Their discoveries represented a significant advance in research and formed the basis of experimental work in genetic engineering in the 1970s. Outline Watson and Crick’s discoveries about the structure of DNA.

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parent

parent

parent

parent

daughter

daughter

FIGURE 6.28 DNA replication.

DNA and the production of polypeptides DNA is the genetic or inheritable material in cells. It can be replicated and the information it carries be passed on to new cells. The genetic information is organised into units known as genes. A gene is a certain sequence of bases along a DNA strand. Each gene contains the coded information required to make polypeptides for the cell. DNA does not make proteins for the cell directly, but it provides the information for the cell to then synthesise or manufacture the protein. This is done via two processes: transcription and translation. ●



BIOFACT A little-known Swiss biochemist, Friedrich Miescher, discovered the DNA molecule in 1868, but for the next 75 years it was thought to be nothing more than an interesting molecule made up of four constantly repeating identical nucleotides.

Transcription is the process by which the information on the DNA is copied onto an RNA molecule. Translation is the process by which the information now on the RNA molecule is used to make a new polypeptide chain.

The genetic code To manufacture a protein, information is required about the number, type and sequence of amino acids that make up the polypeptides of a protein molecule (see Chapter 2, p. 60). This information is on the DNA strand in code. The genetic code is the sequence of bases along the DNA strand. A set of three bases codes for one amino acid. A triplet of bases is called a codon. Almost all organisms share the same genetic code. Sixty-one codons specify one of the 20 amino acids. One of these, methionine (codon AUG) is the start codon for synthesis of a protein. There are three stop codons: UAA, UAG and UGA.

BIOFACT There are four bases in DNA. With a triplet code there are 64 ways of combining the four bases. There are, however, only 20 amino acids found in cells, so more than one triplet codes for the same amino acid. (see Figure 6.29). This type of code where, in most instances, there is more than one codon for each amino acid, is known as a degenerate or redundant code.

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Genetic code in circular form

The genetic code is more or less universal: a codon codes for the same amino acids in all living things

Phe Leu

Gly Glu

Ser

Asp UCAG UC AG UC AG A C G U

Ala

G A C U G A C U G A C U G A C U

Val

Arg Ser Lys

A

U

C

U

G

U

G

A

C

G U C

A C CU

U G

A

U G A C U G AC

C

U

Cys Stop Trp

Read the code radially from the centre, e.g. serine is coded by UCU, UCC, UCA or UCG.

Leu

Pro

Met

Arg Iso

FIGURE 6.29 The genetic code.

Ribonucleic acids are single stranded. The sugar in the sugar-phosphate backbone is ribose and the base thymine (T) is replaced by uracil (U). A polypeptide is a single chain of many amino acids linked according to the type, number and sequence of amino acids. A protein is one or more polypeptides folded or twisted to form a uniquely shaped molecule with a specific cellular function.

BIOFACT Less than 5% of DNA contains the genes that codes for proteins. The remainder, over 95%, has been referred to as non-coding regions or junk DNA. These non-coding regions are either the sections called introns found within genes, or the sections found between genes on a DNA strand. They are all the DNA that has no known function. Recent research has already established that there are patterns in the non-coding regions, and scientists are looking for potential functions. One suggestion is that the non-coding regions form the complex operating system that controls the activity of the genes or coding regions.

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A

A G U C A G U C A G U C A G

This listing of codons is of mRNA. In RNA uracil (U) replaces thymine (T).

His

Thr

300

G

Stop

U C

A

C

GA

Asn

G

Tyr

Arg

In addition, some codons stand for 'START' and some for 'STOP', signalling the end of a peptide/protein chain.

Amino acid abbreviations: alanine Ala arginine Arg asparagine Asn aspartic acid Asp cysteine Cys glutamine Gln glutamic acid Glu glycine Gly histidine His isoleucine Iso leucine Leu lysine Lys methionine Met phenylalanine Phe proline Pro serine Ser threonine Thr tryptophan Trp tyrosine Tyr valine Val

Protein synthesis Production of a protein involves the following: DNA A gene on the DNA strand provides the information required to make the polypeptide in the form of a designated sequence of bases. Messenger RNA (mRNA) This is a type of ribonucleic acid (see Chapter 2, p. xx). It carries the information from the DNA in the nucleus to ribosomes in the cytoplasm. Transfer RNA (tRNA) This ribonucleic acid brings amino acids to the ribosome to be linked together to build the polypeptide chain. There are over 20 types of tRNA, a different type for each amino acid. tRNA has a distinctive clover-leaf shape. Each type of tRNA contains an anticodon or triplet of bases which recognises, and is complementary to, a codon on the mRNA. Ribosomes A ribosome is made up of two sub-units (see Chapter 2, p. xx). It acts as the site for polypeptide synthesis in the cytoplasm. It contains three active binding sites, which hold the mRNA strand and two tRNA molecules together temporarily during the linking of amino acids to make the polypeptide chain. Enzymes As with all chemical processes within the cell, enzymes are involved in catalysing the reactions.

1 Transcription a The double DNA strand in the nucleus unwinds in the area of the gene containing the information about the protein to be made. The enzyme RNA polymerase moves along the strand linking complementary RNA nucleotides together to form an mRNA strand. The ‘start’ codon and a ‘stop’ codon control the length of the mRNA strand. b After the whole gene has been copied, the mRNA strand is modified so that it consists only of the base sequence that will code for the protein. This is because most genes contain non-coding regions known as introns. The regions coding for the protein are known as exons. While

still in the nucleus, the introns are cut out (spliced) from the strand and the exons joined together. The modified mRNA then moves from the nucleus into the cytoplasm.

3' 5'

2 Activation of amino acids

amino acid

tRNA

In the cytoplasm, an enzyme (aminoacyl-tRNA synthetase) attaches amino acids to tRNA molecules. Each type of amino acid is attached to its specific tRNA.

AGC UCG

3'

3 Translation a The mRNA strand binds on to a ribosome at the end with the ‘start’ codon AUG. A tRNA carrying the amino acid methionine at one end and the anticodon UAC at the other, binds to the ‘start’ codon on the mRNA within the ribosome.

anticodon codon 5'

mRNA

FIGURE 6.30 Binding of tRNA.

A second tRNA binds to the next codon. Its amino acid links with a peptide bond to the first amino acid. b The first tRNA is released from the ribosome. The ribosome moves along the mRNA strand one codon at a time. Two tRNAs at a time are temporarily bound within the ribosome and their amino acids linked together. A polypeptide chain forms. c When a ‘stop’ codon is reached the polypeptide chain is released into the cytoplasm. d A polypeptide chain is only the primary structure of a protein. Each protein has a particular conformation or shape formed by the twisting or folding of its polypeptide chains (see Chapter 2, p. 61). Proteins are vital components of a cell. If the DNA sequences are changed by mutation, protein production will change. If no protein or a different protein is made then a cell’s structure or activities may also change. In this way any variation in the genetic material will be expressed by the cell. Variation shown by an organism is the basis on which natural selection can act.

base sequence in DNA template strand

BIOFACT DNA replication is fast and accurate. About 500 nucleotides can be replicated per second with, on average, only one mistake per billion bases.

G C T A A A T C C T A G C G A T T T A G G A T C

TRANSCRIPTION (copying)

base sequence in mRNA

G C U A A A U C C U A G

TRANSLATION (change language)

amino acid sequence in polypeptide

Ala

Lys

Ser

stop translation

FIGURE 6.31 Polypeptide synthesis (translation).

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Mutations • Mutations are the continual source of variation on which natural selection can act. • Cancers are due to a genetic change in a single cell. • Mutations that occur in body cells are known as somatic mutations. • Mutations may generate new alleles in an organism. • Changes in DNA sequences can result in changes in cell activity.

BIOFACT Agriculture and horticulture workers look out for spontaneous mutations in plants that may be useful or interesting for humans, such as seedless fruits.

Variation arises in sexually reproducing organisms by the recombination and crossing over of chromosomes in meiosis and the fusion of two haploid sets of chromosomes in fertilisation (see Chapter 3, p. 174). Mutation of the genetic material is another way that variation can arise. Mutations may lead to the generation of new alleles in an organism—they are changes in the DNA information on a chromosome. Most mutations are lethal and the cells with the changed DNA die. If the mutated form survives it increases the variation in a population. If a mutation occurs in the production of eggs and sperm it will be passed on to the next generation. If it occurs in body cells it will affect only that organism.

Mutation rates and environmental factors Changes or mutations can occur spontaneously. It has been estimated that a mistake in DNA replication occurs once in every billion base pairs replicated. This natural rate of mutation, however, can be increased by environmental factors. Environmental factors that induce mutation are known as mutagens.

Radiation During the 20th century, more and more evidence about the mutagenic nature of radiation was collected.

Ultraviolet radiation Ultraviolet (UV) radiation from sunlight is a known mutagen. Ultraviolet radiation can cause bases in a DNA strand to be lost (deletion). Another known effect is to cause the thymine bases in the same strand to link together. Replication can then not occur normally and a cell with DNA damaged in this way will die. The ozone layer around the Earth offers protection from the Sun’s UV radiation. In the 1980s a hole in the ozone layer was identified over Antarctica. It has been demonstrated that many chemicals released into the air as a result of human activities, such as fluorocarbons, carbon monoxide, oxides of nitrogen and sulphur dioxide, have damaged and depleted the ozone layer. The fear is that, as a result, plants and animals will be exposed to more intense doses of UV radiation that will lead to higher mutation rates. In particular, it is known that humans whose skin cells are exposed to high doses of ultraviolet radiation show an increased incidence of skin cancers.

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Ionising radiation Radiation from the radioactive materials of nuclear reactions and X-rays are mutagens. They can break DNA strands, or, if at a high enough energy level, break up whole chromosomes. Mutation or cell death is the result, depending on the amount of damage to the genetic material. Survivors of the Hiroshima atomic bomb in 1945 are still showing the effects of such radiation today. Studies have shown that the victims and their descendants suffered immediate damage to the DNA in their cells. In the years that followed many died from leukaemia, thereby demonstrating how DNA damage can appear years after exposure to the mutagens. The nuclear accident at Chernobyl in 1986 directly caused many deaths, but the full effect of the release of the radiation might not yet have been seen. In 2003, the effects of this disaster were revised— scientists believed they had been ‘massively underestimated’. It is now described as the greatest environmental catastrophe to date because radiation with a long half-life has spread into food, soil, land and waterways. An estimated 9 million people live in contaminated areas. The long-term effects have been disastrous. For example, two in every three calves born in the first five years were stillborn, and it is estimated that half a million people will die prematurely from radiation-induced cancers. In the past X-rays were used for many purposes. For example, in the 1950s and 1960s X-ray machines were used in shoe shops to measure children’s feet for correct shoe size. Since then we have learned that Xrays are mutagenic, and today they are used only with great care by doctors, dentists and scientists.

FIGURE 6.32 In the 1950s and 1960s people had their feet X-rayed when buying new shoes to ensure a proper fit. Today we avoid any unnecessary exposure to radiation.

The Chernobyl disaster The effects of the Chernobyl nuclear accident in the Ukraine in April 1986 will remain for many years. Research indicates that the incidence of cancer caused by mutations induced by the radiation is expected to peak in 2005. The radioactive fallout has affected almost 300 000 square kilometres of land in the Ukraine,

Belarus and Russia, and radioactive material continues to spread, usually by floodwaters into the reservoirs and catchment areas of those countries. It is not clear how many people have died as a result of the Chernobyl accident, but Ukraine authorities estimate that about 32 000 deaths have already occurred.

FIGURE 6.33 Treatment of material from the Chernobyl reactor site. Note the lack of precautions against contamination. Many workers later suffered the effects of exposure to radiation.

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Chemical mutagens Many chemicals are known as mutagens. Chemicals may act in different ways on DNA. In some instances chemicals may cause the bases in DNA to take on a different chemical shape by altering the usual double bonds and position of the hydrogen atoms in the molecule. Changed bases do not pair as usual during replication, resulting in the production of a mutated DNA strand. This change is known as a tautomeric shift. Another possible chemical change is deamination. This is when a DNA base has an amino group in its molecule replaced by a different group. During replication the usual pairings do not occur, so a mutated DNA strand is produced. Some known chemical mutagens include dieldrin, dioxin, ethyl methanesulfonate (EMS), nitrous acid and acridine dyes. Because of increased industrial activity over the (a)

CH3 H

(b)

C

C

N

N C

H H

O

C

H

N

tautomeric shift

H

O C

N

N

C H deamination

N C N

C

N

C N

H

H

H

C

adenine—pairs with thymine

CH3 C

N C

H

H

usual form pairs with adenine

H

N C

O thymine (usual form)

H

last 200 years, many thousands of chemicals made and used by humans have been released into the environment. Many of these are now known to contain mutagens. They include cigarettes and some medications, pesticides, cleaning products, food additives and preservatives and hair dyes. As a result of accumulated evidence many harmful products have been withdrawn from sale and use. Today strict tests are applied before a product can be marketed and sold. Warnings must be given and strict safety procedures put in place when potentially harmful substances are used. Some mutagens are also carcinogens and may cause cancer. One example is dioxin, a component of Agent Orange, a chemical that was used in a defoliant in the Vietnam War. People who came into contact with the defoliant have been shown to have higher than normal rates of cancers and mutations.

O C

C N

C N

H

C H

hypoxanthine—pairs with cytosine

C

O thymine (rare form) changed form pairs with guanine

FIGURE 6.34 Two types of chemical mutations. (a) A tautomeric shift, where a changed base cannot pair as usual. (b) A deamination, where an amino group is replaced by a different group.

Induced mutations are those resulting from exposure to mutagens, such as radiation, either deliberately or accidentally. In agricultural and horticultural breeding programs plants are often deliberately exposed to mutagens. Scientists look for desirable changes, such as resistance to specific diseases, that appear in the offspring. Dominant gene mutations are expressed immediately. Recessive mutations may remain hidden for several generations. 304

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Beadle and Tatum: one gene, one polypeptide In 1931, George Beadle was studying inheritance in the fruit fly Drosophila in the laboratory of Thomas Morgan. The results of his experiments indicated that eye colour in the fly is the result of a long series of chemical reactions, and that genes somehow affect these results. Beadle found that mutant eye colour in Drosophila was caused by a change in one protein. He concluded that genes must influence heredity chemically. In 1941, Beadle and Edward Tatum published the results of their experiments with a bread mould, Neurospora crassa, that provided evidence of a link between genes and proteins. They used X-rays to produce millions of mutated strains of the mould. Each strain lacked the ability to produce one of the essential nutrients (an amino acid or a vitamin) that would be needed to grow normally. This inability

was caused by the absence of the necessary enzyme. By growing different strains with different combinations of nutrients, Beadle and Tatum were able to establish which enzyme was lacking in each mutant strain. They also found that each genetic mutation was at a specific site on the mould’s chromosomes. They concluded that different sites were associated with each enzyme. This led to the famous ‘one gene, one enzyme’ hypothesis. This hypothesis has since been modified to ‘one gene, one polypeptide’, because although enzymes are proteins, many proteins are not enzymes. For example, haemoglobin is a protein made up of more than one polypeptide. A polypeptide is a single chain of many amino acids linked according to the type, number and sequence of amino acids.

1

2

What did Beadle and Tatum’s experiments with bread mould show?

Explain the importance of this work.

Darwin revisited Darwin’s theory of evolution by natural selection can be explained and expanded by the genetic information we now have. The source of variation in a population comes from: • the random fusion of gametes in sexual reproduction • crossing over of pieces of homologous chromosomes during meiosis • random assortment of chromosome pairs in meiosis • mutations of chromosomes and genes. This genetic variation is expressed in the phenotype of an organism. In any population in a given environment some phenotypes survive and reproduce better than others. Over time, natural selection will operate to change the proportions of certain genes in a population. The characteristics of a population are the expression of the genes of the individuals in that population. As a result of natural selection, individuals with particular genotypes become more common: the frequency of particular genes increases and some other genes may be eliminated. If one species is distributed in two separate areas and change by natural selection continues, the genotypes may become so different that it is not possible for breeding to occur successfully between the two groups after a long period of time. The development of new species can occur as a result of isolation. The mechanisms that isolate two groups and their gene pools may be geographic or behavioural. Geographic isolation occurs when, for example, a mountain range or a large body of water separates the two groups. Behavioural isolation occurs when the organisms do not breed

Geographic isolation occurs when, for example, a mountain range or a large body of water separates the two groups. Behavioural isolation occurs when the organisms do not breed even if they occupy the same region. Blueprint of life 305

even if they occupy the same region: they might come into season at different times of year, or they might have different mating calls or courtship patterns. Whatever the isolating mechanism, when groups no longer share a pool of common genes, they are likely to become separate species over time.

Genetic drift and the founder effect A group of individuals that share a common gene pool is called a population. The gene pool represents the genetic constitution of an entire population, being composed of the genes of all the individuals in a population. Genetic drift occurs when random changes in the numbers and types of genes in a small, isolated population take place. Over a very long time, changes in gene pools may lead to the evolution of new species. When a small population moves away from its original population and becomes geographically isolated, the genes of its members are no longer available to the original population; that is, the ‘gene flow’ stops. After separating, each population will experience different mutations and different environmental selection pressures. As a result, the small population will have different gene frequencies compared to the original population. In other words, the new population will show features that are not typical of the original population. This type of genetic drift is called the ‘founder effect’.

BIOFACT Two major periods of extinction have occurred. One was 250 million years ago, when 80% of marine invertebrates became extinct. The other was 65 million years ago, when the dinosaurs together with about 50% of marine invertebrates disappeared.

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Natural selection versus punctuated equilibrium Darwin’s theory of evolution by natural selection proposes that populations change slowly and gradually over time. In the fossil record we should therefore expect to see a history, for any one group of organisms, of a long sequence including intermediate forms, recording the gradual change from ancestor to descendent. In fact, there are very few instances of this. The evolution of the horse is one example; this species has developed gradually over 40 million years. The fact that there are so few examples of gradual evolution has been related to the small chance of fossilisation of organisms, and the small chance of finding them. If an environment remains stable for many years we would expect there to be no change in the organisms living there. It is only when conditions change that natural selection occurs. The fossil record, in fact, records episodes of stability followed by episodes of mass extinctions and rapid change. The fossil record suggests that most species appear suddenly, show little change for millions of years and then become extinct. In 1972, Niles Eldredge and Stephen Jay Gould put forward a theory to explain this phenomenon. They called it punctuated equilibrium. Punctuated equilibrium proposes that, instead of a gradual transition over millions of years, there have been periods of rapid evolutionary change followed by long periods of stability or equilibrium. ‘Rapid change’ from a geological perspective can mean over several hundred thousand years.

Gould and Eldredge’s proposal suggests that when rapid change occurs in an environment, organisms either move out of the area or, if the change is sudden, die out. Populations of organisms living on the edge of, or some distance away from, the disturbed environment may survive in small isolated pockets. Small isolated populations change at a faster rate than large populations. These new forms migrate and appear suddenly as new species at different locations. They may compete with or coexist with the original species. Examples include: • Globorotalia, a marine microfossil—A second species suddenly appears as a fossil in the Indian and Atlantic Oceans but a transitional form between the two has been found in the South Pacific Ocean. The two forms coexist today. • A trilobite genus Phacops—A new species suddenly appeared in the fossil record but transitional forms between the two have been found in one location in New Hampshire, USA. • dinosaurs such as Tyrannosaurus and Stegoceras—These dinosaurs remained unchanged for 5 million years in the Judith River Formation in Montana, USA, but new species ‘suddenly’ appeared 500 000 years later. Further research revealed that sea levels rose and drowned the Judith River Formation for 500 000 years. The dinosaur species that had been living there had moved out and changed. In other areas of Montana transitional forms have been found. The changed dinosaurs, not Tyrannosaurus but Daspletosaurus, and not Stegoceras but Pachycephalosaurus, returned to Judith River when sea levels dropped.

Questions 1

2 3

4

5

Outline the contributions made by Watson and Crick, and by Franklin and Wilkins, in determining the structure of the DNA molecule.

6

Match each of the terms in the left column with the correct description in the right column. genetic code

site of protein synthesis

Describe the significance of DNA replication.

tRNA

triplet of three bases

a Describe the processes of transcription and translation. b Explain the relationship between proteins and polypeptides.

mRNA

sequence of bases along a DNA strand

codon

type of ribonucleic acid that delivers correct amino acid sequence for construction of protein polypeptide at the ribosome

ribosome

type of ribonucleic acid that carries information from nuclear DNA to ribosome in cytoplasm

a Define the term ‘mutation’. b Explain how mutations are important to the process of evolution. c Describe the circumstances in which mutations can affect i individuals in the next generation ii only the organism in which the mutation occurs. a What is a mutagen? b Outline the evidence that radiation is mutagenic. c Describe the evidence required to decide that a substance is mutagenic.

7

a Describe the concept of ‘punctuated equilibrum’. b Compare punctuated equilibrium with the traditional view of gradual transition.

8

Explain what is meant by the ‘one gene, one polypeptide’ hypothesis.

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F u r ther questions 1

Describe the evidence gathered by Watson and Crick that pointed to the complementarity of the nucleotide bases adenine, guanine, cytosine and thymine.

2

Use a flow chart to model the process through which the DNA of cells controls the manufacture of cellular proteins.

3

Draw a concept map demonstrating your understanding of the factors involved in the process of evolution. Include sources of variation, natural selection, isolating mechanisms and genetic drift.

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4

Outline the potential impact on the evolution of living organisms from mutagens released into the environment by human activity.

5

Outline research that led to the development of the ‘one gene, one polypeptide’ hypothesis.

6.5

Reproductive technologies and genetic engineering OBJECTIVES When you have completed this section you should be able to: ● explain how the genetic composition of a population can be altered by reproductive techniques, including artificial insemination, artificial pollination and cloning ● describe how and why transgenic species are produced, using examples ● use examples to discuss the potential impact of reproductive technologies on genetic diversity.

Manipulating the gene pool Ever since humans first tamed animals and grew crops they have been controlling the breeding of the organisms in their care. The aim has always been to improve the characteristics of the species for human use and purposes; for example, to make the organism larger, faster-growing, more disease-resistant, or tastier. Think of the many types of fruit and vegetables available today and the different sorts of domesticated animals such as dogs, cats, cows and horses. Selective breeding means deliberately mating or crossing individuals of the same species with the characteristics required. Offspring possessing the required characteristics will, in turn, be selectively bred, while all offspring not possessing the required features will be discarded. Over generations, the preferred characteristics come to dominate because of the deliberate changes that have been made to the genetic composition of the local population. Controlled breeding experiments are slow and take many generations.

activities ● ●

Cloning Transgenic species— debating the issue

Selective breeding creates change in species. It provides further evidence for evolution by natural selection.

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Artificial insemination and pollination Artificial insemination is the injection of male semen into a female. Animal breeders, particularly of large domestic animals such as cows, sheep and horses, commonly use this technique. The sperm is collected from a male with desirable characteristics. It can be easily transported over large distances and used to inseminate many females and many more offspring can be produced than by normal mating. Plants can be artificially pollinated, often by hand, by dusting fertile stigmas with the pollen from plants with the desired characteristics. Artificial insemination and artificial pollination techniques allow genetic changes to quickly become widespread within populations.

Cloning Cloning is a method of producing genetically identical organisms. A clone is a collection of genetically identical copies.

Cloning is a method of producing genetically identical organisms. A clone is a collection of genetically identical copies. Genes, cells or whole organisms may be clones. All forms of asexual reproduction produce offspring that are clones (see Chapter 4, p. 193). The cloning of plants by artificial techniques such as cutting and grafting have been used for many years to produce identical plants for crops and gardens. More recently, tissue culture techniques have been used to produce clones of plants with required characteristics. The cloning of animals has offered a challenge to scientists, particularly to clone large domestic animals. In February 1997 the successful breeding of a cloned sheep was announced by researchers at the Roslin Institute in Edinburgh, Scotland. The technique they used is known as nuclear transfer technology.

section of root tip

break cells free in blender

grow cells on nutrient medium

incubate under controlled conditions

FIGURE 6.35 (a) Tissue culture in plants. (b) A plant grown from tissue culture.

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undifferentiated cell growth (a)

genetically identical plants

The cloned sheep, named Dolly, was produced by first culturing adult sheep mammary tissue (taken from the sheep’s udder) in the laboratory. The nucleus from one of these cells was then taken and transferred to an enucleated egg cell. The egg was then implanted into a female sheep where it grew and developed and a healthy lamb was born. All the cells of the lamb were genetically identical to the sheep which provided the mammary tissue. Dolly died of a lung infection in February 2003 at the age of six. During her life she gave normal birth to four healthy lambs. Since Dolly, scientists have cloned not only sheep but mice, rabbits, cats, goats, pigs, cattle, mules and horses. While success rates are still low, animal cloning could become an accepted technology for improving animal breeds by providing genetic copies of animals with the desired features more rapidly than by traditional breeding techniques. There are concerns, however, about applying this technique to humans. Most countries have strict controls over experiments with human cells.

BIOFACT Frogs and mice have been cloned using nuclear transfer technology. The nucleus from a body cell is inserted into an enucleated egg cell (one which has had its nucleus removed), and the egg cell is then allowed to develop. The resulting organism is a clone of the organism that contributed the nucleus.

FIGURE 6.36 Cloning of animals such as Dolly the sheep using nuclear transfer technology is an example of genetic engineering. The entire genome from one organism is inserted into a cell of another organism of the same species.

clone of sheep X

remove DNA from unfertilised egg

fuse cells implant in surrogate early embryo with donor DNA

culture

remove udder cell from sheep X

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Genetic engineering Modern biotechnology uses a wide number of techniques, often referred to as genetic engineering.

BIOFACT A large range of pharmaceutical products are produced by genetically engineered micro-organisms, including: • antibiotics for the treatment of infections • human insulin for the treatment of diabetes • Human Growth Hormone for treating dwarfism and other growth defects, and for the possible treatment of burns, fractures and wounds • human vaccines to combat diseases, such as influenza B and hepatitis B • Tissue Plasminogen Activator (TPA), which reduces the risk of heart attack and stroke • alpha and gamma interferons for the treatment of viral infections and cancer • Bovine Growth Hormone, which increases milk production in dairy cows • Colony Stimulating Factor, for the treatment of leukaemia • vitamin B2 (riboflavin) and vitamin B12 (cobalbumin), used in medicine and as food additives.

The terms transgenic and genetically modified are interchangeable.

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Biotechnology is the use of various techniques to change living organisms at the molecular level to produce useful products or provide services. Modern biotechnology uses a wide number of techniques, often referred to as genetic engineering. Genetic engineering permanently alters the genetic blueprint of an organism. Desirable genes from one organism are isolated and then inserted into the cells of another organism. New DNA made by combining different DNA pieces is known as recombinant DNA.

Production of a transgenic species Genetically modified (GM) organisms are organisms that have had their genetic make-up deliberately modified by either traditional breeding techniques, stimulation of mutation or genetic engineering. A transgenic organism is one that contains a new piece of DNA spliced into a chromosome (recombinant DNA) in each of its cells. The new piece of DNA usually contains a gene that enables the organism to produce a different protein. The inserted DNA may come from a different species or from a different organism of the same species. In genetic engineering, the production of a transgenic species involves several steps: 1 A useful gene and the chromosome it is on are identified. 2 The gene is isolated or ‘cut out’ of its DNA strand. 3 Separate DNA sequences for regulation may have to be added to ensure the gene will work. 4 In some circumstances, multiple copies of the gene may be made. 5 The gene is inserted into the cell of another organism. Sometimes a vector is used to do this. The method of insertion that is used depends on the type of cell. Once the gene has been inserted, it needs to become part of the genetic material of that organism, and must be able to be expressed. The genetically modified organism is not a transgenic species unless it is able to pass on its genetic modification to the next generation. For unicellular organisms such as bacteria, this requires successful cell division to occur. If the gene has been inserted into an animal egg cell, then a whole new organism needs to grow and develop and, in turn, be fertile itself when adult. Transgenic plants are often produced by tissue culture to generate a complete new plant. Transgenic species have been developed to improve some farmed plant and animal species. For example, pest-resistant maize, soya bean, canola and cotton crops are now widely available. (See box on p. 321.) Pigs, poultry, sheep, cattle and goats have also been developed for particular characteristics, such as disease resistance or improved quality or quantity of their meat—for example, leaner pork meat or larger turkey breast fillets. (See also p. 314.)

Some applications of genetic engineering Molecular detectives Forensic scientists investigate the scientific evidence needed to solve crimes. The ability to detect variation between individuals at the DNA level has provided forensic scientists with an important new tool. The technique is called DNA profiling or fingerprinting, and enables criminals to be identified from the DNA in traces of body tissues left at the crime scene. DNA can be extracted from tissue such as blood, skin, semen or hair. DNA fingerprinting requires an extract of DNA to be taken from the tissue sample found, and another from a suspect’s blood cells. The DNA is then treated with an enzyme that cuts either side of a repeated sequence. This produces a mixture of DNA fragments of differing lengths. The fragments are then separated according to size by gel electrophoresis, in which an electric current flows

(a) Obtain tissue sample from scene of crime (blood, sperm, hair with follicle cells, skin, etc.) and tissue sample from suspect.

across a flat plate of gel containing the solution of DNA. DNA moves towards the positive electrode because it is negatively charged. Smaller fragments of DNA move faster. After separation, the DNA fragments are transferred to a nylon membrane. The fragments can then be identified by treating them with radioactive probes, which are pieces of single-stranded DNA with base sequences that may be compliementary to those in the repeating sequences of the samples. If a probe matches the repeating sequences in the DNA being tested, the two pieces of DNA bind together. An X-ray picture is then made of the pattern of bands. This ‘bar code’ pattern is the DNA fingerprint. DNA profiling can also be used in paternal testing, monitoring bone marrow transplants, animal conservation and animal behaviour research.

(b) Extract DNA from each sample by treating with chemicals.

(e) DNA fragments are transferred from gel to a nylon membrane in a method known as Southern blotting. Membrane takes up DNA bands.

(c) Cut long DNA molecules into fragments using restriction enzymes. The DNA of different people will be cut in different places, producing different combinations of fragment lengths.

(d) Fragments are sorted by length using a method called electrophoresis. (An electric current pulls the fragments through a slab of gel. The smaller the fragment, the faster it moves through the gel. When the current stops, the fragments stop in separate bands.) DNA fragments from suspect and from scene of the crime are run through the gel in parallel lanes. Victim

Evidence

Suspect

Match (f) Membrane is bathed in solution containing DNA probes. A probe is a short piece of DNA made in the laboratory, containing a radioactive isotope. Each probe contains a sequence that is complementary to a sequence in the sample DNA. The probes 'stick' to the complementary sequence. Excess probe is washed off.

(g) Bands that the probes have 'stuck' to are revealed when the membrane is placed against X-ray film. Radiation from probe-marked bands registers on X-ray film as dark bands.

(h) Patterns of bands in each lane are compared to those in other lanes. If patterns are the same, it means there is a high probability that cell in the evidence sample came from the suspect.

FIGURE 6.37 DNA fingerprinting as an aid in forensic science.

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Advances in medical treatment Detecting genetic diseases Geneticists can detect genes which cause genetic conditions from small samples of tissue obtained from adults, or from embryos using amniocentesis. ‘Gene probes’ recognise DNA sequences associated with genetic diseases. For example, in vitro embryos are often tested for genetic abnormalities such as cystic fibrosis, Down syndrome and haemophilia. In 1991, a gene that causes an inherited form of breast cancer was found on chromosome 17 (the BRCA1 gene). Another type of breast cancer is associated with a mutation of the BRCA2 gene on chromosome 13. About 5% of families with breast cancer have a mutation of the P53 tumour-supressor gene on chromosome 17 and may develop a range of other tumours as well as breast cancer. Identifying and isolating these genes is the first step in preventing these diseases using gene therapy.

Gene therapy In gene therapy, defective genes are replaced with normal ones. For example, if a person suffering from cystic fibrosis could receive the normal allele for the protein that is not functioning correctly, the disease would no longer be present. Successful experiments have been performed on rats and mice, so it is hoped that humans with cystic fibrosis might be able to be successfully treated in this way in the near future. Research is progressing toward a genetic cure for haemophilia, a disease in which the blood is unable to clot at the site of an injury. People with haemophilia may suffer severe bleeds even after minor events; for example, an action such as jumping can cause a painful bleed into the knees, requiring injections of clotting factor. In a joint trial between the Sydney Cancer Centre and two US hospitals, patients with haemophilia B were injected with an artificial gene for the protein they lack. This protein is a clotting factor known as Factor IX. The gene is packaged in a virus, which carries it to the liver cells where it can produce the missing protein. Similar research is planned for Factor VIII, the protein lacking in haemophilia A. Scientists are now working on modifying the genes in adult stem cells taken from the blood of healthy people to develop cures for a range of diseases. Before the new treatments are available to humans they are trialled in other animals, such as mice or dogs then non-human primates. A team of scientists in Seoul, South Korea, has bred mice containing human stem cells (‘hu-mice’) which they

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hope will provide experimental animals for trials in which the results can be accepted as human clinical trials.

Rapid detection of infectious diseases By using gene probes, infectious diseases can be detected in a number of ways. A sample of blood or a swab from a wound can be tested by a polymerase chain reaction, or PCR (see p. 317). This amplifies the suspected gene sequence, producing millions of copies that can be rapidly detected.

Blood screening New blood screening technology called the ‘nucleic acid test’ has been operating in Australia since April 2000. This technology makes blood supplies much safer by improving the rates of detection of HIV and Hepatitis C infected blood. Australia was one of the first countries to use this technology.

Advances in agriculture Scientists are aiming to use gene technology to produce genetically modified farm animals that are larger and leaner, more resistant to disease, and more productive. For example, CSIRO in Australia has been conducting research on transgenic sheep. These sheep have been genetically modified by the insertion of an extra copy of growth hormone gene. These sheep usually grow larger than normal sheep, are leaner, produce more milk and, in some breeds, more wool.

FIGURE 6.38 Sheep genetically modified with an extra copy of growth hormone gene.

In another CSIRO project, pea plants have been genetically modified by introducing a gene which gives them increased resistance to a weevil that causes major damage in pea crops. This could increase yields by up to 30% and also reduce the use of chemical pesticides.

Transgenic technologies are also being used to improve the nutritional value of pasture crops and to produce high-yield crops so that more can be produced from the area of land available for cropping.

1

Explain the usefulness of DNA fingerprinting in forensic science.

3

2

Outline two examples of the ways gene technology can be used in medicine.

Outline two potential benefits to farmers of genetically modified crops and livestock.

Hybridisation Hybridisation is the production of hybrids. A hybrid is an individual resulting from two genetically unlike parents. Animals and plants that are heterozygous for many alleles are often more healthy, vigorous and fertile than homozygous individuals. This is known as ‘hybrid vigour’. New varieties of crop plants such as wheat and corn have been produced in this way. A cross between two different species is not always possible. The chromosomes from each parent may be incompatible and no offspring results. In some instances offspring are produced that are sterile or infertile. For example, a mating between a horse and a donkey produces sterile offspring known as mules.

Hybridisation is the production of hybrids. A hybrid is an individual resulting from two genetically unlike parents.

FIGURE 6.39 A mule is the offspring that results from the mating of a horse and a donkey. Mules are sterile and so cannot produce offspring.

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Some processes used to produce a transgenic species— gene technology Isolating genes using restriction enzymes Restriction enzymes are sometimes called ‘gene shears’ or ‘gene scissors’

Once a useful gene has been identified it can be isolated by ‘cutting’ it out of its DNA strand. Special restriction enzymes, also known as restriction endonucleases, are used to do this. Restriction enzymes are found in bacteria. More than 800 different types are known. Each type cuts at a specific point in a sequence of nucleotides. The cut ends are known as ‘sticky ends’. restriction enzyme recognition site DNA sequence C – G – G – A – A – T – T – C – T –C 3' ~ G – C – C – T – T – A – A – G – A – G 3' ~ 5' restriction enzyme cuts here

~ ~

5'

5' A – A – T– T A–A– T– –C– T3–' T G– –C~ two DNA A– ' fragments G~ 3 5' FIGURE 6.40 Action of a restriction enzyme.

5' ~ C – G – G 3' 3' ~ G – C – C – T – T – A – A 5'

Making recombinant DNA If the DNA strands from two different organisms are cut using the same restriction enzyme and the DNA pieces from each are then mixed together, matching sticky ends will be attracted to each other and connect up. This process is called ‘annealing’. DNA ligases are sealing enzymes found in all living organisms. They help to make and repair DNA. If DNA ligases are added to annealed DNA fragments they help to seal and strengthen the bonds of the new recombinant molecules.

Making transgenes An isolated gene cannot function if it is transferred alone: it needs a promoter sequence attached to ensure the gene will work.

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A gene contains the code for a specific protein. When a new protein is to be made, the messenger RNA copies only the code (see p. 300). However, on the DNA strand are also control or promoter sequences that switch the gene ‘on’ or ‘off’. They regulate when a protein is made, how much is made and in which cells of the organism the gene will function. An isolated gene cannot function if it is transferred alone: it needs a promoter sequence attached to ensure the gene will work.

donor DNA (extracted from human cells)

carrier DNA (plasmid) G A

A

T

T C

G A

A

T

C T

T

A A G

C T

T

A A G

T C

both DNAs are cut with the same restriction enzyme e.g. EcoRI G C T

A T

A

T

T C

A A

G

A

C T

G

T

A

A A

T

T C G

mixing results in annealing G

A

C T

A T

T

T C

A A

G

DNA ligase seals the join plasmid

'nick'

G A

A

T

C T

T

A A G

T C

human DNA FIGURE 6.41 Constructing a DNA molecule.

recombinant (chimeric) DNA molecule

Making copies of genes For genetic engineering to be possible on a large scale, multiple copies of useful genes need to be made. Polymerase chain reactions (PCR) do this. In this process, the DNA molecule containing the required gene, a large quantity of the four nucleotide bases, guanine, cytosine, adenine and thymine, the enzyme DNA polymerase plus a large quantity of ‘primers’ are mixed together. Primers are short sequences of nucleotides that start or ‘prime’ the process. The mixture is heated to separate the two strands of the DNA molecule. It is then cooled, during which time the primers bind or anneal to the ends of the DNA strands. DNA polymerase then brings about synthesis of complementary DNA strands. This process to double the number of DNA molecules takes less than 2 minutes. Repeated cycles of heating and cooling can very rapidly produce large quantities of the required DNA.

desired gene region

BIOFACT Dr Kary Mullis developed the PCR technique and was awarded the Nobel Prize in Chemistry in 1993. Today automated PCR replicators can make 100 billion copies of a DNA fragment in a few hours. DNA from a single drop of blood or a hair can assist in forensic science investigations. Traces of DNA from fossils can be multiplied to help establish genetic connections between species. This process inspired the movie Jurassic Park and is the reason scientists predict that one day they may be able to reproduce extinct species.

double-stranded DNA heating separates strands, primers added

multiple cycles produce copies with desired gene region

single DNA strands with primers

DNA polymerase reaction new gene regions built

two copies of DNA with desired gene region FIGURE 6.42 Polymerase chain reaction.

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Inserting genes into bacteria Most bacteria contain small circular pieces of DNA called plasmids. Plasmids can be used as vectors or carriers to transfer transgenes into bacteria (Figure 6.43a). Plasmids are inserted into bacteria by treating the bacteria with cold calcium chloride followed by heat. This ‘softens’ the bacterial wall sufficiently to allow the plasmids to penetrate it. An isolated gene can be cloned rapidly using bacteria. Human genes that code for the production of human insulin and human growth hormone have been inserted into bacteria. The bacteria are then cultured and the hormones produced by these modified bacteria are harvested and used to treat people who require them. People with insulin-dependent diabetes can be treated with human insulin and children with specific growth defects can receive human growth hormone. Bacteriophages (viruses that infect bacteria) can also be used as vectors to insert genes into bacteria (Figure 6.43b). human chromosome

interferon gene bacterial plasmid

bacteriophage DNA

same restriction enzyme splits both types of DNA

Bacteriophage parasitises bacteria, and bacteriophage DNA may be built into host DNA. If so, it is replicated as the bacteria grow and divide.

A A T

AA

TT

TT AA

T

A

A

Genetic engineer can extract DNA, 'cut' it with restriction enzyme, and add a 'foreign' gene, using ligase.

TT

interferon gene joins with plasmid by sticky ends plasmid

interferon gene plasmid is taken up by bacterium

interferon injected into patient

bacteriophage DNA 'foreign' DNA

bacterium each daughter bacterium inherits interferon gene (a)

This relationship is exploited when bacteriophage in induced to take up engineered DNA.

'Foreign' gene is copied too, as the bacterium reproduces.

interferon-producing bacterial population (b)

FIGURE 6.43 Cloning genes using bacteria.

Inserting genes into plant cells Plant cell walls make it difficult to insert genes into plant nuclei. One successful technique uses a soil bacterium, Agrobacterium tumefaciens. This naturally causes disease by inserting a plasmid into a plant cell, resulting in the formation of a tumour (Figure 6.44).

Inserting genes into animal cells Genes can be transferred into animal cells by microinjection using very fine glass micropipettes.

Transferring genes using a particle gun Particle guns can be used to shoot DNA-coated microscopic gold or tungsten pellets directly into animal or plant cells. 318

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isolated plasmid

recombinant plasmid

co-culture of transformed Agrobacterium with isolated plant cells (pieces of plant tissue can also be used)

callus tissue yields healthy plantlets with new traits nutrient medium

cut with same restriction enzyme

recombinant plasmid introduced into Agrobacterium

transformed plant cells grow into a mass of transformed undifferentiated cells plant cell known as a callus FIGURE 6.44 The use of Agrobacterium in gene cloning.

FIGURE 6.45 Micropipette technique for transferring animal genes.

The elusive blue rose Florigene, founded in Melbourne in 1986, is a world leader in the molecular breeding of flowers. Florigene strived to produce the world’s first ‘blue rose’ through genetic engineering. In 1991, scientists at Florigene isolated the blue gene from the petunia flower. Using recombinant DNA techniques, the blue gene may be extracted from the petunia. Biochemical gene splicing techniques are used to isolate or extract the blue gene. Once isolated, the blue gene is introduced into the carnation plant cell by the recombinant plasmid. Once inside the carnation cells, the petunia gene will need to ‘switched on’ so that the cells will produce the enzyme required to synthesise the blue gene. In 1992 Florigene commenced patenting its find throughout the world. This gene encodes an enzyme needed to synthesis blue pigment molecules called delphinidins. Unfortunately, Florigene’s aim to pro-

duce a ‘blue rose’ is yet to be achieved. Scientists have discovered delphinidins are only blue at high pH, and the vacuoles that hold petal pigments in roses are acidic. Scientists believe that with more research, their goal of creating a blue rose will be achieved in the near future. In 1994, the blue gene was successfully implanted into carnations, which resulted in the first carnation to express colour in the range. This flower, which is called ‘Moondust’, was introduced in 1996 as the world’s first genetically modified flower to be sold commercially. ‘Moondust’, is a light mauve minicarnation. In 1997, ‘Moonshadow’ was developed as Florigene’s second carnation to contain the blue gene, which in this case was expressed as a rich violet colour. In 1998, ‘Moonshadow’ was launched in Australia and a year later in Europe, Japan and the United States.

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FIGURE 6.46 Genetically engineered flowers are already making an impact in the cut-flower trade.

Identify the technique used to isolate the blue gene from petunias.

Ethics and transgenic species Many social, economic and ethical implications issues arise from the use of new reproduction technologies and genetic engineering. Public concern is raising many questions that need to be answered.

Food safety and health • Are genetically modified plants and animals safe to eat? • Has the nutritional value of genetically modified plants and animals been changed?

Environmental protection • What effect could genetically modified organisms have on natural ecosystems? Could they cause damage or spread uncontrollably? • Will there be a loss of genetic biodiversity through the use of genetically modified organisms? Is there a danger in relying on just a few varieties?

Regulating issues • Should there be more government regulation to protect farmers, consumers and the environment?

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• Should genetically modified foods be clearly labelled?

Social and economic effects • What effect will genetically modified crops and livestock have on farming practices? • Could biotechnology companies with patents on genetically modified organisms and monopolies on their use eventually control world food production? • Could the money spent on genetic engineering be better used on more basic human problems such as housing, health and nutrition?

Ethical and moral issues • Why should we stop using these technologies if they help treat diseases and increase food production? • Should we be tampering with nature in this way? • Is it right to change living organisms for commercial gain?

Potential impacts on genetic diversity The fear that arises from the use of reproductive technologies is that the natural genetic diversity of organisms is being decreased. Already the globalisation of markets has led to fewer varieties being available to more people. If more and more farmers plant genetically modified crops obtained from a global company, then local varieties will die out at an increasing rate. The United Nations’ Food and Agriculture Organization (FAO) has estimated that since 1903 over 20 000 varieties of crop plants have already been lost in the USA alone. Seed banks have been set up in many countries to conserve remaining seed varieties.

BIOFACT Tomatoes—over 4000 varieties known Potatoes—over 3000 varieties known Apples—over 1000 varieties known Check how many varieties are sold by your local supermarket or greengrocer. Cattle—over 700 breeds known Sheep—over 200 breeds known

Bt engineered crops Bacillus thuringiensis (Bt) is a bacterium that naturally produces chemicals that kill many insects. The Bt chemicals are specific to a few species of insects only and do not harm other species. The chemicals are broken down (denatured) in sunlight and so do not remain in soil or water as pollutants. Pesticide sprays containing dead Bt bacteria are used very successfully on crops. Genetic engineering can eliminate the need to spray by inserting the Bt genes that make the Bt toxin directly into plants. Tomatoes, tobacco, corn, potatoes and cotton containing Bt genes have been produced. These plants produce the toxin continuously, so they are more resistant to insect attack and need less spraying with pesticides. Bt engineered crops are sold under licence and with strict conditions for their use. A serious concern is that the target insects will become resistant to the Bt chemicals. The first genetically engineered crop approved for growing in Australia was Bt cotton.

If Bt cotton becomes the standard cotton crop throughout the world, then not only will other varieties be lost but the species itself may become vulnerable if environmental conditions change. Cloning techniques of domestic animals, such as those that produced Dolly (see p. 311), and the genetic modification of livestock for increased size, such as sheep (see p. 314) and pigs, could result in multiple genetic copies becoming widespread in the population, with a subsequent loss of natural variation. Although these techniques speed up the introduction of favourable characteristics into a population, they do so at the expense of a wider pool of genetic variety. There is concern about this reduction in genetic diversity of crops and livestock. Any reduction narrows the capacity of the species to respond to changes in the environment, to disease, to local geographic conditions and even to changes in human demand either in terms of market or production requirements.

BIOFACT Some genetically engineered crops and their modifications are: soy bean herbicide tolerance canola herbicide tolerance corn insect resistance and herbicide tolerance potato insect and virus resistance, and herbicide tolerance sugarbeet herbicide tolerance cotton insect resistance and herbicide tolerance Herbicide tolerance means that farmers can spray greater amounts of herbicides on to their land. Weeds that compete with crops or spoil their quality can be killed without destroying the crop plants. Insect resistance means that the plants produce a toxin or poison that makes them distasteful to insect predators.

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Questions 1

a Define ‘artificial insemination’ and ‘artificial pollination’. b Describe some advantages of artificial insemination and artificial pollination.

2

a What is a clone? b Use a series of dot points to summarise the steps involved in cloning a sheep. c Explain how all the sphagnum plants in an alpine valley might be clones (Hint: see Chapter 4).

3

Explain what is meant by a transgenic organism and give two examples explaining why each was produced.

4

Describe two processes used to produce transgenic species.

5

For a named plant and a named animal that have been selectively bred, explain the potential impact of the plant and animal on genetic diversity.

F u r ther questions 1

2

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Find out about the Human Genome Project. (Include an Internet search in your investigation.) a What is it? b What important information has this project provided? c Outline some potential uses for the information. Genetically modified foods have attracted a great deal of publicity in recent times. a Find out what is involved in developing genetically modified food products such as crop plants and livestock; for example, wheat, tomatoes, pigs. b Explain why food plants and animals are genetically modified. c Outline the advantages and disadvantages of genetically modified food compared with traditionally managed products. d Conduct a survey to investigate community reaction to the idea of genetically modified foods. (Note: It is important to phrase your questions carefully and clearly. Be sure to check your questionnaire with your teacher before conducting the survey.) e Summarise community attitudes to genetically engineered foods.

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f Write your own response to this issue. 3

a Prepare a report on the steps used by the Roslin Institute in Edinburgh, Scotland, to create Dolly, the cloned sheep. You could present your findings as a written report, a computer presentation such as Powerpoint, a brochure or a poster. b Identify specific examples of successful cloning that have been undertaken since Dolly. Include Australian examples. c Outline the advantages and disadvantages of cloning animals.

4

Prepare a list of ethical issues arising from cloning technology about which there is community concern. Choose one of these as the basis of a class debate.

5

a Outline the arguments used to suggest that reproductive technology can adversely affect biodiversity. Are these concerns justified? Explain your reasoning. b Would a plantation of Sydney blue gums have a higher biodiversity than a plantation of pine trees? Explain your answer.

Chapter summar y Practical activities 6.1



Modelling natural selection



Change in a species Vertebrate forelimbs Technology and change Theories of evolution







6.2

6.3



Family trees

● ●

Genetics problems Hybridisation



A model of meiosis



Codominance and sex linkage The effect of the environment on phenotype



6.4



A model for polypeptide synthesis



‘One gene—one protein’ Mutations The development of resistance Solving the structure of DNA

● ●



6.5



Cloning



Transgenic species—debating the issue

6.1 • Changes in the physical or chemical conditions in the environment or competition for resources can bring about evolution by natural selection in plants and animals. • Evidence from the study of palaeontology, biogeography, comparative embryology, comparative anatomy and biochemistry all support the theory of evolution. • Adaptive radiation, divergent evolution and convergent evolution can be explained using Darwin and Wallace’s theory of evolution by natural selection and isolation. 6.2 • Gregor Mendel conducted breeding experiments, especially with pea plants, that helped advance our knowledge of the inheritance of characteristics. • Mendel’s successful experimental techniques involved studying easily observed, clear cut characteristics in pure-breeding plants. He studied these characteristics separately and, in his breeding experiments, deliberately crossed plants to ensure that self-pollination could not occur. He counted and kept careful records of his results, repeating the same cross many times. • In a monohybrid cross involving simple dominance, the first generation of offspring all show the characteristic of one parent—the dominant form. The parental form not expressed is known as the recessive form. In the F2 generation both dominant and recessive forms are shown in the offspring in a 3 : 1 ratio. Mendel called the units of inheritance controlling a characteristic ‘factors’. Today we call them genes. • A homozygous genotype has two identical forms of the gene for a particular characteristic, e.g. TT. A heterozygous genotype has two different forms of the gene for a particular characteristic, eg. Tt. • Alleles are genes for the same characteristic. For example, for the characteristic of height, there may be an allele for tallness and an allele for shortness. Each individual has a pair of genes, each one an allele, for each characteristic. • The genotype of an organism is its genetic make up—the composition of its genes. The phenotype of an organism is its appearance. An individual may show the dominant characteristic, eg. tallness, but if it is genetically heterozygous it will contain one dominant allele and one recessive allele. Although it will appear tall, it will not breed true for tallness. • When Mendel’s work was published, little was known about cells and nothing was known about the processes of mitosis and meiosis. It was not until other scientists independently reached similar conclusions that the importance of Mendel’s experiments and conclusions was recognised. 6.3 • Sutton and Boveri identified the importance of chromosomes in inheritance, that they carry the units of inheritance (genes) and they occur in pairs.

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• DNA is a double-stranded molecule twisted into a helix. Each strand comprises a sugar-phosphate backbone and attached bases. There are four bases: adenine (A), thymine (T), cytosine (C) and guanine (G). Each stand is connected to a complementary strand by pairing the bases A–T and G–C. • The inheritance of genes, as demonstrated in Mendel’s experiments, can be explained by the structure and behaviour of chromosomes during meiosis when the paired strands separate and each gamete contains only a single set of genes. • Gamete formation and sexual reproduction provide the opportunity for new combinations of genes to occur resulting in genetic variation in the offspring. • The inheritance of sex-linked genes and alleles that show codominance do not produce simple Mendelian ratios. In sex linkage the characteristic appears associated with one sex rather than by random recombination and in codominance both alleles are expressed in the offspring. • Our understanding of sex linkage was the result of Morgan’s work on the inheritance of eye colour in fruit flies (Drosophila). He deduced from the ratios he found in the F2 generation when no white-eyed females appeared that the gene for red eye colour was on the X chromosome. • In codominance a third phenotype is found as a result of crossing two different homozygous genotypes. The third phenotype is the heterozygous genotype that reflects the expression of both alleles. • The environment can affect the degree to which a gene may be expressed in an individual. For example, soil type and weather conditions may affect plant growth, food type and availability may affect animal growth. 6.4 • In DNA replication an identical copy of a DNA molecule is made. The significance of this is that identical copies of the genes can be made and passed on to new cells. • Genes contain the coded information required to make polypeptides for a cell. The production of polypeptides is carried out via two processes: transcription and translation. • A protein is one or more polypeptides folded or twisted to form a uniquely shaped molecule with a specific cellular function. • A mutation is a change in the DNA information on a chromosome. Any change in base sequence will affect the genes and may lead to the generation of new alleles. Mutations provide a source of variation in organisms. • Radiation can cause mutation. Evidence has accumulated of the increased mutation rate in organisms exposed to various forms of radiation such as ultraviolet, X-rays and radiation from radioactive sources. • Understanding how variation can arise in organisms provides support for Darwin’s theory of evolution by natural selection. • Darwin’s theory of evolution suggests that evolution occurs gradually over time. The theory of punctuated equilibrium proposes that long periods of stability are punctuated by periods of rapid evolutionary change.

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6.5 • Artificial insemination, artificial pollination and cloning are reproductive techniques for the selective breeding of deliberately chosen characteristics that may alter the genetic composition of a population. • A transgenic species is produced by insertion of a gene or genes from one type of organism into the cells of another. An example of one of the processes used is the use of restriction enzymes to ‘cut’ out useful genes from a DNA strand. • Reproductive technologies have the potential to reduce variation and limit the genetic diversity of a species.

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EXAM - STYLE QUESTIONS Multiple choice 1 What is the impact of the discoveries of fossils of ‘transitional’ forms of living things which appear to have features of two major groups of organisms? A They cause scientists to revise their classification systems. B They are usually considered to be hoaxes. C They are only useful when the age of the specimens has been determined. D They support the idea that one type of living thing may have developed from another. 2 Natural selection over many generations can result in similar adaptations in unrelated species that live in similar environments. Which of the following is the correct term to describe this? A divergent evolution B convergent evolution C parallel evolution D adaptive evolution. 3 Which of the following is the expected proportion of offspring from a cross between a homozygous short pea plant (tt) and a heterozygous tall pea plant (Tt)? A all short B all tall C half tall, half short D three-quarters tall, one-quarter short. 4 Which statement best describes dominant characteristics? A They are more common than recessive characteristics. B They mask recessive characteristics. C They are masked by recessive characteristics. D They occur in equal proportions to recessive characteristics. 5 What is the chemical composition of repeating units of DNA? A sugar–base–phosphate B lipoprotein C complex carbohydrates D deoxyribonucleic acid. 6 Which of the following processes describes how variability of offspring can be produced through sexual reproduction? A crossing over during gamete formation B a random fusion of unique gametes during fertilisation C both A and B D neither A nor B.

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7 Which of the following processes is made possible by the complementary nature of the bases in DNA? A the inheritance of coded information by offspring from their parents B the exact copying of the two strands of the molecule during replication C pairing of adenine only to guanine, and thymine only to cytosine D coding of one nucleotide to the next along the length of the molecule 8 How does the manufacture of proteins in cells occur? A It occurs directly by the DNA in the nucleus. B It occurs by the process of transcription. C It occurs by the process of translation. D It occurs as a result of the processes of transcription and translation. 9 Which of the following statements best describes mutations? A They are the result of cancers. B They always affect the next generation. C They are changes in the genetic material of an organism. D They are changes in the chromosome number of a species. 10 Which of the following definitions of transgenic organisms is correct? A They are created when the DNA from an organism of one species is inserted into the DNA of an organism of another species. B They are hybrids produced when a male and female from different species interbreed. C They are produced as a result of mutation. D all of the above. 11 Study the following diagram of a duplicated chromosome.

Which of the following does this chromosome represent? A A cell undergoing mitosis. B Replication of homologous chromosomes. C Crossing-over during meiosis. D A chiasma event during a mitotic division.

Short answers 1 Study the diagram illustrating the stages of embryological development for different kinds of organisms.

fertilised egg early cell division

a Rewrite the letters corresponding to each stage so that the order correctly reflects the behaviour of chromosomes during the process of meiosis. b How do the daughter cells resulting from meiosis differ from those produced in mitosis? 4 Examine the DNA strand shown. X

A C

late foetal

A

C

B

D

Y G G

chicken

pig

monkey

human C

a Explain how comparative embryology supports the theory of evolution. b Describe how the theory of evolution of life on Earth is supported by two of the following areas of study. i palaeontology ii biogeography iii comparative anatomy iv biochemistry. Use specific examples to illustrate your answers. 2 The litter resulting from the mating of two black guinea pigs consists of four black and one white offspring. a Explain the inheritance pattern observed. b Suggest genotypes for the parents and the offspring in this cross. c Use a Punnett square to show the expected ratio of genotypes and phenotypes. 3 The following diagram illustrates the stages of meiosis. However, the stages are shown in incorrect order.

A

B

C

D

E

F

T

a Identify the molecule represented by structure i X ii Y iii A, C, G and T. b Draw the complementary strand for the DNA sequence shown. 5 The following tree diagram illustrates a possible evolutionary pathway for the some of the finch species that Darwin observed on the Galapagos Islands. insect eaters ancestral group

seed eaters cactus eaters

Explain how the theory of natural selection can account for the evolution of the different finch species. In your answer, discuss the effects of genetic variation, adaptation, isolation and time. 6 a Seals (mammals) and turtles (reptiles) both inhabit the sea and have flippers for locomotion. Explain why they are not classified in the same phylum despite these similar characteristics. b Isolated islands often have distinctive flora and fauna. Australia has a large number of kangaroo species. Explain how the theory of evolution accounts for this.

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7 a Define ‘transgenic species’. b Use a specific example to explain how a transgenic species can be produced. c Outline two ethical issues arising from the technology that makes the production of transgenic species possible. 8 Haemophilia is a recessive sex-linked condition. In an affected person the normal clotting of blood during injury does not occur and the individual experiences excessive bleeding. a Explain what is meant by sex linkage. b Consider the case in which a woman who is a carrier of the disease marries a normal man. Use a Punnett square and appropriate notation to illustrate the possible genotypes and phenotypes you would expect in their offspring. c What is the probability of these two people having an i affected child ii affected son iii affected daughter iv unaffected child? 9 George Beadle and Edward Tatum conducted experiments to investigate the relationship between genes and proteins. a What is a mutation? b Describe the experiment they undertook using the bread mould Neurospora crassa that led them to the ‘one gene, one polypeptide’ hypothesis. c What is meant by the ‘one gene, one polypeptide’ hypothesis? d i List two different kinds of mutagenic agents. ii How do mutagens affect DNA?

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10 The following pedigree shows the inheritance of a particular autosomal disease in humans.

affected male I 1

affected female

2

II 1

2

3

4

5

6

7

8

9

3

4

III 1

2

a Is the disease inherited as a dominant or recessive trait? Give reasons for your answer. b Use appropriate notation to assign genotypes to individuals I–1, I–2, II–2, II–7 and III–1. c Suggest possible genotypes for individual II–3. d Use a Punnett square to show what proportion of offspring produced by parents I–1 and I–2 are expected to have the disease. e Suppose individual II–9 has no family history of the disease. What is the probability of this couple having an affected child? Explain.

Chapter 7

THE SEARCH FOR BETTER HEALTH

Recall the last time you were unwell—how did you feel? Did you require medication or did you get better unaided? It is often easy to describe what being unwell is like but it is not so easy to define good health. A person’s sense of biological, psychological and social well-being is often used to describe health. That is, we measure health in terms of being active and free from pain, feeling happy and thinking clearly, relaxing and getting on socially with others. To maintain health, repair of the body’s cells and tissues must take place, suitable nutrients must be obtained, wastes must be removed, and disease-causing organisms must be combated. In addition, perfect copies of genetic material must be replicated and the expression of the genetic code must be carried out effectively. Over time, humans have learned to recognise the symptons of disease in themselves and other organisms around them. They have learned to identify many of the causes of disease, and through trial and error, experience, and experimentation have learned how to prevent and treat many diseases. There remain many diseases, however, where both cause and effective treatment remain a mystery. Increasing research and technological advances are assisting scientists to understand the causes of disease and the approach to take in the treatment and management of disease. Better communication and effective public education campaigns are also used to inform people about health issues. Examples of these include vaccination and safe sex practices.

This chapter increases students’ understanding of the history, nature and practice of biology and the applications and uses of biology, and the implication of biology for society and the environment.

7.1

What is a healthy organism? OBJECTIVES When you have completed this section you should be able to: ● discuss the difficulties associated with defining health and disease ● outline how the function of genes and mitosis are involved in the health of an individual ● define cell differentiation and cell specialisation, and outline how these are involved in maintaining the health of an individual.

Defining health and disease activity ●

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Defining ‘health’ is not easy because it has many components, and some of these are very subjective. Most of us would describe health in terms of a person’s sense of biological, psychological and social well-being. Biological health includes being active and free from pain; psychological health includes feeling happy and thinking clearly; and sociological health is necessary for getting on with others and being able to work and relax. Each of these factors has a different meaning for different people—levels of physical activity, sensitivity to pain, inner happiness, work ethic, and so on—and these can vary for an individual at different times. ‘Disease’ is similarly difficult to define because it is a state of impaired functioning or poor health. It depends on an individual’s normal level of functioning and on what they expect their quality of life to be.

If we broaden our discussion to include other animals and plants, the definitions can be made a little simpler: health can be defined as a state of normal functioning, and disease as a state of impaired functioning.

(a)

An indication of disease noticed by a patient is called a symptom.

(b)

FIGURE 7.1 (a) Healthy and (b) diseased wombats. In the wild, diseased animals are easy prey for introduced predators such as foxes, cats and dogs.

The maintenance of health Mitosis and genes The maintenance of health of an organism is assisted by the maintenance and repair of the body’s cells and tissues. The function of genes is to ensure that the correct proteins are produced in a cell to enable all cellular processes to continue (see Chapter 6, p. 299). Mitosis enables the genetic material to be copied correctly when new cells are formed. The ability to produce new, genetically correct cells enables an organism not only to grow, but also to repair damage to body parts (see Chapter 2, p. 94). Changes or mutations in the genetic material may occur during the life of an organism (see Chapter 6, p. 302). In most instances these changes are damaging to healthy cells and result in cell death, an inability to function correctly, or uncontrolled cell growth (cancer). Healthy cells that are growing and dividing normally have their cell cycle carefully regulated. This control is brought about by the proteins produced by different types of genes. DNA repair genes produce enzymes that ensure the DNA is accurately copied in the S phase of the cell cycle. If these genes mutate, the enzymes produced do not function correctly and, typically, the replicated chromosomes are incomplete, so that chromosome fragments are found in the cell. Two other types of genes, proto-oncogenes and tumour suppressor genes, play a complementary role in regulating the cell cycle. Proto-oncogenes produce proteins that stimulate cell growth and cell division. Tumour suppressor genes produce proteins that slow down or stop cell growth and cell division. In a healthy cell, this balance results in the control of the cell cycle.

G2 second growth stage S DNA synthesis

M mitosis

G1 first growth stage

G0 quiescence (non-division)

FIGURE 7.2 The cell cycle.

The search for better health 331

Cancer is the disruption of the normal orderly and regulated cycle of cell replication and division. Uncontrolled growth leads to the formation of tumours. (See also p. 99 and p. 370.)

BIOFACT Cancer is the most common cause of death in Australia, accounting for more than 35 000 deaths each year. 82 000 new cases of cancer are diagnosed in Australia each year. In 15–44 year olds, the commonest cancers are melanoma and breast cancer. Overall, the commonest fatal cancers are lung, breast, prostate and colon cancers.

Different types of cells become specialised for different functions within a multicellular organism.

This control breaks down if the proto oncogenes or tumour suppressor genes are themselves damaged or mutated. Mutated proto-oncogenes are called oncogenes. Oncogenes cause uncontrolled cell replication. Little or no time is spent in the G2 phase, so the cell cycle is kept continuously active. Oncogenes show genetic dominance. Only one allele of the pair needs to mutate for the abnormality to be expressed and rapid cell division to occur. When tumour suppressor genes mutate, they lose their ability to control cell division. The rate of cell division increases, and uncontrolled growth occurs. Tumour suppressor genes tend to be recessive. Both alleles must mutate before rapid growth occurs. A single mutated suppressor gene may be inherited, but a second mutation needs to occur before uncontrolled growth and a tumour develop.

Cycle stopper: the p53 gene The p53 gene, located on chromosome 17 in humans, plays a critical role in controlling damage to genetic material. In a normal cell cycle, p53 is inactive. But if the DNA in a cell is damaged in any way, the gene becomes active and produces the p53 protein. This protein acts to stop the cell cycle during G1. Stopping the cell cycle allows the DNA to be repaired. Once this is done, cell replication and division recommence. So what happens if the p53 gene itself mutates? It will not be able to stop the cell cycle, so any DNA that is damaged cannot be repaired. Instead, the cell cycle continues in an uncontrolled way, causing the accumulation of mutated or damaged DNA in cells and resulting in tumours. Tobacco smoke, several viruses and some fungal toxins are known to alter the p53 gene. About 50% of cancers in humans contain a p53 mutation.

Cell specialisation accompanies cell differentiation Different types of cells become specialised for different functions within a multicellular organism. Mammals have different tissues, such as blood, nerves, muscles and bones. These tissues work together in a healthy body in a controlled and coordinated way (see Chapter 2, p. 66).

Healthy cells can adapt to changes Changes can occur in the internal environment; for example, when blood calcium levels fall, bones release their calcium and may themselves become soft to prevent the more serious consequences of low calcium levels in the body. Or there may be changes in size or number in response to other factors such as increased physical activity (muscles increase in size) or changes in altitude (at high altitudes the number of red blood cells increases). These processes occur as a result of the healthy functioning of the genes, the production of perfect copies of the genetic material, and their expression in the processes of cell differentiation and specialisation. If these processes are impaired, the organism will not be healthy. 332

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Questions 1

Define the word ‘disease’.

2

Outline the difficulties in defining health and disease.

3

Briefly outline how each of the following contribute to the health maintenance of individuals: a function of genes b mitosis

c cell differentiation d cell specialisation. 4

Describe an example of how the body can adapt to changes in the internal environment to ensure that an individual’s health is maintained.

F u r ther questions 1

2

Use different kinds of references, such as a dictionary and medical encyclopaedia, to help you construct meaningful definitions of the terms ‘health’ and ‘disease’. Prepare a questionnaire addressing community views about what it means to be healthy. Include each of the following points: a What does it mean to be healthy/unwell? b Importance of different factors to health, for example ● frequency, level and type of physical activity ● diet ● work ● sleep ● having interests outside school or work ● pain ● happiness. c How do people perceive their own level of health? Preparing the questionnaire: ● It is important to present questionnaires in a particular way to ensure people will be happy to participate and will answer as honestly as possible. ● Ensure that your questionnaire is anonymous. ● Phrase questions so that they are not too personal.







It is a good idea to provide tick the box options, including ‘other’ and space for further comment. You may choose to use a sliding scale of how strongly people agree or disagree with a statement you make about a factor related to health. Have your teacher check your questionnaire before handing it to participants.

Issuing the questionnaire: Invite a range of different people to participate in your questionnaire. For example, younger students, older students, parents, grandparents, and adults in a range of different occupations. Analysing the responses: Are there noticeable differences between responses from people in different age groups, occupations? ● Summarise the differences you notice. ● How do the participants’ perceptions of their own level of health compare with their definition or understanding of what is good health? ● How do the participants’ perceptions of good health compare with the definition provided by an authority, such as a reference book on the topic of health? ●

Undertake the questionnaire yourself.

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7.2

The importance of cleanliness OBJECTIVES When you have completed this section you should be able to: ● explain the difference between non-infectious and infectious diseases, giving examples of each ● explain the importance of cleanliness in food handling and storage, clean water and personal hygiene in the control of disease ● discuss the factors that contribute to health and disease ● explain when an organism is called a pathogen.

Infectious and non-infectious disease activities ● ●

Identifying microorganisms Fit to drink

• Microscopic means too small to be seen with the unaided eye. • Macroscopic means can be seen with the unaided eye.

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Disease, or impaired functioning of an organism, may come from within the organism itself. It may also be the result of damage caused by another organism or by adverse non-living factors in the environment. Disease can be infectious or non-infectious. Infectious disease is caused by an infecting organism, which could be microscopic or macroscopic. A pathogen is an infective agent that causes disease. Some examples of pathogens that affect humans are given in Tables 7.1 and 7.2. Non-infectious diseases may develop as a result of genetic inheritance, nutritional deficiencies or environmental factors. Some examples are given in Table 7.3.

TABLE 7.1 Diseases caused by microscopic pathogens in humans.

Prions

V i ruses

Bacteria

P rotozoans

Fungi

Creutzfeld-Jacob disease (CJD)

influenza

tonsilitis

amoebic dysentry

ringworm

Fatal familial insomnia (FFI)

herpes

tuberculosis

giardia

dandruff

Kuru

poliomyelitis

gonorrhea

malaria

tinea

Alpers syndrome

AIDS

tetanus

sleeping sickness

thrush

TABLE 7.2 Some macroscopic organisms which can cause disease in humans.

E x t e rn a l p a r a s i t e s (ectoparasites)

I n t e rn a l p a r a s i t e s (endoparasites)

head louse

tapeworm

body louse

flukes

crab louse

threadworm pinworm

bed bug

hook worm

flea

filarial worm

tick

roundworm

itch mite

whipworm giardia parasite malarial parasite

TABLE 7.3 Categories and examples of non-infectious disease in humans.

C a t e g o ry

Examples

Inherited

haemophilia Down syndrome cystic fibrosis phenylketonuria Huntington’s disease

Nutritional

scurvy beri-beri anorexia nervosa kwashiorkor tooth decay

Environmental

skin cancer lung cancer heavy metal poisoning asbestosis hearing loss stress asthma

Non-infectious diseases may develop as a result of genetic inheritance, nutritional deficiencies or environmental factors.

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Contributing factors Three interacting factors contribute to health and disease: 1 the host organism 2 the agent of disease 3 the environment.

Three interacting factors contribute to health and disease: 1 the host organism 2 the agent of disease 3 the environment.

The host Organisms vary in their resistance to infection; individuals vary, and the resistance of a particular individual can vary with time.

Interpersonal variations A healthy person might resist an infection which is devastating to someone whose defences are poor for some reason. For example, elderly people are often more prone to illness than younger people; and AIDS sufferers have very little resistance, so that the infections they catch often have unusually severe effects.

Personal variations People under stress may succumb to an infection (e.g. a common cold) that they might resist at another time.

Resistance and immunity Whether or not people suffer disease as a result of infection by a particular pathogen depends partly on their ability to fight the infection. Humans have non-specific and specific defences with which to fight diseases; this protection and immunity is discussed in detail in Section 7.7.

Behaviour By adopting healthy lifestyles we can reduce our susceptibility to many diseases (e.g. through a better diet and exercise), and by participating in screening programs we may reduce the severity of a disease (e.g. by early detection of melanoma).

The agent of disease Specificity

FIGURE 7.3 Keeping fit and taking part in healthy activities reduces our susceptibility to many diseases.

Most infective agents infect only one species—humans do not usually share infections with other organisms. For example, cat influenza is not passed to humans, nor is human influenza passed to pets. A pathogen can usually grow only in a particular host, and only within certain tissues of the host.

Infective dose It is possible that contact with a pathogen does not result in disease because the dose is too small. An infective dose is one which is in sufficient quantity to cause disease. The virulence of a pathogen is its ability to cause disease.

Effect on host Most pathogens stimulate defensive reactions in the host, including increased white blood cell production, and they are often accompanied by fever and tiredness. Depending on the pathogen, the infection will be localised in one part of the body (e.g. cold sores), widespread (e.g. influenza), intense (e.g. poliomyelitis), or trivial (e.g. head lice). 336

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The environment The nature of the environment will affect the likelihood of a pathogen growing and being passed from one host to another. Standards of housing, water supply, sewerage and air pollution are all important in this regard. Many infectious agents are spread in crowded unhygienic conditions.

BIOFACT Legionnaire’s disease, which can cause death, is spread by breathing in airborne droplets containing the Legionella bacteria.These bacteria normally occur in soil, but can thrive in other warm, moist environments. The water towers of air-conditioning plants in buildings provide suitable conditions for their growth and have been the source of many outbreaks of the disease.

FIGURE 7.4 With no running water or hygienic sewage and rubbish disposal, people living in slums risk contracting serious diseases such as cholera and typhoid fever.

Cleanliness and contamination Control measures can stop the spread of infectious diseases, particularly if we know how a disease is transmitted (see p. 341). General hygiene and cleanliness are important in reducing the transmission of infectious diseases. Cleanliness may be at the personal level, such as hand washing, and, for a society, may include government legislation and controls regulating public health issues such as sewage disposal, water purity and the handling of food for sale. Many ancient civilisations recognised the importance of cleanliness in food, water and personal hygiene. The third book of the Old Testament (Leviticus, Chapters 11 and 13) sets down hygienic practices that included personal cleanliness, protection of water and food supplies, and protection from the spread of infectious diseases, particularly the identification and treatment of leprosy. The Minoans and Cretans had drainage systems and flushing toilets, and the ancient Romans had public baths, sewers and carefully maintained water supplies. The Chinese were writing about health and medicine as long ago as 2500 BC. They believed that good health was the correct balance of two energy forces in the body, Yin and Yang. Disease resulted from an imbalance, which could be treated by acupuncture and herbal treatments.

Cleanliness is important because it is the way that the spread of infectious disease can be prevented or minimised.

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Food practices Contamination of food may be visible, such as dirt or insects, or it may be invisible to the naked eye, such as contamination by microorganisms. Diseases caused by contaminated food include salmonella, clostridial food poisoning and botulism. Hygienic food-handling reduces the spread of diseases such as food poisoning. There are many general hygiene rules relating to food that we are all familiar with, such as: • Use clean utensils and plates. • Do not sneeze or cough over food. • Do not use food that has fallen onto the floor. • Wash your hands after using the toilet. • Cover any cuts or sores before handling food. • Store perishable foods in a refrigerator.

FIGURE 7.5 Good personal hygiene means washing hands before handling and eating food. Using tongs when serving food and wearing protective gloves and clothing minimises the chances of transmitting diseases.

In New South Wales, the Food Act 1989 and the Food (General) Regulation 1997 regulate all matters relating to food hygiene and the protection of food from contamination for the safe handling of food for sale. Their requirements are detailed, and inspection of any premises selling food must be allowed. Clause 26 of the Food Regulation states that food handlers must be clean and wear clean clothing, and that they must wash their hands before commencing work, after handling a handkerchief or tissues, and after visiting the lavatory. Clause 27 says: ‘Do not urinate, defecate, spit or smoke in a place used for food, or sit, stand or lie on food preparation surfaces.’

Universal Precautions Universal Precautions is an infection control method that was developed to reduce the spread of serious infections. For example, rather than determining which person is contagious, hospital staff treat every patient as being potentially infected with the human immunodeficiency virus (HIV), hepatitis B (HBV), and other blood-borne pathogens. The Precautions are a set of guidelines to be followed when there is contact, or the possibility of contact, with blood and other body fluids, wastes, cells, and tissues that may carry germs and bacteria. Many of the infection control practices are different from one hospital to another, and different again to those in place in health-care centres or laboratories, but they usually include the following: 1 Avoiding contact with blood and body fluids by using disposable latex gloves, gowns, aprons, masks and protective eyewear. 2 Avoiding injuries caused by needles, scalpels and other sharp instruments or tools. 3 Having proper and sufficient supplies of equipment to treat the patient. For what reasons were these guidelines developed?

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4 Immediately cleaning and disinfecting contaminated surfaces with a bleach–water mixture or other disinfectants. 5 Using disposable absorbent material to control bleeding. 6 Disposing of blood-stained clothing and other waste in sealed plastic bags, using lined rubbish bins with lids. 7 Washing hands for at least 30 seconds after contact with blood, body fluids contaminated with blood, or contaminated clothing or wastes. 8 Covering cuts and scratches with bandages until they have healed. 9 Not eating, drinking, smoking or applying cosmetics or contact lenses when working in laboratories. 10 Placing used syringes and needles and other disposable sharp instruments in punctureresistant containers for disposal. 11 Using a mechanical device (forceps) to place contaminated surgical equipment into appropriate containers for autoclaving (sterilising).

Sydney’s water: problems and solutions Water quality became an important issue for people living in Sydney because of the problems experienced between late July and mid September 1998. Water is supplied to homes and businesses by an extensive network administered by water authorities established to oversee the flow and quality of our water. In Sydney, Sydney Water is the authority with this responsibility. Domestic water quality must comply with strict standards and guidelines. The Australian Drinking Water Guidelines are set by the National Health and Medical Research Council and the Agricultural and Resource Management Council of Australia and New Zealand. Sydney’s water quality monitoring program is controlled by NSW Health. The Licence Regulator examines Sydney Water’s methods of monitoring and testing water quality each year, and consults NSW Health. Water must be continually tested for microbiological, chemical and physical quality. The problems experienced by Sydney has focused attention on two protozoans: cryptosporidium and giardia. These parasites are usually transmitted through water contaminated with faeces of an animal or person, or by direct contact with a carrier or contaminated food. The symptoms they cause are abdominal cramps, diarrhoea, nausea and vomiting. Some Australian water catchments are ‘closed’; that is, they are protected from contamination by humans, domestic animals and livestock, and from agriculture, timber harvesting and other human activities. In contrast, Sydney’s catchments were open to multiple uses, including grazing and agri-

culture. If the catchment areas are protected, there is less risk of faecal contamination and other dangers, and therefore an outbreak of cryptosporidium and giardia is less likely. Sydney Water blamed the contamination of its water on either the existence of a dead animal in a water catchment or a defect in the water filtration system. Water filtration and disinfection is the method used by Australian water authorities to maintain high water quality. Chlorination is the principal method of disinfection used by most authorities, including Sydney Water. Chlorination is very effective in killing most protozoans and other disease-causing organisms. The water is filtered, then disinfected with small amounts of chlorine. At each treatment plant and other specified points of the water supply, careful monitoring maintains chlorine levels at a minimum. Another disinfection method used is chloramination, which is a modified form of chlorination in which a small amount of ammonia is added to the water just before the chlorine. This results in the formation of chloramines, which provide a longerlasting means of disinfection. During Sydney’s water quality crisis, US experts advised Sydney Water that future contamination could be avoided if an ozone filtration plant was installed in Sydney’s water system, at a cost of about $100 million. It appears that ozone is the only chemical that can eliminate the single-celled organisms that contaminated Sydney’s water. It is used in many US water supply systems. Ozone can be produced from generators fed by air or oxygen.

FIGURE 7.6 The cryptosporidium parasite.

1

Name the two parasites that have polluted Sydney’s water supplies in the past.

2

What is the main source of contamination?

3

What two processes are used to purify the water supply?

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BIOFACT The water reclamation and management scheme at the Olympic site at Homebush Bay is an example of the latest technologies in water re-use systems for reclaiming and recycling sewage and stormwater. The scheme involves ● a water reclamation plant to harvest water from sewage ● a stormwater storage reservoir ● a water treatment plant to filter and disinfect water from the reclamation plant and the stored stormwater ● a separate supply system to pipe water from the treatment plant to the Olympic site, local parks and the suburb of Newington. The recycled water is clear and odourless but is not for drinking. It is used for toilet flushing and irrigation. This has three major benefits: ● drinking water is not wasted ● the volume of sewage entering the sewer system and ultimately the ocean is reduced ● stormwater is utilised.

Clause 33 says: ‘Do not handle or display food in a way that it can be contaminated. Premises, appliances, vehicles, food must be clean, free of vermin and protected from contamination.’

Water supplies The provision of clean water and the disposal of waste water and sewage is a public health issue. Water supplied to houses must be pure and safe to drink. Water is usually filtered and chlorinated by a licensed operator before it can be supplied to people. Similarly, waste water and sewage collected from people’s properties can only be released by an operator with a licence to discharge effluent. Diseases that may be contracted from polluted water include typhoid, cholera and giardia.

Personal hygiene Australians have high standards of personal hygiene. Think of some of the ways you keep yourself and your home clean. Personal hygiene is about washing ourselves, hands, hair and body, regularly. Hands should always be washed after using the toilet, before making or eating food, after handling dogs or other animals, and after contact with someone sick or something dirty. Personal hygiene also includes brushing and flossing teeth, wearing clean clothes, being careful not to sneeze or cough on others, putting used personal items such as tissues in a bin, and keeping our homes clean, especially kitchen and bathroom areas. Doing these things helps us avoid infectious diseases, and prevents their spread.

Pathogens BIOFACT Antiseptics and disinfectants are chemicals that kill most bacteria and fungi and some viruses, but they do not usually destroy spores. They act unselectively on any microorganisms they contact. Antiseptics and disinfectants reduce the number of pathogens present. Sterilisation by boiling in water for 10–20 minutes removes nearly all living things present, including bacterial spores. Chlorine kills bacteria in water. The maximum level of chlorine in drinking water should be 5 mg per litre according to the National Health and Medical Research Council.

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Organisms are called pathogens when they cause disease. To cause disease, organisms need the right conditions to multiply and be transmitted. Cleanliness and hygienic practices discourage the growth and spread of microorganisms. Practices such as heating food to a high enough temperature, pasteurising, preserving, freezing and irradiation can ensure that microorganisms are destroyed. Chemicals such as chlorine, detergents and disinfectants either kill or discourage growth and therefore reduce the numbers of potentially pathogenic organisms. Some microorganisms flourish in warm weather but die in the cold. For example, tinea, a fungal infection of the skin, flares up in summer when conditions are hot and humid; colder, drier weather is not suitable for its growth and reproduction, and few people suffer the disease in winter. Certain diseases are known as tropical diseases because they originate and flourish in tropical climates where temperatures are high all year round and the rainfall is also high. Examples include malaria, giardia and kala-azar (leishmaniasis). In cold conditions the organisms do not survive. Crowded, dirty, unsanitary conditions encourage the spread of disease. After natural disasters such as earthquakes and floods there is always a concern about the spread of disease, because fresh water supplies, cooking facilities and sewage disposal might not be available to the survivors.

Organisms are called pathogens when they cause disease.

FIGURE 7.7 A serious tinea infection.

How diseases are spread Airborne Dust and droplets in the air may carry microorganisms. Colds and influenza are spread by inhaling contaminated droplets in the air from people coughing and sneezing. Contaminated dust from clothes and bedding can spread disease. Contact Diseases spread by contact, either direct or indirect, are called contagious diseases. Athlete’s foot is a fungal disease that can be picked up in showers and changing rooms. Contact with bodily fluids such as saliva or blood from an infected person can also transmit disease. Bleeding cuts and scratches should always be covered. Faeces Contact with faecal matter from an infected person or animal, either from contamination of food or the water supply, may spread disease. Typhoid and cholera may be spread in this way. Proper disposal of sewage is important, as are personal hygiene practices such as washing hands after going to the toilet and before handling food. By other organisms Other organisms, known as vectors, may transmit diseases. For example, malaria is transmitted by mosquitoes of the genus Anopheles (see pp. 347–348).

BIOFACT Giardia is caused by the parasite Giardia lamblia. It is contracted by drinking contaminated water or by close contact with an infected person. The parasite interferes with the body’s ability to absorb fats and vitamins, causing diarrhoea, nausea and other symptoms. The disease is treated by the administration of drugs such as quinacrine and metronidazole.

BIOFACT Ebola is an RNA virus that was first identified in Zaire, Africa, in 1976. The disease is spread by close personal contact (or contaminated needles). Ebola is a virulent disease—the death rate is about 90%. There is no cure.

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Questions b Identify other hygiene rules relating to food are important in preventing the spread of disease.

1

Explain the difference between infectious and noninfectious diseases.

2

What is a pathogen? Give examples of different kinds of pathogens that cause disease in humans.

5

List some practices that help to ensure foods are not spoilt by pathogenic organisms.

3

List three types of non-infectious disease, and name an example of each.

6

4

a Explain how washing hands before handling food is likely to help control disease.

Identify three personal hygiene practices and for each, explain how this practice helps prevent disease.

7

Outline the ways in which disease-causing microorganisms are spread.

4

a Find out what government regulations are in place concerning ● handling food and money in food outlets ● storing foods by food outlets ● processing of food by food manufacturers ● sewage treatment and disposal ● disposing of disposable nappies. b Why do such regulations exist?

5

Sydney suffered a water crisis in July 1998 when the disease-causing parasites Giardia and Cryptosporidium were discovered in the water supply. Do some library or Internet research of journal and newspaper articles covering the water crisis. a Describe the symptoms of giardia and cryptosporidium infection. b How did the city’s drinking water become infected? c Outline the precautions residents were advised to undertake in order to ensure that the drinking water was safe to consume. d Find out how water for household use is usually treated before it reaches your home.

6

Prepare your own set of regulations about hygiene in your kitchen at home. Include statements addressing ● the handling of food items ● the preparation of different food items on particular kitchen surfaces ● storage of different kinds of foods.

7

Explain how the transmission of a disease is affected by features of a the host organism b the agent of disease c the environment.

F u r ther questions 1

Australia has experienced some serious outbreaks of food poisoning related to poor hygiene standards in the handling of foods in recent times. Two notable examples include the contamination of smallgoods (salami) and peanut butter. Choose one of these cases for research. a Name the pathogen responsible for the food poisoning. b Describe the symptoms suffered by the victims. c How many casualties were there? d How did the food products come to be contaminated in the first place? e What regulations need to be in place and enforced to ensure such incidents do not recur?

2

a Do some library or Internet research to find out about ‘Typhoid Mary’. i Who was she? ii When and where did she live? iii What was her occupation? iv Why was she eventually imprisoned? b What restrictions are placed on people in Australia today in terms of their occuption, if they have suffered from or are carriers of the same disease?

3

Re-read the information on page 348 about Universal Precautions. a Explain the importance of each of the control practices described. b What problems could arise if these are not followed? c Why is it important to treat every patient as though they were potentially infected with particular blood-borne pathogens?

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7.3

The search for microbes as causes of disease OBJECTIVES When you have completed this section you should be able to: ● describe the contributions of Pasteur and Koch to our understanding of infectious diseases ● distinguish between different kinds of pathogens, including prions, viruses, bacteria, protozoans, fungi and macroparasites ● identify the role of antibiotics in treating disease ● name one disease caused by each type of pathogen.

Plagues and epidemics In Europe, waves of infectious diseases such as cholera and plague in the Middle Ages resulted in attempts by governments to control the spread of diseases by introducing laws about public health and infectious diseases. As medical awareness of the nature and transmission of diseases increased, legislation was brought in to apply this information to the control of diseases affecting public health.

activities ● ● ● ●

Pasteur’s experiment Understanding disease— an historic perspective An infectious disease The role of antibiotics

The dirt theory In England in 1842, the ‘Report of Inquiry into the Sanitary Condition of the Labouring Population of Great Britain’ stressed the need for centralised administration of health matters. The report clearly linked dirty, overcrowded living conditions with disease.

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BIOFACT ●



An epidemic is a large-scale, temporary increase in the numbers of organisms suffering from an infectious disease. An epidemic with symptoms similar to pneumonia was identified in southern China in 2002. Now known as SARS (severe acute respiratory syndrome), this disease spread rapidly to several other countries (see page 350). A plague is similar but originates from the disease known as bubonic plague or black death that swept Europe in the 14th century. Today the term is used for other temporary sweeping occurrences, not necessarily infectious, such as plagues of locusts or mice.

A microbe is any microscopic organism. Some microbes are the agents of disease, including microscopic fungi, bacteria and viruses. The scientific study of microbes is called microbiology.

Public Health Acts were passed in Britain, Australia and the USA in the second half of the 19th century as awareness grew of the possible sources of diseases and their transmission. Public health legislation includes provision for • the notification of communicable diseases • the isolation of infected persons and premises • compulsory treatment of certain diseases • penalties for the intentional spread of disease.

Microbes as the cause of infectious diseases Until the middle of the 19th century most people thought that living things came into existence by spontaneous generation—that is, they came into existence directly from non-living matter. For example, the ancient Greeks believed that rats arose from garbage, and the Egyptians believed that snakes came from mud. When bacteria were discovered in 1676, they were also thought to be spontaneously generated. In about 1680 Italian scientist Francesco Redi conducted an experiment that disproved spontaneous generation, but most scientists could not bring themselves to believe his findings. The continuing belief in spontaneous generation prevented people from understanding what caused diseases and how they were transmitted. The two scientists who contributed most to understanding the causes of diseases were Louis Pasteur and Robert Koch.

Louis Pasteur Louis Pasteur discovered that most infectious diseases are caused by microorganisms, or germs. This became known as the germ theory of disease.

BIOFACT In 1832 the Quarantine Act was passed in the colony of New South Wales, providing powers of detention and isolation as a response to the threat of new immigrants bringing cholera into the country. New South Wales’s most recent Public Health Act was passed in 1991.

BIOFACT Rabies is caused by a virus. Pasteur identified that there was an agent causing the disease, but it was too small to be seen using a light microscope.

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Louis Pasteur (1825–1895), a French chemist, is considered to be one of the greatest scientists of all time. He discovered that most infectious diseases are caused by microorganisms, or germs. This became known as the germ theory of disease. In his research on fermentation he examined samples from fermenting wines under the microscope and identified and described the microorganisms that cause fermentation. His demonstration that living organisms are present in the air (see Figure 7.8) destroyed the theory of spontaneous generation. During his work for the wine industry in France, Pasteur showed that the microbes causing wine to spoil could be killed by heating the wine to 55°C for several minutes. This technique, now applied to many products including milk and beer, is known as ‘pasteurisation’. In his work with the silk industry, he established that a disease called ‘pebrine’ which afflicted silkworms was caused by a microorganism. It was both contagious and hereditary; that is, it could be passed on from parent to offspring in the eggs. Selecting disease-free eggs was the solution to controlling the disease. Pasteur also demonstrated that anthrax, a disease of cattle, sheep and horses, was caused by a rod-shaped bacterium now known as Bacillus anthracis. He developed the technique of using a weakened strain of the disease and injecting (inoculating) animals with it. In a classic experiment he took 50 sheep and inoculated 25 of them with his weakened form of the disease. Several days later he inoculated all 50 with a strong dose of the disease. He predicted that the 25 untreated sheep would die from anthrax, and they did. The other 25 survived. Today we call this process vaccination (see p. 363). Pasteur developed vaccines against anthrax, chicken cholera, and swine erysipelas. In 1885 he used a vaccine developed against rabies for the first time on a human and saved the life of a young boy bitten by an infected dog.

Pasteur’s flasks Pasteur’s early research suggested the existence of spores (reproductive cells). He hypothesised that these spores were carried in the air, where they were inactive, but developed into active microorganisms when nutrients were available. But how could he test his theory? Pasteur designed a brilliant experiment, using a specially designed flask that had a long S-shaped neck (Figure 7.8). This flask allowed air to enter, but dust and spores would be trapped in the curve of the neck and never reach the broth. Other flasks he used in the experiment were completely open to the air. broth poured into two identical flasks

neck of one flask broken to admit air

neck of other flask not broken

Pasteur put some broth into each flask, then boiled the broth thoroughly to ensure that the heat killed any organisms in it. Any spores clinging to the glass were killed by treating it with steam. As Pasteur has predicted, the broth in the S-neck flask did not become contaminated with bacteria, but the broth in the open flasks did. This proved that the organisms that contaminated the broth must be carried in the air and are not generated spontaneously. Pasteur’s flasks are on display at the Pasteur Institute in Paris. After nearly 150 years, the broth in the S-neck flask is still free of bacterial growth.

necks of flasks bent in an S-shape, and broth boiled to kill existing organisms

dust and microorganisms enter

dust and microorganisms trapped in S-bend

broth in open-necked flask becomes infected broth in S-necked flask remains uninfected

FIGURE 7.8 Pasteur’s experiment. Note the long S-shaped neck of the flask, which was a critical part of the apparatus.

1 What conclusion did Pasteur draw from this experiment?

Robert Koch Robert Koch (1843–1910), a German scientist, also studied the cause of the disease anthrax. He eventually succeeded in isolating the bacterium which causes the disease from the blood of dying animals. He examined the blood under a microscope and identified active rodshaped cells and resting spores. He established that the blood of animals with the disease always contained these microorganisms while the blood of healthy animals did not. He also found that if blood from an infected animal was injected into a healthy animal it caused the disease. To finally demonstrate that it was the bacterium and not any other component in the blood that caused the disease, he separated the bacteria from the blood and they alone were injected. This also resulted in the disease. Koch described the criteria which must be met if we are to be sure that a particular microorganism causes a particular disease. These are now called Koch’s Postulates.

BIOFACT Pasteur’s proof that contaminating organisms were carried in the air inspired the work of Joseph Lister (1827–1912), a British surgeon. Until the mid 19th century, many people who underwent surgery survived the operation but died from infection of their wounds. Lister was the first to apply antiseptic treatment for wounds, using carbolic acid, and he introduced antiseptic surgery and the prevention of infection by insisting on absolute cleanliness in the operating theatre.

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BIOFACT Between 1891 and 1899, Robert Koch investigated many human and animal diseases, including malaria. He had just come to the conclusion that malaria was transmitted by mosquitoes when a British bacteriologist, Ronald Ross, published the research that proved this was so.

Koch’s postulates 1 The specific microorganism must be present in every host with the disease. 2 The specific microorganism must be isolated from the host and grown in a pure culture. 3 A potential host, when inoculated with the microorganism, must develop the same symptoms as the original host. 4 The specific microorganism must be able to be isolated from the second host and identified as the same species as originally cultured. The work of Pasteur and Koch laid the foundations for the scientific study of microorganisms, including the culture techniques and the rules and procedures now used in infection control, such as the sterilisation of instruments.

Tuberculosis unravelled Robert Koch was the first person to separate different bacteria and grow them as ‘pure cultures’. He selected bacteria according to their shape and transferred them onto a solid medium, first gelatin and later nutrient agar, in a flat dish invented by a colleague, J. R. Petri. On 24 March 1882, at a meeting of the Physiological Society of Berlin, Koch announced that he had succeeded in isolating the tubercle baccillus Mycobacterium tuberculosis—the cause of tuberculosis or TB, which killed one in seven people during

the 19th century. One strain of tuberculosis, called bovine tuberculosis because it occurs in cattle, can be transmitted to humans in unpasteurised milk. Koch continued his worked on tuberculosis, and in 1905 was awarded the Nobel Prize in Physiology or Medicine for developing tuberculin to test for the disease. Today the risk of tuberculosis is greatly reduced as a result of milk pasteurisation, vaccinations and chest X-rays. Drug-based treatments and surgical procedures that can cure the disease are also widely available.

FIGURE 7.9 The Mantoux test for turberculosis involes the injection of tuberculin. If the site of injection swells to more than 10 mm wide within 48 hours, the test is considered positive for tuberculosis. (That is, the patient has been infected by the pathogen, which might or might not be active.)

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What type of microorganism causes TB?

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2

What three measures have reduced the incidence of TB in Australia?

Malaria Malaria is a serious disease causing the deaths of over two million people world wide each year. Malaria is caused by protozoan parasites of the genus Plasmodium. The parasites live in mammalian red blood cells and liver cells. Plasmodium is transmitted by a vector or carrier, the female Anopheles mosquito. The mosquito transfers the parasite when feeding on blood. The life cycle of the parasite is complex, involving asexual and sexual multiplication in both the mosquito and the human (Figure 7.10). Humans suffer attacks of fever when red cells containing the parasite burst, releasing merozoites or larvae into the blood stream. The malarial parasite was first identified in the stomach of an Anopheles mosquito by Ronald Ross, a British bacteriologist. Ross became a world authority

on malaria, and won the Nobel Prize in Medicine in 1902 for his discoveries. Despite many programs to eradicate it, malaria remains a major health problem world-wide in tropical and sub-tropical areas, particularly in Africa. Although Australia is free of malaria, Australian scientists are active in continued research into this disease. Dr Alan Cowman was awarded the Walter and Eliza Hall Institute’s 1990 Burnet Prize for his work on malaria. He made discoveries to explain why the malarial parasite can elude the human defence system and why it can become resistant to almost all anti-malarial drugs. Currently there is research in Australia to help produce a vaccine against malaria.

asexual multiplication

gut wall zygote migrates to midgut wall

larvae migrate to salivary gland

fertilisation occurs in gut

female mosquito picks up gametes while feeding

infected mosquito injects larvae through skin

MOSQUITO HUMAN blood vessels in skin

formation of gametes

merozoites

asexual multiplication in liver cells

asexual multiplication in red blood cells

asexual multiplication in red blood cells

enters red blood cell

FIGURE 7.10 The cycle of infection by the Plasmodium parasite.

Name the host, vector, parasite and major symptom of malaria.

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Agents of disease Organisms can suffer from both infectious and non-infectious diseases. Bacteria, fungi and viruses can sometimes cause infectious disease and may be transmitted in the air or by carriers (vectors).

virus bacterial cell

virus penetrates bacterial cell and injects DNA

new virus DNA made from chemicals found inside bacterium

new virus heads and tails are made

bacterium bursts, releasing new viruses

FIGURE 7.11 The mechanism of infection by a virus.

Viruses are not cellular. Their structure is simply nuclear material, DNA or RNA, enclosed in a protein coat.

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An infectious disease is one that is caused by a pathogen and that can be passed from one organism to another. Many types of organisms may be pathogens. Some pathogens and examples of the diseases they cause are described below. Many animals, including worms, insects, birds, wallabies and humans, may also damage plants. While worms and insects may live parasitically on plants and in large numbers may kill the plant, other animals are usually feeding on them. At times, however, this may seriously impair the plant’s functioning—that is, cause disease.

Prions Prions are infectious agents that cause brain disease in mammals, including humans. They are proteins that have been altered from their normal shape in the body to an abnormal shape, although the chemical make-up of the protein remains the same. Prions present in an animal cause disease in that animal. A prion can also convert other similar but normal proteins into the abnormal prion shape. Prions can also be passed from one animal to another, infecting them with the disease. Characteristics of prion diseases are loss of motor control, dementia, wasting and eventual death. At post mortem, all animals that die from prion diseases have large vacuoles in the brain. Some diseases caused by prions are scrapie in sheep, bovine spongiform encephalopathy or mad cow disease in cattle, and Creutzfeldt– Jakob disease in humans.

Viruses Viruses have been isolated and identified only in the last fifty years. They are very small. A typical virus is about 100 nm wide and is visible only with the electron microscope. Viruses have been found in all types of cells, both eucaryotic and procaryotic. Viruses are not cellular. Their structure is simply nuclear material, DNA or RNA, enclosed in a protein coat (see Figure 7.12a). It is even possible to crystallise viruses—this is not possible with living cells without destroying them. Viruses reproduce only inside another cell, called the host cell. Once inside, the nuclear material from the virus causes the cytoplasm of the host cell to produce new viruses. This results in the death of the host cell. The new viruses escape from the resulting dead cell. At the moment there are no cures for diseases caused by viruses, but we can reduce their prevalence in humans and other animals by vaccination (see p. 363). In this way, many previously injurious or fatal diseases can now be controlled. Some of the diseases caused by virus infections in humans are smallpox, measles, warts, influenza and AIDS. Many animals suffer from coughs and colds caused by viruses. Different herpes viruses cause disease in humans and domestic animals. Diseases in other animals include Newcastle disease (fowl pest) in poultry, Akabane disease in sheep, cattle and goats, and distemper in dogs, including dingoes and foxes.

BIOFACT

bacteriophage protein coat

DNA (a)

50 nm

influenza virus

tobacco mosaic virus

adenovirus: causes colds

35

0

nm

mumps virus

Ross River fever is a viral disease transmitted by mosquitoes. The usual hosts of the virus are possums and bandicoots. Humans ‘catch’ the disease when they are bitten by a mosquito that has previously picked up the virus from biting a possum or a bandicoot or other infected mammal. Symptoms include a rash, extreme tiredness and muscle pain. The name comes from the Ross River near Townsville, Queensland, where the first cases were identified.

100 nm (b)

80 nm (c)

25 nm (d)

(e)

Examples of diseases caused by viruses in plants include tobacco mosaic virus, potato leaf roll virus and woodiness virus in passionfruit vines.

FIGURE 7.12 Some different types and shapes of viruses: (a) bacteriophage that attacks bacteria, (b) mumps virus, (c) influenza virus, (d) tobacco mosaic virus, (e) adenovirus that causes common colds.

BIOFACT AIDS will eventually overtake bubonic plague and Spanish flu as the most devasting disease in human history. There are 16 000 new infections every day, or about 5 per minute. By June 1998 there were about 31 million people affected with the AIDS virus world wide; 12 000 of these live in Australia and New Zealand.

FIGURE 7.13 Not all viruses are destructive. Many variegated flowers owe their beauty to infection by ‘mosaic’ viruses.

BIOFACT

Bacteria Bacteria are procaryotic cells (see Chapter 3, p. 124). Most bacteria live freely, but some are parasites or commensals in or on other organisms (see Chapter 1, p. 22). Some are pathogens of animals and plants, causing serious disease.

Chlamydia causes four common diseases in koalas: ● conjunctivitis, which can cause blindness ● pneumonia ● urinary tract infections ● reproductive tract infections, sometimes leading to infertility.

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Some examples of bacterial disease in animals include tetanus, pneumonia, tuberculosis and chlamydia, foot rot and Johne’s disease. Some examples of bacterial disease in plants include crown gall, bacterial blossom blight, some leaf spot diseases and some rot diseases such as black leg, a stem rot disease of potatoes.

BIOFACT Severe acute respiratory syndrome (SARS) is a new human disease first identified in China in 2002. It is caused by a coronovirus. Coronoviruses are a group of RNAcontaining viruses that have a crownlike (corona) or halo appearance when seen under the microscope. The disease is transmitted by contaminated droplets in the air and possibly also by close contact with an infected person. Symptoms include fever and, after 2 to 7 days, a dry cough. Some people develop severe lung infections and require intensive hospital treatment. There is no cure for the disease, but outside the body the virus can be inactivated by disinfectants and killed by moderate (56°C) heat.

Protozoans Protozoans are single-celled, animal-like eucaryotic organisms. Many are free-living, but some are internal parasites of animals and cause disease. Some examples of diseases in animals caused by protozoans include coccidiosis in many domestic animals, and sleeping sickness, giardia and amoebic dysentery in humans.

Fungi Fungi are eucaryotic organisms. Some fungi, such as yeasts, are unicellular organisms. Most, however, are composed of a system of microscopic tubular filaments or threads which branch and spread to form a structure known as a mycelium. The threads of a mycelium are known as hyphae (singular hypha). The mycelium produces fruiting bodies which contain thousands of spores by which the fungus reproduces. Mushrooms and toadstools are the fruiting bodies of certain fungi (see Chapter 4, p. 193).

Phytophthora root rot Technical name Phytophthora root rot or cinnamon ‘fungus’. Cause The oomycete Phytophthora cinnamomi. Oomycetes are organisms that cause plant blights and downy mildews. They were once thought to be fungi but were reclassified as protists because of their motile cells with flagella. There is some

section of infected potato leaf

sporangium

debate about whether this disease was introduced into Australia or is a naturally occurring species that has spread beyond its natural range. Whatever is the case, it has caused enormous devastation to many areas. Transmission Phytophthora has a complex life cycle (Figure 7.14). It may be transmitted from plant

germinating spore spore water film

hyphae surface of potato leaf hyphae invade leaf

sporangia containing spores sporangiophore next season

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infection of tuber

FIGURE 7.14 Phytophthora spores germinate to form hyphae which invade leaves. Some of these hyphae grow out through stomates and produce further spores.

to plant in water and soil, by wind or by vectors. It thrives in wet conditions with poor soil drainage, and is easily transported in wet soil attached to boots, tyres and bulldozer tracks. The zoospores that are produced swim through the soil water to plant roots, which they enter and infect. The organisms grow throughout the plant. Some hyphae in the leaves grow out through the stomates and produce chlamydospores. These have thick cell walls and can survive dry conditions. They may be blown by the wind or transported by other organisms, including humans, for great distances. On finding suitable moist conditions, the spores germinate and produce the zoospores that swim to find another plant. Host response Plants vary in their response to infection by Phytophthora. Some plants are resistant or show no signs of the disease. Some grasses and sedges are resistant. Button-grass shows no sign apart from a reduced number of flowers. Other plants may be highly susceptible and are quickly killed, and do not regrow in infected soil. Over 1000 Australian species have been identified as being susceptible to infection, including grass-trees, many small shrubs and herbs in heathlands, banksias, waratahs, and many forest trees. The spores that penetrate the plant roots germinate, sending out threads or hyphae that branch and spread throughout the plant. Xylem and phloem channels become blocked, stopping the flow of water and nutrients to the plant. This is a serious disease that causes death to most susceptible plants that are infected. Major symptoms Plant roots become black and brittle. Leaves become discoloured, often appear-

ing red or yellow, and may wilt or drop. Any new leaves are small and pale. Tree branches die back. Treatment Spraying with a fungicide may increase the plant’s resistance to attack, but does not kill the Phytophthora. This is only possible in cultivated areas. In the wild there is no practical treatment at present. Prevention In the case of known susceptible cultivated plants, horticulturalists should ensure that they use only disease-free plants. Soil, gravel and water from known infected areas should not be used, and these areas should be quarantined. Soil and gravel can be fumigated before use. Planting should be in well-drained soil and not in areas likely to be flooded. None of these preventative measures is possible in a natural environment. Areas considered to be at risk in natural environments, such as many areas in National Parks, should be targeted for special management control. Control Phytophthora may be spread by air, water, gravel and vectors. It is consequently very difficult to control. In natural environments the most feasible control is to control the movement of people through areas not yet infected and targeted for special care. This may mean restricting access, controlling the development of roads and tracks in the area, closing roads and tracks in wet weather, and washing soil from all items prior to entry to the area. Spraying foliage with the chemical phosphonate can increase disease resistance in plants. It is cheap, water-soluble, and non-toxic to animals. Measures such as these are already in operation in some conservation reserves in Victoria, Tasmania and Western Australia.

FIGURE 7.15 Dieback of understorey plants, including grass-trees, in a massive woodland. The loss of understorey plants has been shown to significantly affect populations of small mammals because of the loss of habitat and food resources.

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Some examples of fungal diseases in animals include ringworm in humans and many domestic animals, and aspergillosis in birds and humans. In plants fungi are responsible for rusts, mildews, moulds and blights.

(a)

(b)

FIGURE 7.16 (a) Downy mildew on daisies. (b) A patient with ringworm.

Macroparasites Macroparasites are large parasites that can be seen with the naked eye. They may be external (ectoparasites) or internal (endoparasites) parasites. External macroparasites of animals include lice, mites, tick and fleas. Internal parasites include tapeworms, roundworms and flukes. Macroparasites of plants are known to gardeners as pests. They include mites, aphids, borers, lerp insects, scale insects and many types of parasitic wasps. Several of the viruses carried by insects cause diseases in humans that are notifiable (see p. 349, 381), including Ross River fever, dengue fever and Murray Valley encephalitis.

(a)

FIGURE 7.17 Three macroparasites: (a) bush tick embedded in skin, (b) tapeworm, (c) aphids.

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(b)

(c)

Hepatitis B Medical name Serum hepatitis. Cause The hepatitis B virus (HBV), which has a circular DNA molecule surrounded by an envelope of lipid, protein and carbohydrate. Transmission The virus spreads by close contact in which blood or bodily fluids are exchanged. The most common ways are needle-stick injuries or unsafe sexual activity. The virus can also infect the baby of an infected mother during birth. The baby will then be a carrier of the virus. Host response After a long incubation period (average 6 to 8 weeks) with no symptoms, the acute disease occurs and lasts for weeks or even months. Three outcomes of this infection are possible: ● About 90% of people recover completely. ● 1% follow a severe and fatal course called fulminating hepatitis. ● About 3% suffer chronic hepatitis, which can lead to cirrhosis of the liver and then to liver cancer. In 1% of these people, cirrhosis or cancer is fatal. Major symptoms Symptoms may include fever, painful joints, jaundice, nausea and a tender liver.

Treatment The patient should rest at home and also rest their liver by avoiding fatty foods, alcohol and any other drugs that are toxic to the liver. People with chronic hepatitis can be given interferon, an antiviral drug. Prevention Avoid contact with blood or bodily fluids of anyone known or suspected of carrying the HB virus. People with whom the sufferer has had sex should be immunised. Control Education about the disease is essential if people are to understand the risks associated with unsafe sexual and needle-sharing practices. Vaccination is recommended for people at risk, such as health workers and intravenous drug users. Vaccination is also recommended for babies. Three doses are required: at birth, at 1 month and at 6–12 months. Vaccination is now available free in Australia to all children over 13 years of age. Three doses are given over a period of 6 months.

The role of antibiotics Antibiotics are substances capable of destroying or inhibiting the growth of bacteria. They are chemicals that act selectively on the pathogen without destroying the host. They are not effective against viruses. The first antibiotic, penicillin, was discovered by Alexander Fleming in 1928. It was first purified by Howard Florey and Ernst Chain in the 1930s, and became available for medical use in 1941. Penicillin revolutionised the management of infectious diseases: previously serious and often fatal diseases could be cured. For their work, Fleming, Florey and Chain were jointly awarded the Nobel Prize in Medicine. As well as penicillin, other groups of antibiotics have been developed, including cephalosporins, macrolides, tetracyclines and aminoglycosides. Different antibiotic groups target different types of bacteria. ‘Broad-spectrum’ antibiotics act on a wide range of bacteria; ‘narrow-spectrum’ antibiotics act on only one or two. Antibiotics work internally at the cellular level. They interfere with, damage or destroy the cells of the microorganism. For example, penicillin contains a special ring-shaped molecule in its structure which gives it its bacteriocidal properties. It inhibits the formation of the bacterial cell wall. Cephalosporins act in a similar way. Amphotericin destroys cell membranes. Some antibiotics act on the bacterial ribosome, inhibiting protein synthesis while others interfere with DNA synthesis or other metabolic pathways, resulting in cell death. Because of the widespread use of antibiotics, some bacteria have evolved resistant strains. These resistant strains, sometimes termed

Antibiotics are known as ‘bacteriocidal’ if they kill the bacteria, or ‘bacteriostatic’ if they inhibit its growth.

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‘super’ bacteria, are a problem when treating diseases. For example, after years of exposure, some bacteria have evolved an enzyme which destroys the ring-shaped molecule in penicillin. In response, scientists have developed chemicals to inhibit the enzyme. You may be prescribed augmentin (penicillin + clavulanic acid) instead of just penicillin if you have certain staphylococcal infections. Since the widespread use of cephalosporins, Methicillin-resistant Staphylococcus aureus (MRSA) has also evolved. Since the development of the antibiotic vancomycin, vancomycin-resistant enterococci (VRE) bacteria have appeared. In response, the search for new antibiotics continues.

FIGURE 7.18 A wide range of broad-sprectrum and narrow-spectum antibiotics are available.

Questions 1

What is a microbe?

2

Draw a timeline to identify and summarise developments in our understanding and management of disease-causing microbes. Include the work of Louis Pasteur and Robert Koch.

3

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Consider each of the following different kinds of organisms: a prions b viruses c bacteria d protozoans e fungi.

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For each, answer the questions that follow: i Describe the organism. ii List some diseases caused by the organism. 4

a What is a macroparasite? b Make a list of macroparasites that infect i animals ii plants. In each case indicate whether the organism is an external or an internal parasite.

5

a What is an antibiotic? b Briefly outline the role of antibiotics in the management of infectious diseases.

F u r ther questions 1

2

a Find out what is meant by a ‘notifiable disease’. b Name the notifiable diseases listed under the Health Act in Australia. c Do some library or Internet research to find how hospitals manage these diseases. a What is meant by the term ‘pasteurisation’? b How did the term come about? c Read the information provided on milk cartons or bottles in your home. How is milk treated before you buy it? d Why is milk pasteurised?

3

Why is it important to ensure instruments used in surgery and dentistry are sterilised?

4

Choose an example from the work of either Louis Pasteur or Robert Koch to describe how the causal agent of an infectious disease was identified.

5

Outline how Koch’s Postulates can be used to identify an organism that is the cause of an infectious disease.

6

Undertake a research investigation of one of the following infectious diseases: tetanus pneumonia anthrax fleas aphids encephalopathy (mad cow disease) measles distemper Creutzfeldt–Jakob disease crown gall tuberculosis chlamydia giardia amoebic dysentery ringworm liver fluke mildew potato blight myxomatosis calicivirus tinea ebola Include the following points in your research: a cause of the disease b host c mode of transmission d symptoms e host response f treatment g prevention h control







References such as Biology texts, medical encyclopaedia or the Internet may be useful. Present your findings as a brochure for public information. Share your research findings with your class. Overhead transparencies or a computer-based presentation such as Powerpoint will be a useful way of presenting information to your class.

7

Find out how Howard Florey and Alexander Fleming came upon the discovery of penicillin.

8

In 1997 a four-month-old baby boy in Japan became infected with Staphylococcus aureus, commonly known as golden staph. This is a serious bacterial infection that can lead to the death of the patient. None of the conventional antibiotics used to combat the disease were successful. The boy was subsequently treated with methicillin and then vancomycin, the strongest available antibiotic. Nothing worked. The boy eventually recovered when he was treated with experimental antibiotics. Do some library or Internet research about multidrug-resistant antibiotics. Address the following questions. a What are multi-drug-resistant bacteria? b How do bacteria develop resistance to antibiotics? c Explain why the strongest antibiotics are not the first ones used to combat serious infections such as golden staph. d Is the way we use antibiotics a factor in the problem of resistance observed in bacteria? Explain. e Suggest what the consequences of overusing antibiotics might be in our society. What solutions can you offer?

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7.4

Protecting the body: defence barriers OBJECTIVES When you have completed this section you should be able to: ● explain what is meant by the body’s defence barriers ● outline the role of skin, mucous membranes, cilia, chemical barriers and other body secretions ● define the term ‘antigen’ ● explain why organ transplants provoke an immune response ● describe defence adaptations including inflammation, phagocytosis, macrophage, lymph system and cell death to seal off a pathogen.

activity ●

Microflora—maintaining the balance

The body has various defence mechanisms against pathogens. Firstly, defence mechanisms protect the body at possible entry points. This protection is non-specific and aims to prevent any pathogens from entering the body. Secondly, defence mechanisms operate when pathogens have succeeded in entering the body. Most of these internal second-line defences are also non-specific. Thirdly, defences are specific and mediated by lymphocytes.

Defence barriers—preventing entr y The skin barrier The skin forms a tough outer barrier covering the body. The outer layers of skin contain keratin, and microorganisms cannot penetrate it unless it is broken. The skin has its own population of normally harmless bacteria living as commensals (see p. 23). Their presence helps keep invading pathogens from multiplying. Sebaceous glands in the skin secrete sebum. Lipids in the sebum are broken down by the skin’s harmless bacteria to produce acids which inhibit the growth of some bacteria and fungi (Figure 7.19). If the skin is broken, the blood-clotting mechanism very quickly forms a seal across the wound to prevent the entry of pathogens. 356

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Mucous membranes Mucous membranes line the digestive, respiratory, reproductive and urinary tracts with a thick, slimy mucus. The mucous membrane must allow the exchange of substances when needed and also protect against invasion. This protection is aided by the presence of an antibody called IgA in mucus which reacts with potential pathogens, preventing them from invading the surface. Fluids such as saliva, tears and nasal secretions wash over the mucous membranes. These fluids contain lysozyme, which breaks down the cell walls of some types of bacteria.

Cilia Cilia are minute hairs that project from the cells lining the respiratory surfaces of the nose, trachea and bronchial tubes. The cilia beat and sweep the mucus along, so that any particles breathed in and trapped in the mucus are transported to the nose opening or to the pharynx where they are coughed out or swallowed (Figure 7.20).

Chemical barriers Chemical barriers are provided by conditions that make the surface inhospitable for the potential pathogens. For example in the alimentary canal, pathogens entering the body with food and drink are usually destroyed by the acid environment in the stomach (Figure 7.21) or the alkaline environment in the small intestine.

Other secretions Other body secretions also protect the body from invasion. For example: ● There are populations of harmless microorganisms in the vagina. They act on cells shed from the walls of the vagina to create acid conditions which inhibit the growth of some bacteria and fungi. ● Urine is a sterile, acid fluid. It flushes the ureters, bladder and urethra and helps prevent the growth of microorganisms. ● Tears contain lysozomes that destroy bacteria.

Specific responses— the immune response The body’s immune response is its reaction to invasion by foreign materials. These may be viruses, bacteria, toxins or other foreign proteins. These substances are identified as foreign by the body, which then responds by trying to destroy them. Substances which trigger this reaction are known as antigens. Transplants are being used more and more to treat people whose own tissues are diseased, particularly kidneys, liver, heart, lung and bone marrow. However, an introduced organ contains proteins (antigens) that are recognised as foreign to the patient and so stimulates the production of antibodies that attack and possibly destroy the new tissue. In the early days of organ transplants, rejection was a constant problem. Nowadays, new drugs have overcome many of these problems (see p. 364).

hair surface of skin

layer of sebum dead cells

living cells of epidermis sebaceous gland

FIGURE 7.19 Skin protection. Dead cells on the surface form a barrier to the entry of pathogens. Sebum inhibits the growth of some bacteria and fungi.

flow of mucus and trapped material

rows of cells lining trachea

cilia

mucus-producing cell

FIGURE 7.20 Protection of respiratory surfaces. Sticky mucus is produced and traps dust particles and pathogens. Cilia sweep the mucus to the back of the throat, where it is swallowed or coughed out.

stomach acid

gastric pit cells lining stomach

gastric gland in stomach wall FIGURE 7.21 Stomach protection. Glands in the stomach produce concentrated acid that kills most pathogens.

Antigens are molecules that trigger the immune response.

BIOFACT Interferon is a protein produced by cells when they become infected with a virus. It is secreted by the infected cell and binds on to healthy neighbouring cells, thus preventing any virus from replicating in those cells. In addition to providing a non-specific protective function against virus infection, interferon also assists in the immune response to specific viruses.

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Defence adaptations— non-specific responses Inflammation response When any body tissue is damaged, such as by the invasion of a pathogen, the area becomes red, hot, swollen and painful. The blood circulation to that part is increased, and the blood vessels dilate and become leaky. This helps confine the pathogen while an increased number of white blood cells helps destroy it; dead cells and toxins (poisons) can be removed quickly and repair of the tissues can begin. The inflammation responses are mediated by chemicals such as histamine and prostaglandins released from the damaged tissues.

Phagocytosis Phagocytes are white blood cells which can actively move from the blood to the tissues where they ingest and destroy any foreign material including pathogens.

Phagocytes are white blood cells which can actively move from the blood to the tissues where they ingest and destroy any foreign material including pathogens (see Chapter 5, p. 229). This action is called phagocytosis. In acute inflammation (lasting hours or days) the main phagocytes are called neutrophils. In chronic inflammation (weeks or months) the main phagocytes are called macrophages.

FIGURE 7.22 Phagocytes engulfing foreign bodies in the blood stream.

Lymphocytes are white blood cells that become active in chronic inflammation.

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The lymph system within the tissues consists of lymph capillaries which unite to form larger vessels similar to veins (see Chapter 5, p. 233). These vessels transport tissue fluid away from the cells towards the heart. At various points along the lymph vessels are lymph nodes or glands where lymphocytes are made and added to the lymph as it flows through.

Lymph nodes also engulf and destroy bacteria and other foreign materials and cell debris circulating through them. Lymph nodes may become tender when there is an infection nearby. They are inflamed and enlarged by the bacteria and toxins they accumulate as they fight the invaders. Lymphocytes are white blood cells that become active in chronic inflammation. There are two main types—B cells and T cells involved in the immune response (see p. 361).

Sealing off the pathogen When the body is unable to neutralise an antigen, a particular type of chronic inflammation involving both macrophages and lymphocytes may occur. This reaction forms a cluster of cells called a granuloma in which a central core of dead tissue is surrounded by layers of macrophages, then lymphocytes, then fibroblasts which produce a tough outer wall. Granulomas are produced in tuberculosis and leprosy. The body’s immune response is its reaction to invasion by foreign materials. Substances which trigger this reaction are known as antigens.

FIGURE 7.23 In tuberculosis, granulomas form around sites of infection in the lungs, preventing the enclosed bacteria from causing further infection. In this chest X-ray of an infected person, the granulomas show up as dark spots.

Questions 1

a Outline the difference between non-specific and specific defence mechanisms. b When do non-specific defence mechanisms operate, compared with specific defence mechanisms?

Defence barrier

Location in body

2

Explain what happens in the inflammatory response to help the body overcome invading pathogens.

3

Complete the table summarising the defence barriers in the body.

How defence barrier prevents entry of pathogens

Skin Mucous membranes Cilia Chemical barriers Other secretions

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4

What is phagocytosis? What types of cells are responsible for this activity in the body? In what circumstances does this process occur?

5

a Is interferon an example of a non-specific or specific defence mechanism?

b How does interferon operate to control infection in the body? 6

Define the term antigen.

F u r ther questions 1

Prepare a concept map to demonstrate your understanding of the non-specific defence mechanisms involved in defence discussed in this section. Include: skin inflammation phagocytes mucous membranes blood vessels granuloma cilia white blood cells neutrophils chemical barriers dead cells lymph other secretions toxins

2

a Find out some of the different kinds of microflora that live in the gut. b Describe the relationship between the microflora and the host. c i How do the microflora benefit? ii How does the host benefit? d Describe a disease that occurs in humans as a result of an imbalance of microflora in the gut.

3

Each year some Australian city beaches are temporarily closed because of high levels of Escherichia coli (E. coli) in the water. E. coli is a bacterium that lives in the human bowel. In this relationship the bacterium causes no harm to humans. In fact, both bacterium and human benefit from their co-existence. a Outline the advantage of this symbiotic relationship to i the bacteria ii humans.

Blood type

Antigens present on red blood cells

A B AB O

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b How do ocean beaches periodically become infected with high levels of E. coli? c Why are high levels of E. coli in the water around beaches detrimental to human health? d Describe the symptoms suffered by people suffering from E. coli infection. 4

During early research on the human immunodeficiency virus (HIV), researchers were surprised to discover that some apparently healthy individuals tested positively for antibodies against the virus. What does this mean for those individuals?

5

Why is it that a Rhesus negative mother who has just given birth to a Rhesus positive baby must receive an injection of Rhesus negative antibodies?

6

a Visit your local Blood Bank to find out i the differences between blood groups ii the reasons why blood type must be carefully determined before being used for transfusions. b Use the information you have obtained to complete the following table, summarising the compatibility of blood groups. c i Which blood type is often referred to as a ‘universal donor’? Why? ii Which blood type is often referred to as a ‘universal recipient’? Why?

Can donate blood to blood type

Can receive blood f rom blood type

7.5

The immune response OBJECTIVES When you have completed this section you should be able to: ● identify the components of the immune response ● define the term ‘antibody’ ● describe the role of antibodies, T cells and B cells in the immune response ● describe the different kinds of T cells and their respective roles ● outline the interaction between B and T cells ● outline the role of vaccination in preventing infection ● explain why the immune response in organ transplant patients is deliberately suppressed.

The immune responses When we are exposed to an antigen for the first time, our body responds by producing lymphocytes. Lymphocytes are a type of white blood cell; the two main types are T cells and B cells (also called T lymphocytes and B lymphocytes). Antibodies (also called immunoglobulins) are produced in the lymph nodes by B cells in response to a specific antigen. They are proteins that bind to the antigens. The antigen–antibody complex activates the production of a series of proteins (called complement) that results in bacteria being ingested and destroyed and histamine being released, resulting in inflammatory changes.

activity ●

Vaccination programs

T cells T cells are lymphocytes which form in the bone marrow and mature and develop in the thymus gland. They remain inactive in the blood and lymph until they come in contact with an antigen. The antigen binds onto the T cell, activating it to multiply (make clones). T cells control the cell-mediated response, in which various types of T cells destroy the antigen or the foreign cell. Other T cells stimulate the activity of B lymphocytes and macrophages. Some T cell clones remain in the body as memory cells (Figure 7.24).

T cells are lymphocytes which mature and develop in the thymus gland. They remain inactive in the blood and lymph until they come in contact with an antigen.

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T CELLS

B CELLS special lymphocytes present in the blood and lymph

Natural killer cells large lymphocytes that destroy infected cells

ANTIGENS foreign substances that enter the blood and stimulate lymphocytes

Helper T cells (T4 cells) cells that help: T cell clones copies of T cells

i B cells produce antibodies ii the formation of cytotoxic T cells iii the inflammatory response

differentation

Cytotoxic T cells produce chemicals that destroy antigens and attract phagocytes

Suppressor T cells (T8 cells) reduce output: i of antibodies from plasma cells ii of chemicals from cytotoxic cells INTERFERONS protect cells around an infected cell from viral invasion

B cell clones copies of B cells differentation

Amplifier T cells stimulate other T and B cells

Memory T Memory B cells cells remain in the lymph nodes or circulation and confer long-term immunity

Hypersensitivity T cells increase the activity of other white cells: i phagocytes ii cells causing allergy and hypersensitivity responses

Plasma cells secrete antibodies into the blood

ANTIBODIES combine with antigens and destroy them

FIGURE 7.24 The acquired immune response. The natural immunity we are born with is supplemented by an acquired immunity, which results from contact with a pathogen and is specific to that disease. The antigen stimulates the production of lymphocytes that produce antibodies and interferon, which destroy the antigen.

Types of T cells and their roles Cytotoxic T cells destroy the cells which carry foreign antigens, thus removing from the body any foreign proteins, recognised as non-self, such as bacteria and transplanted tissues. Natural killer cells are special cytotoxic T cells which destroy any abnormal host cells, for example cancer cells in the body or cells infected by viruses. Natural killer cells also produce interferon. Helper T cells secrete chemicals called interleukins, which regulate both cytotoxic T cell functions and B cell functions. Inducer T cells and suppressor T cells also regulate B cells and T cells. Memory T cells recognise an antigen when it reappears and have helper T cell functions, quickly producing a large amount of antibody to the antigen. 362

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B cells B cells mature and develop in the bone marrow of humans. They control the humoral (blood) response in which B cells present in the blood and lymph are activated by the presence of antigens. Activated B cells clone themselves and then differentiate, either into plasma cells that send antibodies into the blood or into memory cells. This process usually occurs in the lymph nodes.

Interactions between B and T cells An antigen that enters the body travels in the blood to a lymphoid tissue (lymph node or spleen) where it is ingested and processed by a macrophage. The macrophage then displays fragments of the antigen on its outer membrane, and these are recognised by helper T cells and B cells. The different cells are able to collaborate because they are close to each other and because they are regulated by cytokines. Cytokines are proteins or polysaccharides secreted by T cells and macrophages. They signal other cells to initiate the immune response, for example, for a B cell to transform into a plasma cell.

level of antibody in blood

secondary immune response

faster larger longer

primary immune response

10 20 30 primary infection

10 20 30 days later infection

FIGURE 7.25 Following sensitisation of the immune system by primary infection or immunisation, the secondary immune response is faster, higher and longer.

Memory cells Some of the cloned and differentiated T and B cells remain in the body for a long time, as do some antibodies. The lymphocytes carry receptors that recognise antigens and provide a ready defence against subsequent invasions. The antigen may be eliminated quite rapidly next time, sometimes even before any symptoms appear.

B cells control the humoral (blood) response in which B cells present in the blood and lymph are activated by the presence of antigens.

Immunity and immunisation programs Once a pathogen has infected the body and then been destroyed by it, the infected person is said to be immune to that disease. This immunity might last only a short time, or for the rest of the person’s life.

Vaccination Vaccination, or immunisation, is the process of making people resistant to infection caused by a pathogen. It involves giving them an injection or oral dose of a vaccine which produces immunity either actively or passively. Vaccines are preparations from weakened or dead infective microorganisms that are injected into the body with the intention of provoking immunity to a disease without producing the symptoms. Some vaccines confer immunity for life (e.g. the measles vaccine), others for shorter times (e.g. the tetanus vaccine which gives immunity for 10 years). With the latter, booster injections are given at various intervals. For example, tetanus and polio vaccinations are given to babies, then booster injections are given at ages 5 and 15. Active immunisation involves the injection of an antigen in the form of a vaccine. This stimulates the production of antibodies and T and B memory cells specific to that antigen (or that type of antigen), many of which

BIOFACT Smallpox has been eradicated from the world: tuberculosis could be eradicated by similar control methods of screening and vaccination.

Vaccination, or immunisation, is the process of making people resistant to infection caused by a pathogen.

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FIGURE 7.26 Children can be immunised against many diseases that in the past caused serious epidemics.

remain in the body for a long time. Active immunisation is used to protect against measles, tuberculosis, polio, diphtheria, and whooping cough. Passive immunity involves the injection of antibodies that another organism has produced in response to infection by a particular pathogen. Passive immunity does not provide long-term protection and carries the risk of stimulating reactions against other foreign blood proteins that might be present in the vaccine. However, passive immunity is invaluable in providing immediate protection if people have no immunity themselves to a disease they have come into contact with. This would apply, for example, to a pregnant woman who has come in contact with rubella (German measles) or a person who comes in contact with hepatitis. In these cases, the patients are injected with human serum or with gamma globulins (antibodies) from human serum. Childhood immunisation is recommended for diphtheria, influenza B, measles, mumps, pertussis (whooping cough), rubella, polio and tetanus (Table 7.4). In Australia in 1997 and 1998 no cases of diphtheria were reported; no cases of polio have been reported since 1986.

Deliberate suppression of the immune system Babies are not able to make their own antibodies until they are 1–3 months old. They must depend on the passive immunity from antibodies their mothers provide. Some antibodies cross the placenta, providing immunity before birth; other antibodies are present in colostrum (first milk).

Suppression of the immune response in organ transplant patients is necessary to prevent rejection of the new organ. Because blood drains from transplanted organs into the recipient’s circulation, the body recognises the foreign tissue cells and produces antibodies in response. Cytoxic T cells, natural killer cells, macrophages and B cells can all be involved and cause a number of serious reactions that destroy the tissue. Rejection is reduced by matching the transplanted tissue proteins to the recipient’s proteins as closely as possible, and by giving drugs such as antilymphocyte globulin (ALG) which suppress the immune response. Clearly, the danger of this therapy lies in the inability of the patient to resist any infection they may meet, and the use of suppressive drugs must be carefully balanced against the risk of life-threatening infection.

BIOFACT

TABLE 7.4 Australian Standard Childhood Vaccination Schedule (recommended). (Source: Australian Immunisation Handbook (8th edition), September 2003, Commonwealth Department of Health and Ageing.)

BIOFACT

Meningococcal C is a rare but serious illness. Those most at risk are young children and young adults. In New South Wales between 200 and 250 people contract the disease each year and in 2002 there were 19 deaths. The disease progresses rapidly. Symptoms include fever, headache, nausea and vomiting, joint pains, neck stiffness and a distinctive rash of red-purple spots or bruises. A national meningococcal C vaccination program started in August 2003 to vaccinate all 1–19 year olds over the next four years. In New South Wales it is expected that 1.2 million school children will have received the free vaccination by 2005.

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AGE

VACCINE

Birth 2 months 4 months 6 months 12 months 18 months 4 years 10–13 years 15–17 years

HepB HepB HepB HepB HepB

DTPa DTPa DTPa

Hib Hib Hib Hib

IPV IPV IPV

7vPCV 7vPCV 7vPCV MMR

Men.C VZV

DTPa HepB

IPV

23vPPV

MMR VZV

dTpa

Key HepB — hepatitis B, available separately or combined with DTPa or with Hib; four doses needed in all (given at age 13 if earlier doses were missed) DTPa — diphtheria, tetanus, pertussis (whooping cough) dTpa — booster against pertussis for adolescents and adults Hib — haemophilus influenza b IPV — poliomyelitis vaccine (injection is preferred to the oral vaccine if available) MMR — measles, mumps, rubella (German measles) Men.C — meningococcal C (recommended) VZV — Varicella (for children with no history of chicken pox (Aboriginal and Torres Strait Islanders only) 7vPCV, 23vPPV — pneumococcal vaccines

A great Australian scientist: Macfarlane Burnet Sir Frank Macfarlane Burnet (1899–1985) was an Australian physician and virologist who made a huge contribution to the understanding of immunological responses. He was born in Traralgon, Victoria, in 1899. In 1922 he graduated from the University of Melbourne with a Bachelor of Medicine and Surgery. The next year he started his long career of research at the Walter and Eliza Hall Institute for Medical Research in Melbourne. In 1926 he left for London to undertake studies at London University and work at the Lister Insitute in London. In 1928 he had earned a Doctor of Philosophy degree, and returned to the Walter and Eliza Hall Institute for a short time. Before long he was back in London, at the National Institute for Medical Research in London, where he stayed until 1934, when he again returned to the Walter and Eliza Hall Institute, becoming Director in 1944. In 1951 he was knighted, and in 1960 he won the Nobel Prize in Medicine and Physiology with Dr Peter Medawar. In 1965 he was appointed President of the Australian Academy of Science. By his death in 1985 he had published more than 500 papers and 30 books. Macfarlane Burnet was one of the founders of the science of immunology. He spent most of his working life studying viruses. His work included research on poliomyelitis, Q fever (caused by Coxiella burnetii, which was named after him), cholera vibrio, myxomatosis, herpes, Murray Valley encephalitis and smallpox-like viruses. In 1935 he isolated a strain of influenza A virus in Australia. In 1946 he developed an experimental technique that enabled him to isolate and cultivate human influenza virus in chicken eggs. This method produced a high concentration of the virus, enabling it to be used to develop a vaccine. Burnet was also interested in the prevention of virus infections. This interest led to the work for which he won the Nobel Prize. He investigated the reasons why the immune system fights off foreign invaders such as viruses, while not reacting against its own cells. He examined how the body recognises itself and is able to use its

immune system to respond to foreign substances, such as viruses, without invading its own cells in the process. He concluded that the ability to recognise self-substances cannot be inherited, but is gradually acquired in the course of fetal development. Due to constant contact with cells in the body at an early age, the developing immunity-producing tissue learns to recognise and remember its own pattern. Burnet predicted (correctly) that if tissue from another body were introduced to a foetus at the right time, it would learn not to reject this foreign tissue. An English scientist, Dr Peter Medawar (1912– 1987), became interested in Burnet’s prediction and commenced a series of experiments in which he injected tissue from one type of mouse (donor mice) into mouse embryos of another type. When the embryos developed into normal mice, he gave them skin grafts from the donor mice. The grafts were not rejected. However, mice that had not been treated when they were foetuses always rejected the grafts. Burnet and Medawar’s work in this area has played a major role in organ transplants, and led in 1960 to the award of the Nobel Prize in Physiology or Medicine to both men. By 1960 Burnet had also developed the clonal selection theory. This important theory has helped us to better understand the immune system and how our bodies learn to distinguish between self and nonself.

FIGURE 7.27 Sir Frank Macfarlane Burnet during his time at the Walter and Eliza Hall Institute in Melbourne.

1

What sort of microorganisms did Macfarlance Burnet study?

2

What branch of medical science did he work in? Name a vaccine he developed.

3

For what research did he share the Nobel Prize in Physiology or Medicine in 1960?

The search for better health 365

When the immune system fails It is possible for the immune system to malfunction. It may over-function, resulting in hypersensitivity reactions or auto-immune diseases; or it may underfunction, resulting in immune deficiency diseases.

Hypersensitivity reactions These occur when the body’s immune reaction attacks the body tissues instead of the foreign material. There are several types of hypersensitive reactions—allergies are one such reaction. In an allergic reaction, antibodies are produced that are different from ‘normal’ antibodies, and these cause cell damage and the release of chemicals (histamines and serotonin) that have two main effects: 1 leaking of fluid from the blood into the tissues, causing swelling, blisters, skin irritation, nose and eye irritation 2 muscle spasm, causing asthma in some people.

Auto-immune diseases Auto-immune diseases occur when T cells mistake the body’s own proteins for antigens. The body’s own chemicals normally do not trigger off any immune responses, but in auto-immune diseases, a particular chemical or tissue is treated as foreign. In attempting to destroy it, the immune response may destroy healthy tissue.

For example, in rheumatoid arthritis, cells in the joints of the hands and feet are not tolerated and local inflammation around these cells is established causing swelling, stiffness and pain. In multiple sclerosis, cells in the brain and spinal cord are attacked causing tingling and numbness in various parts of the body and sometimes uncontrollable muscle movements. (‘Sclerosis’ comes from a Greek word for hardening. In multiple sclerosis the damaged nerves contain hard patches which can be widespread.) In myasthenia gravis the nerve–muscle junctions are attacked causing weakness in the muscles. The earliest signs are in the muscles of the face where the eyelids and the cheeks droop; later, any muscle may be affected.

Immune deficiency diseases Immune deficiency diseases are diseases in which the body’s defences are inadequate and unable to effectively defend it against attack. For example, in acquired immune-deficiency syndrome (AIDS), a virus attacks the T cells causing the body to be susceptible to infections it might otherwise fight off. These infections can be fatal. In Cushing’s syndrome (when the adrenal glands secrete too much cortisol), the immune system is suppressed because the body is unable to make proteins, including lymphoid tissue. Many sufferers of Cushing’s syndrome die from an infection, not from the disease itself.

Questions 1

Briefly summarise what happens in the body during an immune response.

3

Define memory cells. Describe their role in defending the body against disease.

2

a Explain what is meant by the term ‘cell-mediated immune response’. b Explain what is meant by the term ‘humoral (blood) response’.

4

Complete the following table, summarising the role of the different components involved in the immune response.

Immune system component T cell B cell Antigen Antibody

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Kind of structure

P roduced in/by

Role

5

a Define ‘immunisation’. b Draw a flow chart illustrating how the body’s immune system responds to a poliomyelitis vaccination.

6

a Explain the difference between active and passive immunity. b Use an example to outline at least one advantage and one disadvantage of these two processes.

7

a Describe the circumstances in which the activity of the immune system is deliberately suppressed. b Explain the reason for this. c Outline any disadvantages associated with this practice.

8

Explain the difference between an antigen and an antibody. Use diagrams to illustrate the specificity of the antigen–antibody complex.

5

Until recently only girls in Australia were immunised against rubella (also known as German measles). Vaccinations were routinely administered during the early teenage years. a Find out why it was important to immunise girls whilst they were young teenagers. b Today girls and boys are routinely immunised with rubella vaccine. Why is this?

6

Allergies are a type of hypersensitivity of the body. This means the body over-reacts when an antigen stimulates an immune response. How is the allergic response similar to a normal immune response? How is the allergic response different from a normal immune response?

7

Research an auto-immune disease. What are the symptoms of the disease? Describe the long-term effects on the body. Outline the treatments and prognosis for the disease. Some examples of autoimmune diseases are: pernicious anaemia, rheumatoid arthritis.

8

Tuberculosis (sometimes referred to as ‘consumption’) is a kind of pneumonia caused by the bacterium Myobacterium tuberculosis. Sufferers of tuberculosis typically have a persistent cough. Sputum is sometimes stained with blood. If untreated the infection spreads to the lymph glands and other organs including the liver, and eventually affects the brain and lungs. Death eventually results. TB is a very contagious disease passed from one person to another in airborne droplets coughed into the air and then inhaled by the second person. Today TB is a rare disease in developed countries where nutritional standards are high and vaccination programs against TB are in place. a Check Table 7.4. Is tuberculosis listed for routine vaccination? Suggest a reason for this. b In countries where vaccination against TB is not routine, the disease periodically turns up. Outline the potential consequences of this. c Suggest a strategy that could be used to ensure the complete eradication of tuberculosis from Australia.

F u r ther questions 1

Prepare a flow chart summarising the series of events that takes place in the immune system in response to the presence of an invading pathogen, for example the measles virus. Name each component of the immune system that is deployed, describing the role of each in eradicating the pathogen and preventing renewed infection.

2

Visit the Internet site of the Macfarlane Burnet Centre for Medical Research at www.burnet.edu.au/Internet/research a List some of the diseases currently being studied at the centre. b Choose one of the diseases that is being or has been under research at the centre. Write a brief summary of the disease and the results of any research that has been undertaken.

3

4

Obtain a copy of your own childhood vaccination record. Compare your vaccination record with Table 7.4. a List the diseases for which you have been immunised. b Are there any diseases for which you were not immunised? If so, try to find out the reason why. c Outline the reasons why some parents do not have their children routinely immunised. Complete the following table summarising aspects of preventable childhood diseases in Australia.

Disease Diptheria Pertussis Tetanus Poliomyelitis Smallpox Measles Influenza B Mumps Rubella

Kind of pathogen

Symptoms of disease

Treatment

Hepatitis B The search for better health 367

7.6

Epidemiological studies OBJECTIVES When you have completed this section you should be able to: ● define ‘epidemiology’ ● explain how the key features of epidemiology are useful in identifying the cause of lung cancer ● identify the cause of some non-infectious diseases, including examples of inherited diseases and those caused by nutritional deficiencies and environmental factors.

What is epidemiology? activities ● ●

Epidemiological studies Non-infectious diseases

Epidemiology is the study of diseases in populations.

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Epidemiology is the study of diseases that affect many people. It describes the patterns and cause of diseases in populations. The diseases studied include infectious diseases and those related to peoples’ lifestyles and the environment. Epidemiological studies have, for example, established the links between diet and heart disease, smoking and lung cancer as well as causes of diseases; they have been applied to assess factors in the spread of certain infectious diseases and the value of treatments and preventative strategies. Modern methods of epidemiological study involve groups of people, rather than individuals. Large quantities of information and data are collected and analysed statistically in the search for reliable knowledge that can be applied in health services. Cause-and-effect relationships are often very difficult to clarify, particularly in human beings, and we need to think about all the features of the individuals and those of the disease that might be related. We saw at the start of the chapter that many factors may influence the development of a disease. Carefully designed studies can help us distinguish those factors that may be causing a disease from those that are merely chance associations, or associations that are related to a particular factor but not as cause-and-effect.

An example: smoking and health Although it was suspected before 1900 that tobacco smoking was harmful to health, it was not until the 1950s that investigations were well designed and documented. Since that time numerous casecontrol studies and cohort studies have consistently reported the risks of smoking. In 1989 the American Cancer Society published results of an enormous study following people in 50 US states over 4 years and totalling 1.2 million participant-years. They identified the risks of death from various diseases in

Cause of death

Males

smokers compared to non-smokers; in other words, the relative risk. A relative risk of 1 means that the risk for a smoker is the same as that for a non-smoker; in other words, there is no association between smoking and that disease. You can see from Table 7.5 that in all the diseases studied the relative risk is greater than 1—that smoking increases the risk of that disease. For cancers of the lip, mouth, throat, larynx and lung, the relative risks are extremely high.

Females

All causes

2.34

1.32

Coronary heart disease

1.94

1.78

Other heart disease

1.85

1.69

Cerebrovascular disease

2.24

1.84

Other circulatory disease

4.06

3.00

*COPD

9.65

10.47

Other respiratory disease

1.99

2.18

Cancer: lip, mouth, throat

27.48

5.59

Cancer: oesophagus

7.60

10.25

Cancer: pancreas

2.14

2.33

Cancer: larynx

10.48

17.78

Cancer: lung

22.36

11.9

Cancer: kidney

2.95

1.41

Cancer: bladder

2.86

2.58

Cancer: cervix/uterus



* Chronic obstructive pulmonary disease

–2.14 TABLE 7.6 Percentages of cancers in Australia attributed to smoking.

Every year in Australia more than 18 000 people die from smoking-related diseases. Smoking accounts for one in every three cancers diagnosed. It may also be the cause of heart disease, strokes and respiratory diseases such as emphysema. Table 7.6 shows the current proportions of some cancers in Australia that can be attributed to smoking.

1

What advice would you give someone about the health risk of smoking?

TABLE 7.5 Relative risks of various diseases for current smokers in 50 US states (source: American Cancer Society).

Cancer tye

2

Males

Females

Lung

84

77

Mouth and pharynx

57

51

Oesophagus

54

46

Larynx

73

66

Bladder

43

36

Kidney (pelvis)

55

48

Pancreas

24

19

Stomach

14

11

Why is the risk of developing cancers of the lip, mouth, throat, larynx and lungs particularly high for tobacco smokers?

The search for better health 369

What is cancer? The best way of looking at cancer is to consider how cells multiply; that is, by the cell cycle (see Figure 7.2, p. 331), where one cell divides to become two cells. In the cell cycle, a cell starts off either as a resting cell (G0) or in the G1 phase of the cycle. The DNA is then totally copied, and the cell goes into a second stage., G2. All being well, it then goes on to divide. It may continue in cycle or go to the resting phase (G0). Each time it goes through the cycle, a new cell is generated. Most cells which come out of the cycle go on to become mature cells that make up a human being. However, many cells also die. There is a program for cells to die (apoptosis). If the cycling becomes abnormal, and excess abnormal cells are produced, a mass of them will result. This mass is called a tumour, which simply means a lump (Latin derivation). These days, however, by common usage, if someone says they’ve got a tumour, then they really believe they have got cancer. So a mass of abnormal cells resulting from normal cell division (cycling) is what we mean today by a cancer or tumour.

Cancer and genes Cancer is a disease caused by genes that do not work properly, usually because they are damaged (mutated). The genes involved in cancer are those which control cell division, maturation and function. When these genes are normal and function normally, cells divided, mature, function and die in a normal fashion. When these genes are mutated (damaged) and function abnormally, then there can be excessive cell division or cells can fail to die on time. Abnormal cells then accumulate, and if they function abnormally and invade the normal tissues, a cancer results.

Carcinogens Cancer does not happen by accident. There are many agents which damage genes and cause cancer. These agents are called carcinogens. Carcinogens come in three varieties—chemical, physical and biological. Chemical causes of cancer have been known for over two centuries, since tobacco snuff was found to be a cause of cancer of the nose. Since the end of the last century, radiation has been known to be associated with cancer. For many years, there was speculation that cancer could be infectious, and today we know that there are certain human viruses, parasites and bacteria which can cause cancer. For example, the most fatal cancer in the world is stomach cancer, which is associated with infection by a bacterium, Helicobacter pylori, identified about 15 years ago in Perth by Dr Barry Marshall. The second most fatal cancer of women world wide is cancer of the cervix, which we have tried to prevent by doing cervical smears to detect abnormal cells before the cancer develops. We know now that most cancers of the cervix is caused by certain types of the human papilloma (wart) virus. Another common fatal cancer worldwide is cancer of the liver, most of which is caused by the hepatitis B virus. It is rare in Australia because the hepatitis B virus doesn’t usually cause primary cancer of the liver in white (Caucasian) people. However, in Black and Asiatic people, the hepatitis B virus is the major cause of liver cancer. All in all, at least 15% of the 10 million or more new cases of cancer world wide each year are ‘infectious’. Hepatitis B vaccination has already been shown to prevent liver cancer, so vaccine development for these ‘infectious’ cancers is a top priority.

G2 180

projected mortality

DNA copied

CELL DIVIDES mitosis

CELL CYCLE

CELL DIES apoptosis

G1 CELL MATURES

resting G0

a mass of abnormal cells accumulates, forming a tumour

FIGURE 7.28 The cell cycle, also showing cell death and abnormal cell production.

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abnormal cycling: excess abnormal cells produced

Age-standardised rate per 100 000

male 160 140 120 female

screening

100 80

screening 60 1950 1960 1970 1980

1990

Year of death

FIGURE 7.29 Deaths caused by cancer.

2000

2010

For example, if we studied all the patients in a large metropolitan hospital who had heart attacks, we might find they all own a television set, but it is most unlikely that this is a causal factor in their disease! Or we might find that there is a significant relationship between salt intake and a particular nervous disorder, but without a properly designed study we might not be able to say whether (a) is it a chance relationship in which a third factor is involved (e.g. infection with a virus that causes both the nervous disorder and a craving for salt), or (b) eating too much salt brings about the nervous disorder, or (c) people with the nervous disorder are inclined to eat more salt. It is easier to establish a cause-and-effect relationship between pathogens and infectious disease than it is with the factors that cause non-infectious diseases. There are three broad categories of epidemiological studies: ● Descriptive studies which show patterns in the ways diseases happen to be distributed in populations; for example, where a disease occurs geographically or which occupational groups of people suffer a certain disease. ● Analytic studies which are planned investigations designed to test specific hypotheses; for example, a long-term study of a group of people exposed to a particular agent to see if they develop a particular disease (a cohort study), or a comparison of the lifestyle of a group of people with a particular disease to see if they have any risk factors in common (case control study). The control group would be people exposed to the same risk factors who do not have the disease. Analytic studies may be prospective (looking forward) or retrospective (looking back). ● Intervention studies which measure the effectiveness and safety of certain interventions; for example, clinical trials of a new drug or other treatment developed for a disease.

BIOFACT Tobacco smoke contains 43 known carcinogens. One of these—benzo α pyrene—inactivates the p53 gene. This gene delays mitosis until any genetic damage in a cell is repaired.

Two commonly kept statistics in epidemiology are the number of new cases diagnosed each year (incidence) and the number of deaths (mortality).

Lung cancer Lung cancer is a disease caused by the abnormal growth of cells in the lung. In New South Wales in 2001 lung cancer was the most common cause of cancer deaths in people aged 50–84 years old. Epidemiological studies in the 1950s and 1960s established that smoking is a major cause of lung cancer. These studies were retrospective studies of people who died of lung cancer, compared with a control group of non-smokers. About 80% of lung cancers occur in smokers. Further studies have shown a relationship between the risk and severity of lung cancer and the number of cigarettes smoked each day. The risk doubles from tenfold in average smokers to twentyfold in heavy smokers. The length of time a person has smoked is also significant. The risk of contracting lung cancer increases further in people who started at a young age or continued into later life. The risk is stated in terms of ‘pack years’—the number of packets smoked per day multiplied by the number of years the person smoked. The collection and analysis of epidemiological data continues to increase our knowledge of the disease.

BIOFACT Some 2001 New South Wales lung cancer statistics: Incidence Of all new cancer cases, lung cancer was diagnosed in 11% of males and 7% of females. Total: 2698 new cases of lung cancer. Mortality Of all cancer deaths, lung cancer accounted for 22% in males and 14% in females. Total: 2326 deaths from lung cancer

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We know that lung cancer in New South Wales occurs most commonly between ages 40 and 70, with a low survival rate after five years of 12% in males and 15% in females. Detailed studies have shown a variation in the disease for factors such as age, sex, regional area and occupation. Long-term data collection indicates trends or changes in these over time. We know, for example that trends in lung cancer incidence reflect changes in smoking habits and that the incidence of lung cancer in males has fallen in the last ten years but risen in females. Comparisons with data from other countries help give us a global picture of the disease.

Causes of non-infectious diseases Non-infectious diseases are not caused by pathogens; they are not transferable from one organism to another; they have a wide variety of causes; and they include a wide variety of types of disease. They include:

FIGURE 7.30 The Down syndrome chromosomes. Note the extra chromosome in pair 21.

• inherited diseases: these are genetically transmitted in reproduction, e.g. cystic fibrosis, Down syndrome. • nutritional deficiencies: these are the result of incorrect or insufficient diets, e.g. vitamin deficiencies, starvation, anorexia nervosa, anaemia. • environmental diseases: these are the result of environmental factors, perhaps acting on some inherited susceptibility, such as asthma, cancer, heart disease, heavy metal poisoning and drug abuse (see Table 7.7, p. 376).

Inherited diseases There are many inherited or genetically transmitted diseases, including both gene and chromosome abnormalities. They may be minor disorders such as vision defects (e.g. short-sightedness or red–green colour blindness), or major diseases such as thalassaemia or cystic fibrosis. Inherited diseases can often be successfully treated. This may involve surgery, drug treatment or special diets. Faulty genes cannot be corrected at the present time but genetic engineering techniques may enable us to correct them in the future. When a genetic disease runs in the family, genetic counselling is available to couples who wish to investigate the chances of having an affected child. For those already pregnant, the techniques of amniocentesis and chorionic villus sampling can identify an increasing number of genetic abnormalities. These techniques involve the removal of a sample of embryonic cells which are then cultured and the chromosomes examined to determine if any abnormalities are present.

Down syndrome FIGURE 7.31 A child with Down syndrome.

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Down syndrome is caused by the inheritance of one extra chromosome. Normal human cells contain 46 chromosomes. Children born with Down syndrome have 47 (Figure 7.30). This is caused by the incomplete separation of a specific chromosome pair during meiosis. The likelihood of this occurring increases with the age of the mother.

Children born with Down syndrome show mild to severe mental retardation. Their facial characteristics may include a small, flat nose, skin folds at the eye corners which give a slanted appearance to the eyes, a protruding tongue and small ears (Figure 7.31). Neck, arms and fingers are often shorter than normal. About 40% have some sort of heart defect. There is no cure for Down syndrome. Special education programs, however, can assist children. Many can attend normal schools and become capable of living independent lives. Because the chance of having a Down syndrome child increases with age, pregnant women over 35 in Australia can have amniocentesis or chorionic villus sampling in early pregnancy to detect the condition and make a choice about whether to continue with the pregnancy.

Nutritional diseases Deficiency diseases Lack of a vital component in the diet can result in a deficiency disease. For example, the lack of vitamin C can cause scurvy (see p. 374). A lack of the mineral iron results in anaemia (tiredness and a low number of red cells). A lack of protein results in diseases such as kwashiorkor and marasmus, with symptoms which include tissue wasting, anaemia, sores on the skin, and fluid accumulation (oedema) which causes a potbellied appearance (Figure 7.32). Starvation results in symptoms such as muscle and tissue wasting, loose skin, a lowering of the body’s metabolic rate, and swelling due to accumulation of body fluids. Without food, an adult will die in about 2 months. Anorexia nervosa is a psychiatric disease caused by deliberate undereating. It is a serious disease that can have dangerous long-term effects on health.

FIGURE 7.32 Children in poor regions are most likely to suffer from protein deficiency: a child with kwashiorkor, a condition which can be reversed if protein supplements and medical aid can be provided early enough.

Drug abuse A drug is any chemical substance that affects the functioning of the body. Medicines are drugs used deliberately to prevent and treat diseases. We take other drugs deliberately as part of our normal social behaviour, such as caffeine in coffee and tea, ethanol in alcoholic drinks, and nicotine in cigarettes. Other drugs include illegal substances such as heroin and cocaine, and household chemicals such as glues and paint thinners, taken or inhaled deliberately for their temporary effects on the brain and nervous system. Accidental or deliberate misuse of drugs, mostly involving high doses taken over a long period, can cause diseases, as in the following examples: ● smoking tobacco can cause lung cancer ● over-consumption of alcohol can cause alcoholism, a disease that damages many parts of the body, especially the liver and brain ● frequent use of narcotic drugs, such as heroin and cocaine, causes addiction, with painful withdrawal symptoms. Throughout Australia education programs such as ‘Party safe’ are actively promoting responsible behaviours and helping to reduce drink-driving. Visit www.jaybees.com.au/news/party.html

A drug is any chemical substance that affects the functioning of the body.

The search for better health 373

Scurvy: scourge of the high seas When humans first set out on long sea voyages, they were plagued by a terrible disease which came to be known as scurvy. Various parts of the body would swell. The capillaries became fragile and caused bleeding under the skin and within tissues. The gums became swollen and rotten, and teeth fell out. Wounds failed to heal. Death was the inevitable result without treatment. Sailors knew that getting ashore and eating well helped, but how to treat the disease while at sea was a mystery until the 1700s. The death of 626 out of 961 British sailors from scurvy on a 4-year sea voyage to the Pacific in the 1740s inspired a ship’s surgeon in the Royal Navy, James Lind, to conduct what is thought to be the first controlled nutritional study, in 1747. He took 12 sailors suffering from scurvy and controlled their diet for 2 weeks. He divided the 12 men into six groups of two. All were given the same basic meals, but each group was also given a different dietary supplement. The group that got two oranges and two lemons each day recovered immediately. None of the other groups showed any improvement. Lind deduced that something in the citrus fruits was counteracting the cause of scurvy. But in his study Treatise of the Scurvy, published in 1753, he unfortunately concluded that a sailor’s usual diet was adequate and that, to avoid scurvy, sailors only needed to get plenty of fresh air and exercise and avoid damp conditions.

Thus scurvy continued to be a serious problem on long voyages. Not long after Lind published his findings, a surgeon’s mate named David McBride conducted experiments to determine the cause of scurvy. In 1764, in his book Experimental Essays on the Scurvy and other Subjects, he stated his belief that sauerkraut (raw cabbage that is salted and then fermented) and malt were effective in preventing the disease. His brother, Captain John McBride, followed his dietary recommendations while commanding HMS Jason on a long voyage, and found that cases of scurvy almost disappeared. The British Admiralty was still not convinced, and by 1769 it was looking for an opportunity to test McBride’s ideas. The voyage of HM Bark Endeavour to the South Pacific, under the command of James Cook, proved to be ideal for this purpose. The small ship was supplied at Portsmouth with 7680 lbs (more than 3 tonnes!) of sauerkraut and a huge quantity of dried malt. The addition of these to the normal diet was stunningly effective: only five minor cases of scurvy were reported on the whole voyage, which lasted more than 3 years. But it was not an easy task to get the sailors to accept the new diet. When the sailors refused to eat the strange ‘sour krout’, Cook employed a brilliant tactic. He served large helpings of it to the officers but not to the sailors. Soon the sailors became jealous of this ‘privilege’, and demanded that they be given a fair share. Within a week, the sailors were eating so much of it that Cook had to ration it for the rest of the voyage! We now know that scurvy is caused by a lack of vitamin C (ascorbic acid), and that green vegetables and citrus fruits are excellent sources of this vitamin. In later years citrus fruits became the favoured source on ships, because they needed no special cooking or storage and could be obtained in many ports around the world. The Royal Navy preferred limes and lime juice, and so British sailors became known as ‘limeys’—a name that is sometimes still applied to British people today.

FIGURE 7.33 A mild case of scurvy. The symptoms of scurvy can be truly dreadful, and it was once the most feared disease amongst sailors. It accounted for most of the deaths on long voyages, yet its cure is simple.

1

What causes scurvy?

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2

Name three foods that have been used to overcome the deficiency in sailors’ diets.

Cardiovascular disease Cardiovascular disease occurs when the system of the heart and blood vessels is unable to keep up with the demands of the rest of the body for oxygenated blood. There are different forms of cardiovascular disease, and different causes. The chance of developing the disease is increased by certain risk factors, including: ● high levels of cholesterol in the blood ● high blood pressure ● cigarette smoking ● diabetes. These are the four major risk factors for atherosclerosis. Other possible factors are: age, lack of exercise, obesity, stress, rheumatic fever, and genetic inheritance. With age, and a high cholesterol diet, fatty deposits tend to build up in the blood vessels, particularly the arteries. They become thicker, harder and less elastic (Figure 7.34). They are less able to respond to changes in blood pressure and if they become too clogged, blood flow is reduced and vital organs may be deprived of oxygen and nutrients when they need them most, e.g. during exercise. Symptoms may include breathlessness, pain in the chest, abnormal heart rhythm, and swelling of the legs and ankles. Some different types of cardiovascular disease are: ● coronorary thrombosis: a ‘heart attack’ when a blood vessel in the heart becomes blocked ● arteriosclerosis: thickening and hardening of the artery walls ● hypertension: ‘high blood pressure’ brought on by obesity, stress, kidney disease, glandular disorders, a high salt diet or arteriosclerosis

stroke: a failure of the blood supply to the brain, either from a blood clot (thrombosis) or from internal bleeding (haemorrhage) ● rheumatic heart disease: rheumatic fever, a bacterial infection, may leave the heart muscle and valves scarred and weakened. Treatment may include rest, drug treatment to reduce high blood pressure or high cholesterol levels, valve replacement surgery, fitting an electronic pacemaker to control heart rhythm, heart transplants, and bypass operations to re-route the blood and avoid diseased blood vessels. Prevention methods include keeping fit and exercising regularly, no smoking, a low cholesterol and low salt diet, weight control and stress management. The most important management involves adopting a healthy lifestyle from an early age to prevent the diseases developing. ●

normal artery

artery clogged with fatty deposits

FIGURE 7.34 A normal artery (left), and an artery clogged with fatty deposits (right).

Name three lifestyle changes you could make to reduce your risk of cardiovascular disease.

The search for better health 375

BIOFACT The male Sydney funnel-web spider is one of the world’s deadliest spiders. Its venom contains a powerful toxin that attacks the nerves, triggering thousands of uncontrolled electrical impulses. These cause twitching accompanied by sweating and the production of tears and saliva. The venom also affects the blood vessels, which can lead to loss of consciousness and brain damage. Until 1980, when an antivenom was developed by a team led by Dr Struan Sutherland, many deaths were caused by this spider. We now know that there are several other funnel-web species, at least one of which might be more venomous than the Sydney funnel-web. Fortunately, the antivenom is also effective for bites from those spiders.

Environmental diseases Many factors in the environment can cause disease. They include exposure to radiation, heavy metals, pollution in the air, soil or water, loud noise, stress and drug abuse (Table 7.7).

TABLE 7.7 Some environmental factors that can cause disease.

Category

Examples

Mechanical trauma

motor vehicle accidents workplace accidents sports injuries injuries due to violence

Temperature extremes

burns frostbite hypothermia heat stroke

Irradiation

sunburn skin cancers radiation sickness

Chemicals e.g. alcohol heavy metals such as lead and mercury air and water pollution

alcoholism heavy metal poisoning nervous disorders cancers

Excessive noise

hearing loss sleeplessness hypertension

Bites and stings

cardiopulmonary failure respiratory failure blood poisoning tetanus sores, lesions, ulcers

Asthma and climate Australia has the highest incidences of asthma in the world. In Australia it affects 10–15% of adults and 20–25% of children. One theory about why the incidence is so high here is that, because most of us come from European stock, our bodies are not adapted to cope with the pollens of native plants and the spores from native fungi. Climate has a significant effect on the risk of an asthma attack amongst susceptible people. Some events that are known to trigger attacks are: ● thunderstorms ● rapid passage of a low-pressure system ● increased winds Name two climatic and two non-climatic triggers of asthma.

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excessive heat and high humidity spring rains ● dust storms ● periods of long, hot weather ● high levels of air pollution, including cigarette smoke. If the predictions of greenhouse climate change are correct, there is likely to be an increase in the incidence of asthma in Australia, because the predicted increase in temperatures and rainfall will encourage the development of pollens and the growth of fungi and moulds, and dust storms might also be more frequent. ● ●

Taking the ‘heavy’ out of heavy metals: lead and lead poisoning Lead is a ‘heavy metal’. Heavy metals are metals with a density more than five times that of water. Many can accumulate in the tissues of animals, including humans, and cause serious diseases or death. Some other heavy metals are cadmium, chromium, cobalt, gold, mercury, silver, thallium and vanadium. Lead is one of the most toxic heavy metals for humans. Acute lead poisoning is rare; it occurs when a large quantity of lead is accidentally taken into the body. But since the early 1900s there has been a rise in the incidence of low level or chronic lead poisoning as the amount of lead we use has increased and exposure to it has become more common.

Sources of lead Lead can enter the body from several sources: ● Food Canned and processed foods may contain higher levels of lead because of the use of lead-soldered cans. These are now very rare in Australia but are still used in some countries. Crops grown in lead-contaminated soils can also contain high levels of lead. ● Lead-based paints and pesticides Before 1950, paint contained as much as 50% lead. In 1965 the maximum lead level recommended for domestic paints was lowered to 1%, and today the maximum level allowed is 0.25%. Other types of paint may still contain significant levels, and if they are flaky they can produce dust containing lead. These include car enamel paints, lacquers and some primers. Lead arsenate is a pesticide that was commonly used in home gardens until the 1960s. Both the lead and the arsenic in this chemical could cause serious disease. ● Air Older cars and trucks needed lead compounds to keep their engines running properly. Most of this lead is emitted into the air from the exhaust system. In Australia, cars and trucks made since 1986 have had to run on unleaded petrol, but some older vehicles still use leaded petrol. In 1999 a lead-substitute petrol was introduced for older cars, and leaded petrol is now being phased out. ● Water In some parts of the world, drinking water has become contaminated by industrial chemical wastes or lead piping, and has sometimes been found in processed drinks such as beer and soft drinks. The use of lead cartridges in shotguns for duck hunting can also add large amounts of lead to lakes and streams. ● Soil Lead levels may be high in soil (as well as in air and water) in lead mining and processing areas, particularly smelters, putting workers and the local community at risk. ● Hobbies and sports Lead particles may be inhaled or ingested while participating in hobbies and sports that use lead products, such as lead glazes in pottery, lead strips in leadlighting, lead weights in fishing, and lead bullets in pistol shooting.

FIGURE 7.35 Before the introduction of unleaded petrol in the 1980s, all petrol-fuelled motor vehicles emitted large amounts of lead in their exhaust gases. Many people living along busy roads had unacceptably high levels of lead in their blood. Today, almost all of the petrol-fuelled vehicles on Australian roads use unleaded petrol.

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Effects on the body Lead in the body has a toxic effect, particularly in the blood cells, nervous system and kidneys. Symptoms of acute lead poisoning include: a metallic taste in the mouth, nausea and vomiting, diarrhoea, delirium, paralysis and coma. Most people recover, but severe poisoning can cause death. In chronic lead poisoning the early signs are tiredness, headaches and feeling generally unwell. Later, other symptoms may develop including anaemia, a bluish marking (lead line) on the gums, nausea and vomiting, kidney failure, and nervous system damage to the brain and peripheral nerves. Peripheral nerve damage is shown as weakness, numbness and pain in the muscles. Children develop the symptoms at much lower levels of lead than adults, and brain damage is more common, leading to impaired growth, seizures and mental retardation.

Control

FIGURE 7.36 People who make stained glass windows are at risk of ingesting large amounts of lead unless they take special precautions. The lead is in the flexible slotted ‘came’ used to hold the pieces of glass in place, and in the solder used to join the pieces of came. Safety guidelines include wearing gloves and a face mask, using a fan to draw fumes from the work area, using low-lead solders, and not eating or drinking while working with lead.

Treatment is by administering EDTA (ethylene diamine tetraacetic acid). This combines with any lead in the body and is excreted via the kidneys. Prevention includes measures such as: ● Monitoring the concentration of lead in the blood of people at risk. The level of lead detected in the blood is a good measure of the exposure to lead. Blood lead levels are usually expressed in micrograms per decilitre (µg/dL). Levels over 10 µg/dL can affect intellectual development. From estimates made in 1993 the National Health and Medical Council recommended that the target for Australia should be that 90% of children aged 1 to 4 should have a blood lead level of less than 10 µg/dL by the end of 1998. A national survey of lead in children conducted in 1995 by the Australian Institute of Health and Welfare found that this target had already been met. ● Monitoring the levels of lead in the air, soil, dust and water, particularly in urban areas. ● Setting national standards for maximum lead levels in soil, dust, air and water. ● Strictly controlling industrial wastes that have a high lead content. ● Setting down safety standards for the handling of lead and products containing lead. ● Phasing out the use of unleaded petrol in motor vehicles. ● Undertaking public education programs about the risk of absorbing lead from old paints and painted items, hobby products, and some modern products. FIGURE 7.37 The lead shot in shotgun cartridges is responsible for significant contamination of some freshwater lakes where duck hunting is popular. The lead shot sinks to the bottom of the lake, where it can be ingested by bottom-feeding fish and birds, or gradually taken up by other aquatic organisms. In Australia there are moves to replace lead shot with less hazardous metals.

Name three causes of environmental lead pollution and name some strategies that have reduced their impact in recent years in Australia.

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Questions 1

a What is epidemiology? b How is ongoing research in epidemiology important to our society?

2

Describe the three broad categories into which epidemiological studies are classified.

3

Describe how epidemiological studies have contributed to our knowledge of lung cancer.

4

How are non-infectious diseases different from infectious diseases?

5

Non-infectious diseases can be grouped into three categories, based on the causal factor. Name the three categories, explaining the difference in cause between the groups. Give examples of diseases in each category.

F u r ther questions 1

Find out how techniques such as amniocentesis and chorionic villus sampling are used to determine genetic abnormalities in the developing foetus. What is involved in each technique? Outline any risks associated with these procedures. List some of the conditions that can be identified using these technologies.

2

At the present time, we cannot cure inherited diseases. a Explain why this is so. b Describe how genetic engineering techniques might help overcome inherited diseases in the future. c What is gene therapy? How can it be used to treat some inherited diseases? Use an example in your answer.

3

4

Do some library or Internet research to find out about one of the following diseases. asbestosis mesothelioma asthma lung cancer emphysema coronary heart disease diabetes In each case: ● describe the symptoms of the disease ● outline any long-term effects ● describe current treatments and prognoses ● summarise any apparent relationships between the disease and a causal factor indicated by statistics (include relevant statistics). Nuclear power stations are becoming increasingly common for generating electricity for domestic and industrial use around the world. But nuclear accidents involving dangerous releases of radiation,

such as the accident in Japan in September 1999, are all too common. a What is radiation sickness? Describe its symptoms, treatment and prognosis. b What do epidemiological studies suggest about the long-term effects of exposure to high levels of radiation? Information related to the Three Mile Island incident and the Chernobyl disaster of 1986 may be useful. 5

Use reference books to help you complete the following table, summarising aspects of deficiency disease in humans.

Disease

Cause

Symptoms

Treatment

Kwashiorkor Anaemia Scurvy Beri-beri Pellagra Cretinism Marasmus Rickets Obesity Anorexia nervosa Bulimia Starvation

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7.7

Strategies to prevent and control disease OBJECTIVES When you have completed this section you should be able to: ● discuss the importance of Australia’s quarantine practices in preventing the entry and spread of disease in Australia ● outline strategies aimed at controlling and preventing the spread of disease, including public health programs, pesticides and genetic engineering.

Modern knowledge of disease has led to the development of a wide range of strategies to prevent and control disease in humans.

Public health programs activities ● ● ●

Plant diseases Quarantine in action Managing diseases

Public health programs help control and prevent disease by strategies directed at three targets: the pathogen, the host and the environment.

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Public health programs help control and prevent disease by strategies directed at three targets: the pathogen, the host and the environment. There is an increasing emphasis on preventing disease, rather than hoping to cure it when it occurs. The pathogen is controlled by such standardised procedures as sterilisation methods and the Universal Precautions (see p. 338), which provide strict guidelines for health workers in order to reduce the spread of blood-borne viruses (HIV, hepatitis B and C) and other infections. Laws requiring government authorities to be notified of the occurrence of certain diseases, such as leprosy and AIDS, have helped stop their spread, as have laws requiring people with certain diseases such as chicken pox and rubella (German measles) to be quarantined or isolated.

Some public health programs New South Wales introduced a cervical screening program in 1992. It encouraged women, particularly those over 50, to have a check-up, called a Pap test, every two years. Pap tests check for early signs of cancer of the cervix. A central register is kept, and women receive a notice when their test is overdue. After five years of the program the incidence of cervical cancer in New South Wales dropped by nearly one third. Before the introduction of the program the incidence of cervical cancer was a stable rate of about 12.3 per 100 000 women. By 1997 this had dropped to 8.6. The latest figures released by the NSW Cancer Council (1996) indicate that the incidence of melanoma (skin cancer) is still increasing. It is the commonest cause of cancer in males 15 to 54 years old and in females 15 to 29 years old. Prevention programs such as ‘Slip, Slop, Slap’ and ‘Me No Fry’, which have been going for over 20 years, appear to be helping to reduce the incidence of skin cancer. The incidence of melanoma in people 20 to 39 years old has fallen. As people in this group become older, it is hoped that reduction in the incidence of skin cancer will continue.

People are protected by improved awareness from public education campaigns that influence people to make lifestyle changes which improve their health. Some examples are the ‘Quit’ campaign, safe sex programs, healthy heart programs, diet and exercise information, home hygiene publications and child health services. Other initiatives are: ● drink-driving laws which help reduce both alcohol consumption and road accidents ● vaccination schedules for travellers and children (see Table 7.4, p. 364) ● asthma awareness campaigns ● screening programs which improve the early detection of cancers, high blood pressure, tuberculosis, etc.

BIOFACT If a disease is declared to be ‘notifiable’, it means that when a case is found by a health worker, the government health authorities must be informed. Disease notifications provide valuable data on the diseases, such as any increase or decrease in rates of infection, geographical areas where it is prevalent, populations most affected, and effectiveness of control measures. Notifiable communicable diseases include: ● blood-borne diseases (e.g. hepatitis B, C and D) ● diseases that require quarantining (e.g. cholera) ● gastrointestinal diseases . (e.g salmonella) ● sexually transmitted diseases (e.g. chlamydia) ● vaccine preventable diseases (e.g. measles, mumps, tetanus) ● vector-borne diseases (e.g. Ross River fever) ● other diseases (e.g. leprosy, tuberculosis).

FIGURE 7.38 Public education campaigns are an important weapon in the fight against disease.

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BIOFACT Travellers entering Australia cannot bring in the following items: ● eggs and egg products ● dairy products (unless from a country free of foot and mouth) ● uncanned meat products ● live animals and plants ● seeds and nuts ● fresh and frozen fruit and vegetables. The following things must be declared and inspected, and may require treatment: ● food ● animal products, e.g. wool, fur, shells, used animal equipment ● plant material, e.g. wooden, straw or cane articles ● sporting and camping equipment ● footwear or clothing used in rural areas.

Quarantine is a period of isolation to prevent the spread of a contagious disease.

FIGURE 7.39 Quarantine inspections at ports-of-entry by the Australian Quarantine Inspection Service help to prevent plant and animal diseases entering Australia.

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The environment is improved by public health measures such as: ● pollution monitoring programs ● controls in waste dumping and pollution emissions ● clean water supplies ● domestic sanitation and sewerage ● garbage collection ● vermin control ● building designs which includes adequate space, lighting and ventilation ● higher standards of personal and domestic cleanliness ● medical and hospital facilities ● food and hygiene inspection ● quarantine controls ● legislation such as Occupational Health and Safety laws that help create safer workplaces.

Quarantine as a control measure Quarantine is a period of isolation to prevent the spread of a contagious disease. Australia has had strict quarantine laws for most of this century to protect the country’s plants and animals from introduced diseases and from introduced animals and plants. These laws prohibit the entry of items that are considered a risk, as well as enforcing the isolation of plants and animals. Quarantine inspectors are on duty 24 hours a day at all the main points of entry of ships and aircraft into Australia. They are employed by AQIS (Australian Quarantine and Inspection Service) to check the people entering and the items they bring with them, as well as all the cargo and mail arriving in the country. Imported animals face a time in isolation or quarantine before they can be released to live freely in Australia. All imported plant seeds are examined to check that there are no weed seeds mixed in. Imported used vehicles and agricultural machinery are inspected and cleaned if necessary so that no soil, straw or plant matter enters the country. Because of Australia’s strict quarantine laws, Australian plants and animals do not have some of the serious diseases found in other countries. Our livestock is free from diseases such as foot-and-mouth disease, bovine encephalitis (mad cow disease) and rabies. Different States in Australia also have quarantine requirements for animals and plants. Phylloxera is an insect disease of grape vines caused by plant lice of the genus Phylloxera. The disease is present in eastern Australia but not in South Australia or Western Australia. Quarantine regulations forbid the introduction of any part of the vine into South Australia. If you have ever travelled interstate you may have seen notices forbidding you to take fruit across State borders. This is to control the spread of fruit flies, particularly the Mediterranean fruit fly which is in Western Australia and the Queensland fruit fly which is in eastern Australia. The Northern Territory, South Australia and Tasmania do not have these pests, which cause serious damage to fruit.

Pesticides Pesticides are chemicals that can be used to destroy the organisms that directly damage crop or garden plants or cause disease in animals. They are also used to eliminate the vectors of diseases in humans or other animals and plants. Herbivores feeding on plants may occur in such large numbers that they cause severe damage. When huge areas of land are planted with a single crop (monoculture), natural predators can increase to plague proportions because of the abundance of food and pesticides are used to control them. Pesticides may also be used against the vectors of a disease. Potato leaf roll virus is a serious disease of potatoes in Australia that can reduce the potato crop by half. The vector of the disease is a small insect, an aphid. Farmers spray potato crops up to eight times between planting and harvesting to try to control aphids. Mosquitoes act as vectors in the spread of several diseases to humans and other mammals (see p. 347). Eradication of mosquito larvae and adults by spraying with pesticide is actively pursued in most countries. In Australia, local councils administer schemes to control mosquito populations. Pesticides may be administered as sprays, baits or in irrigation water. Pesticides used on farm animals are called drenches and administered either by dipping or jetting (spraying).

Pesticides are chemicals that can be used to destroy the organisms that directly damage crop or garden plants or cause disease in animals.

Pesticide resistance The continued and increased use of pesticides has resulted in the development of genetic resistance in pests that are sprayed. The small percentage that survive are resistant to the effects of the chemical and pass this characteristic on to their offspring. for example, mosquitoes sprayed with DDT developed resistant strains (see Chapter 6, p. 266). Roundworm parasites in sheep and cattle in Australia are controlled by chemical drenches. It has been estimated that resistant parasites have developed on 90% of Australian sheep farms. The larval stage of the Australian sheep blowfly (Lucilia cuprina) causes flystrike in sheep and cattle (Figure 7.40). Many different pesticides have been applied to sheep, mostly by dipping or spraying, to kill any blowfly larvae. If the pesticide is used constantly, the blowfly population develops a resistance to the chemical, and a new one is needed to maintain effectiveness. An example is the use of the pesticide dieldrin. When first used in 1955 it gave 3 months of protection from flystrike. After 2 years of use it gave only 2 weeks protection. The development of resistance to pesticides is a major problem and has led to the development of integrated pest management schemes that use a combination of pesticide applications and biological control techniques.

FIGURE 7.40 Dipping in pesticide is a traditional method of preventing flystrike in sheep and cattle. But eventually the flies develop a resistance to the pesticide, and a new pesticide must be found to replace the old one.

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Flies Most of us use pesticide sprays to control insects such as flies, mosquitoes and cockroaches in our homes. Although flies do not directly cause disease in humans, they can contaminate food by spreading pathogens. Diseases may be spread if flies have previously walked over or eaten infected material such as faeces or rotting food. Pathogens that cause salmonella poisoning, typhoid fever, poliomyelitis and gastrointestinal disorders can all be spread by flies. Disease spread by flies can be controlled in the home by ensuring food is covered, killing adult flies, maintaining clean bathrooms and kitchens, and eliminating potential breeding sites outside, such as dog faeces and open compost heaps.

DDT concentration increases by 10 million times

DDT in fish-eating birds: 26.4 ppm

DDT in large fish: 2.07 ppm

DDT in small fish: 0.23 ppm

DDT in plankton: 0.04 ppm

DDT in water: 0.000005 ppm

FIGURE 7.41 DDT passes along food chains to high level carnivores. Because the amount of biomass decreases at each level in the food chain, DDT is concentrated. DDT is measured in parts per million (ppm).

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Biological magnification (biomagnification) Pesticides are often essential for the efficient production of a healthy agricultural crop, and can be very effective. But their use may also cause environmental problems, such as the accumulation of the pesticide in the food chain and the destruction of organisms other than the ones intended. DDT, now banned in Australia, has been used to control mosquitoes and agricultural pests. It was the first pesticide to demonstrate the process called biological magnification (or biomagnification). DDT does not break down easily, and persists in the environment for many years. It poisons soils on land that has been sprayed. It has been carried by water to all corners of the globe—even Antarctica—and traces have been found in almost every organism tested. DDT becomes concentrated in the food chain, and top-level carnivores accumulate the largest quantities of it in their tissues. In birds, high levels of DDT cause fragile egg shells that break before the young can hatch. In Australia, peregrine falcon numbers declined for this reason.

Genetic engineering Genetic engineering has produced disease-resistant plants and animals. By improving the health of crops and livestock, humans have increased their own food supplies. Use of these techniques remains controversial (see Chapter 6, p. 321) and the potential of genetic engineering has not yet been fully realised. Australian scientists have produced a genetically modified pea that is resistant to the pea weevil, a major pest of pea crops. The gene that confers resistance was isolated from the common kidney bean. The gene produces a protein that blocks digestion when eaten by the larvae, so that they fail to grow into adults. No chemical pesticides need be used on these genetically modified peas. (See p. 314 and 321 for other examples.) Another example is research into the control of bloat in cattle. Bloat is caused by the production of foam in the animal’s rumen when livestock eat clover and lucerne. In large quantities foam causes the animal’s throat to close, preventing gases from the digestive system escaping (see Chapter 3, p. 127). The gases result in pressure building up inside the animal, crushing their internal organs and causing death. Bloat does not occur if the level of compounds known as tannins in pasture is high.

These prevent foaming when plants are consumed. Scientists are working to isolate the gene from a related plant that will ‘switch on’ tannin production in clover and lucerne leaves. In medicine, genetic engineering already provides help in the control of some human diseases. Insulin is manufactured using recombinant DNA technology to help sufferers of diabetes.

Implications for the future The prevention and control of disease is a continuing battle. In some areas there are success stories, such as the eradication of smallpox and the decline of once-common human diseases through vaccination programs; in others there are disasters, such as the spread of cinnamon ‘fungus’ in Australian ecosystems. Genetic engineering techniques hold the promise of increasing the genetic resistance of hosts to pathogens. Trials of genetically modified organisms, including the growing and testing of genetically modified cotton in Australia, will be watched with interest for their success in controlling disease. If successful they will offer another preventative strategy in the fight against disease. Some controls work well for a time but then their effectiveness declines—the virus myxomatosis and its host the rabbit have learned to co-exist, and many bacteria have developed resistance to drugs such as antibiotics. The development of drug resistance in pathogens means that without continued research for new chemicals to destroy them, pathogens will continue to spread disease.

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Questions 1

a Describe the role of the Australian Quarantine and Inspection Service. b Explain how quarantine laws help control the spread of plant and animal diseases in Australia.

2

Describe one example each of how public health programs help to control and prevent disease by targeting the: a pathogen b host c environment.

3

Explain how public education campaigns can be beneficial to the health of the community at large. Use a specific example in your answer.

4

Outline the advantages and disadvantages of using pesticides on food crops.

5

Why are some plants and animals that are used for foods genetically engineered?

3

Do some library or Internet research to find out about public education campaigns run by the NSW Cancer Council, the Heart Foundation or the Motor Accidents Authority. a List the education campaigns currently being run by the organisation you chose. b What is the aim of the education campaign? c Is the campaign directed more specifically at a particular group in the community? If yes, suggest a reason for this. d In what form is the campaign delivered to the public? Example: television advertisements, radio broadcasts, newspaper, billboards, other.

4

Set up a class conference addressing the issue of genetically modified foodstuffs. ● Decide upon the issues that need to be discussed at the conference. ● Prepare a list of interest groups that would be represented. ● Decide upon an interest group that you wish to represent. ● Do some research on the topic of genetically modified foodstuffs. ● Prepare arguments to represent your cause. During the conference it will be important to set up the tables in your classroom in a large circle or oblong so that all representatives are facing each other. Set up cards facing your colleagues so that your interest group is clearly identifiable. You will need a facilitator (e.g. your teacher) to ensure fair time for each speaker. Take notes so that you can ask questions and prepare replies. At the end of the conference write a newspaper editorial discussing the issues raised at the conference and outlining your point of view.

F u r ther questions 1

2

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Australia has very strict quarantine laws to protect against the entry of exotic plant and animal diseases. The stallion Amerique, from the United States of America, who was entered in the 1999 Melbourne Cup, was brought to Australia 3 weeks before the event and kept in quarantine facilities for 2 weeks. After the arrival of the horse in Australia, and before its release from quarantine, it was reported that there had been an outbreak of a serious horse virus, West Nile virus, in New York, about 300 km from Maryland where the horse was trained. The disease had claimed the lives of six people and 13 horses in the weeks leading up to the running of the Melbourne Cup. West Nile virus occurs in birds. It is related to encephalitis and is spread by mosquitoes. Although the disease is rare, tests for its presence in Amerique were not conducted by American or Australian quarantine services. No proven vaccine is currently available against the virus. Amerique was eventually withdrawn from the race, and did not run. a ‘Amerique was trained a long way from the location of the outbreak of West Nile virus, and so had no contact with any infected horses in the New York area. Therefore, he could not be at risk of developing the disease.’ Comment on the validity of this statement. b Comment on the potential problems that would be associated with the introduction of this virus for i the Australian and international racing industry ii the Australian and international community. c Do you think Amerique should have been allowed to race in the 1999 Melbourne Cup? Give reasons for your point of view. Why are some infectious diseases listed as notifiable while others are not? Heinemann Biology

Chapter summar y Practical activities 7.1



Genes and health

7.2



Identifying microorganisms



Fit to drink—the treatment of drinking water

7.3



Pasteur’s experiment



Understanding disease—an historical perspective An infectious disease The role of antibiotics





7.4



Microflora— maintaining the balance

7.5



Vaccination programs

7.6



Epidemiological studies



Non-infectious diseases



Plant diseases Quarantine in action Managing diseases

7.7





7.1 • While there are sometimes difficulties defining the terms ‘health’ and ‘disease’, health can be broadly defined as a state of normal functioning, and disease can be boradly defined as a state of impaired functioning. • The maintenance of health in an organism is assisted by the maintenance and repair of the body’s cells and tissues through such processes as normal gene function, mitosis, cell differentiation and specialisation. 7.2 • An infectious disease is caused by infecting organisms such as bacteria and viruses. A non-infectious disease is caused by other factors, such as genetic inheritance, the environment or nutritional deficiencies. • An organism is described as a pathogen when it causes disease. • Cleanliness in food, water and personal hygiene help to control disease by reducing the risk of infection and by controlling its spread. 7.3 • Pasteur and Koch discovered that infectious diseases are caused by microorganisms. • Pathogens that cause disease plus an example of a disease they cause include: – prions, e.g. Creutzfeld–Jacob Disease – viruses, e.g. influenza – bacteria, e.g. tuberculosis – protozoans, e.g. malaria – fungi, e.g. tinea – macroparasites, e.g hydatid disease caused by a tapeworm. • Antibiotics help in the management of infectious diseases by destroying or inhibiting the growth of bacteria. 7.4 • The human body has a system of defences to prevent and respond to infection. The defence barriers that prevent the entry of pathogens in humans include the skin and mucous membranes. • Antigens are molecules that trigger the immune response. • Organ transplants should trigger an immune response, because the transplanted organ contains antigens that the recipient’s body will recognise as foreign. • The body’s defence adaptations as part of the non-specific immune response include inflammation and the action of phagocytes.

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7.5 • The immune response involves the production of B and T lymphocytes in response to exposure to an antigen. • B lymphocytes produce specific antibodies that destroy bacteria and viruses (antibody mediated immunity) and T lymphocytes produce specific chemicals that are toxic to the antigens (cell-mediated immunity). • Both B and T lymphocytes produce memory cells that confer longer-term immunity by making the response to subsequent exposure to the antigen more rapid and more intense. • In the immune response, B and T lymphocytes interact and collaborate under the regulation of cytokines. There are various T lymphocyte types that have different roles in the immune response. • Vaccination exposes the body to a small amount of dead or weakened antigen in order to safely stimulate the production of memory cells to the antigen. Subsequent exposure to the pathogen results in a rapid, intense immune response. • The immune response is suppressed in organ transplant patients to prevent the rejection of the transplanted organ by T lymphocytes. 7.6 • Epidemiology is the study of diseases in populations. It includes the statistical analysis of data to try to identify cause-and-effect relationships of a disease. The relationship between smoking and lung cancer was established using epidemiological studies. • Non-infectious diseases include inherited diseases such as Down syndrome, nutritional deficiencies such as scurvy, and environmental diseases such as lead poisoning. 7.7 • Australia has quarantine laws to prevent the introduction and spread of infectious plant and animal diseases into Australia. As a result, Australia is free of many major diseases, such as foot-and-mouth disease. There are also quarantine requirements between states to prevent the spread of pests such as fruit fly. • Strategies that can help control or prevent disease include: – public health programs, such as the use of sunscreen promoted by Cancer Councils – the use of pesticides to control disease-causing pathogens or to eliminate the vectors of disease, such as mosquitoes – the production of transgenic species that are disease-resistant, such as Bt cotton.

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EXAM-STYLE QUESTIONS Multiple choice 1 Which of the following is the best definition of disease? A Disease is any condition that impairs the functioning of the body. B Disease is a condition caused by an infectious or non-infectious factor. C Disease is the result of infection by microscopic or macroscopic organisms. D Disease is an interaction between a host, a pathogen and the environment. 2 What happens when potential hosts are exposed to a pathogen? A They usually become ill. B They will become ill if they are already weakened by a pre-existing condition. C They will produce toxins to fight the pathogen. D The result depends on the interaction between the host, the pathogen and the environment. 3 Which of the following statements describes how cleanliness and good hygiene practices assist in disease control? A They destroy pathogens. B They dilute the concentration of the infective dose. C They isolate pathogens. D They limit the spread of pathogens. 4 Which of the following is a list of the causes of infectious diseases of plants and animals? A prions, viruses or environmental factors B nutrient deficiency, bacteria or fungi C prions, viruses, bacteria or fungi D protozoans, bacteria or radiation 5 Which of the following statements applies to the way the skin provides protection from the entry of pathogens? A Secretion from sebaceous glands inhibits the growth microorganisms. B The salt in sweat kills micro-organisms. C The blood clotting mechanism quickly heals broken skin. D The skin forms a waterproof layer. 6 Which of the following is a correct statement about inflammation at the site of infection by a pathogen? A There is painful swelling and reddening of the affected area because red blood cells are able to ‘leak out’ of dilated blood vessels.

B Inflammation is an indication that the body is unable to manage the infection. C Inflammation is an important non-specific defence mechanism that results in the dilation of blood vessels so that white blood cells can ‘leak out’ and destroy the pathogen. D The body must be treated with an antihistamine to reduce the inflammation. 7 Vaccination involves injection of substances to induce immunity to a disease. What are the characteristics of these substances? A They always produce mild symptoms of the disease. B They are themselves antibodies or they stimulate antibody production. C They confer immunity for life. D They are prepared from dead micro-organisms. 8 What are the main outcomes of the immune response to infection? A B lymphocytes produce specific antibodies and T cells produce specific chemicals that destroy the antigens. B Specific antigens engulf and destroy the invading pathogens. C Fevers and sleep are induced, enabling the body to heal itself. D B and T memory cells remain in the body for the rest of the person’s life. 9 Why are public health programs important? A They control the spread of certain infectious diseases. B They prevent the incidence of certain environmentally induced diseases. C They inform the public of some diseases related to life-style. D all of the above 10 Which of the following is a correct statement about Australia’s quarantine procedures? A They ensure that native plants and animals as well as the agricultural industry are completely protected against diseases from other countries. B They prohibit the entry of plants, animals, soils or foodstuffs that might carry exotic diseases. C They require irradiation of all plant and animal imports to kill potential pathogens. D They include an isolation period for all imported animals but not for imported plants.

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Short answer a Distinguish between non-infectious and infectious diseases. b Name two non-infectious diseases and two infectious diseases that you have studied in this course.

2

Describe three rules of hygiene related to food handling and storage that are important in preventing contamination by disease-causing organisms.

3

a Describe how Louis Pasteur tested his theory that disease-causing microbes were carried in the air. b Outline the results of his experiment. c Was his hypothesis supported by his results?

4

a Explain what is meant by a non-specific defence mechanism in the human body. b Name two non-specific defence mechanisms, and describe their role.

5

Malaria is a serious disease world-wide. It is caused by a parasitic protozoan that invades the red blood cells of a mammalian host. a Name the vector that transmits this disease and explain how transmission occurs. b Describe the role played by Ronald Ross in the historical development of our understanding of malaria. c Name two ways in which the spread of malaria can be prevented.

6

An experiment was set up to test the effect of an antibiotic on a disease-causing bacterium. Two nutrient agar plates were made ready with a suitable medium for growth of the bacterium and then sterilised. Both plates were then exposed to the spores of the bacterium. Plate S was then sealed. Plate T was treated with five drops of a particular antibiotic and then sealed. Both plates were incubated for 48 hours at a temperature of 37°C. The diagram below illustrates the experimental results.

c Was the hypothesis supported by the results? Explain. 7

Consider two families with differing views about the merits of immunisation programs. Child X in one family is routinely vaccinated against all of the preventable childhood diseases. Child Y in the second family is not vaccinated at all. Child Y contracts whooping cough and is hospitalised with dangerous complications of the disease, but eventually recovers. Both children develop an immunity against whooping cough. a Explain the role of T cells, B cells and antibodies in the immune response that allows both children to develop immunity against whooping cough. b Outline the difference between active immunisation and passive immunity. c Considering that both children develop an immunity to whooping cough, explain why it is advisable that children be vaccinated. d Child X and child Y could come into contact with whooping cough again, but because they are immune they will not develop the disease. Explain why this is so. Name the particular cells responsible for this, and describe their role.

8

Epidemiological studies indicate a relationship between smoking and the incidence of lung cancer. Study the graph below, which illustrates the incidence and mortality for lung cancer in Australia in recent years. 100 Rate per 100 000 population

1

80

New cases: males

60 Deaths: males 40 New cases: females 20 Deaths: females 0 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000

S

T

a Why was it important to sterilise the nutrient agar plates at the start of the experiment? b Write a hypothesis that was being tested in this experiment.

390 Heinemann Biology

a Compare the incidence and mortality rate of lung cancer between males and females. b In general, how does the incidence of lung cancer compare with the death rate? c Explain how public health programs can be useful in preventing the incidence of this disease.

Glossar y abiotic Relating to the physical parts of the environment, as opposed to the biological (biotic) parts. abundance The number of individuals of a species that occur in a particular area. acidic Relating to a substance that releases hydrogen ions in water. Acidity is measured on the pH scale; acids have a pH below 7. active transport Movement of substances across membranes that requires the expenditure of energy; occurs through selective protein channels. adaptation A feature of an organism that makes the organism suited to its environment and helps it to survive and reproduce. An adaptation can be physiological, structural or behavioural. aestivation Remaining in a state of torpor during unfavourable seasons. alkaline Relating to a substance that releases hydrogen ions in water; alkalis have a pH scale; acids have a pH below 7. allele An alternative form of a gene. allelopathy The production of specific biomolecules by one plant that can harm or benefit another plant. alveoli Terminal air sacs of lungs, where gas exchange takes place. ambient temperature The temperature of the environment. amino acid An organic compound containing an amino group (–NH2) and a carboxyl group (–COOH) at opposite ends of the molecule. Linked amino acids form the peptide chains in protein molecules. aneuploidy An abnormal number of chromosomes in an individual, caused by an accident during meiosis when a pair of chromosomes fail to separate. anoxic Without oxygen. anther Structure at the tip of the stamen that contains chambers called pollen sacs in which pollen grains develop following meiosis. antibiotic A substance that inhibits the growth of bacteria. antibiotic resistance The ability of a bacterium to survive an antibiotic. Resistance develops in bacterial populations by natural selection. antibody A substance produced by the body’s immune system (B lymphocytes) in response to the presence of a foreign antigen. antigen Substance capable of binding with an antibody produced by the immune system. apoplastic loading The loading of nutrients into the phloem via a pathway within the cell walls (apoplast).

archaea Type of procaryote with no murein in cell wall, branched lipids in cell membrane, not sensitive to common antibiotics. artery A blood vessel with thick, elastic walls, through which blood flows from the heart to the other body parts. artificial insemination The injection of male semen into a female of that species; often used by animal breeders. artificial pollination Fertile stigmas are dusted with pollen from selected plants, to give desired characteristics; usually done by hand. ATP (adenosine triphosphate) Molecules of ATP provide energy for immediate use by the cell; produced during glycolysis and cellular respiration. atria (sing. atrium) Chambers of heart into which blood returns from the body or the lungs before passing into the ventricles. autotroph ‘An organism which uses simple inorganic materials from the environment for its nutrition (e.g. green plant). bacteria A type of procaryote with murein in the cell walls. B cells Lymphocytes that produce large quantities of antibodies when stimulated by a particular antigen; control the humoral (blood) immune response. behavioural adaptation Animals or plants behave in some way which improves their chance of survival and reproduction. behavioural isolation Isolation between populations of a species that results from a difference in the behaviour, such as a difference in courtship behaviour. binomial system The standard system for naming species, in which each name consists of two parts. The first is the name of the genus, and the second is the name of the particular species. For example, Eucalyptus regnans. biodiversity The variety of all the living things on earth. biogeography The study of the distribution of organisms. biomass pyramid The biomass of plants eaten is much grater than the biomass of animals it produces, for each level in a food chain, causing a pyramid shape. biomass Amount of new growth (plant and animal tissue) that accumulates in an ecosystem; usually measured as mass per unit area (g/m2) or the equivalent amount of chemical energy bound in the mass of tissue (kJ/m2). biotechnology Deliberately changing living organisms at a molecular level, to produce more useful products.

Glossary 391

biotic Relating to the biological parts of the environment, as opposed to the abiotic (physical) parts. blood Specialised fluid, often containing cells, that is circulated to provide internal transport in animals. buds Regions of potential growth, containing meristematic cells, in leaf axils. They may not develop. cambium Region of rapidly dividing cells (vascular cambium and cork cambium) which produces secondary growth in woody plants. cancer Malignant tumour resulting from uncontrolled cell division of abnormal cells. capillarity The ability of water to rise within a narrow tube, such as xylem vessels, without an input of energy. capillary Tiny blood vessel, with wall only one cell thick and across which exchange occurs between blood and tissues. capture–recapture A method of estimating the size of a population of animals, by capturing some, tagging them, and then capturing another sample. The proportion of tagged animals in the second sample can be used to estimate the total population size. carnivore Animal that catches live prey for food; also called a predator. carpels Inside the female reproductive system (the pistil); each carpel is made up of a stigma, a style and an ovary. catalyst A substance that increases the rate of a reaction, without being consumed in the reaction; includes enzymes. cell membrane Phospholipid layer that encloses the contents of a cell and controls the movement of substances into and out of the cell. cell sap The fluid inside a plant cell vacuole. It is mostly water, with dissolved sugars, salts and sometimes pigments. cell wall Cellulose wall outside the cell membrane of plant cells. cells The smallest structural and organisational units of which all living things are built. cellulose Complex carbohydrate molecule that is very strong; forms plant cell walls. centrioles A pair of organelles found in animal cells; consist of hollow cylinders of fibres used in spindle formation in mitosis. centromere Part of the chromosome that attaches to the spindle during cell division, and where the two chromatids of a double-stranded chromosome are joined. chenopod An Australian family of salt-tolerant plants: saltbushes, bluebushes and glassworts. chloroplast Green organelle containing chlorophyll, present in some plant cells, in which photosynthesis takes place. Composed of many folded layers of membrane.

392 Glossary

chromatid One of the two daughter strands of a replicated chromosome which are joined by a single centromere; separates and becomes a daughter chromosome. chromosomes Darkly staining structures in the nucleus that are composed largely of DNA and which carry the hereditary information of the cell in the form of genes. Found in constant numbers in body cells of a particular species. cilia The hair-like structures on the surfaces of some eucaryotic cells, consisting of a ‘9 + 2’ arrangement of microtubules enclosed by an extension of the cell membrane. Cilia move with an oar-like motion and are usually shorter and more numerous than flagella. classification Grouping organisms on the basis of features they have in common and naming these groups in a hierarchical system of kingdoms, phyla or divisions, classes, orders, families, genera and species. See also taxonomy. cleavage Multiple mitotic cell divisions of the zygote which divide it into many smaller cells, each of which contains a portion of the egg cytoplasm. clones New individuals that arise from ordinary body cells of an organism and therefore carry identical genetic information to the parent organism. closed circulatory system A circulation system in which a specialised fluid carrying nutrients (such as blood) is circulated through the body in a closed system of vessels. codominant alleles Alleles from different homozygous parents that are both expressed in heterozygous offspring, producing a third phenotype. coenzyme Very small molecule that combines with an enzyme and is necessary for its activity. commensalism A relationship between two organisms which benefits one but does not harm the other, e.g. epiphytes growing on rainforest trees. community All the living organisms in a habitat; the living part of an ecosystem. companion cells In flowering plants, the small cells that occur next to sieve-tube cells in phloem, and arising from the same parent cell. comparative anatomy Study of the differences and similarities in structure between different organisms. comparative embryology The study of embryos of different species, looking for similarities and differences between them, to discover common ancestry. competition The struggle between organisms for an environmental resource that is in limited supply, e.g. water. condensation A reaction in which two organic molecules combine to form a larger molecule and a smaller molecule (often water). convergent evolution The process of natural selection over many generations which results in similar adaptations in species which live in similar environments.

creation myths Explanations of the origins of the Universe and everything in it, by ancient cultures. creationism The belief that the Earth, and everything on it, was created by a being, rather than evolving gradually from the coalescence of space debris. crossing over The exchange of chromosomal material between members of a chromosome pair during meiosis. cross-pollination The transfer of pollen from the stamen of one flower to the stigma of another flower. cuticle A waterproof layer secreted by the epidermis. cyanobacteria Microscopic single-celled procaryotic organisms containing chlorophyll; found in wet and damp situations. cytokines A group of plant hormones that, in the presence of auxin, stimulate the division of plant cells. cytokinesis The division of cytoplasm during mitosis or meiosis, as distinct from the division of the nucleus. cytoplasm Fluid content of a cell, made up mostly of water; includes ions, enzymes, food molecules and organelles other than the nucleus. daughter cells The two new cells created when a cell divides by mitosis. deamination The conversion in the liver of excess amino acids to urea by the removal of ‘amino’ groups (NH2). decomposer Bacteria and fungi that consume and break down dead plants and animals and their waste products (organic matter) into soluble organic molecules (such as sugars) and eventually into inorganic nutrients (e.g. phosphorus, carbon dioxide). denaturation Irreversible change in protein structure, usually as a result of heating above a certain critical temperature. differentiation The process of change from an unspecialised cell to a specialised cell. diffusion The passive movement of molecules from where they are more concentrated to where they are less concentrated. digestion The breakdown of food into a form that can be used by an organism for metabolism; involves mechanical digestion and chemical digestion. diploid cells Cells containing two of each type of chromosome found in a species (2n); e.g. the diploid number in humans is 46. diploid Containing the full set of chromosome pairs, as in body cells. disaccharides Simple carbohydrates which are double units of sugar. distribution The area or range of locations in which a species can be found.

divergent evolution The evolution in a species from its original form to a variety of new forms or species that are adapted to different environments or ways of life. DNA (deoxyribonucleic acid) Molecule that is the carrier of genetic information in the cell; found in chromosomes. dominant (1) In ecology, an individual having greater access to resources than other individuals in the group. (2) In genetics, a gene that is expressed equally in the heterozygous or homozygous state. dominant phenotype The set of observable characteristics that is dominant in a population, resulting from the expression of one or more dominant genes. ductless glands Part of the body’s endocrine system; they secrete hormones. ecology The study of the interrelationships between living things and between living things and their environment. ecosystem System formed by organisms interacting with one another and their physical environment. ectothermic In animals, having a body temperature that is more or less determined by the temperature of the surrounding environment. effector A muscle or gland which responds to a stimulus. electron microscope A microscope in which a beam of electrons is used to form an image of an object. embryo A developing plant or animal formed from a fertilised egg. enantiostasis A balance of metabolic and physiological functions in the body, in response to variations in the environment (e.g. human blood usually stays at the same temperature, whether the air temperature is freezing or 40˚C. endocrine system System of internal control in animals that involves the release of specific chemicals, called hormones, into the blood. endocytosis Active transport of large molecules across a membrane using temporary vacuoles (pouches) formed by the membrane. endoplasmic reticulum Layers of intracellular membranes; may be rough endoplasmic reticulum (associated with ribosomes) or smooth endoplasmic reticulum (lacking ribosomes). endothermic In animals, having a relatively constant body temperature that is usually higher than the temperature of the surrounding environment. environment Non-living and living surroundings of an organism. enzyme Protein molecule that acts as a biological catalyst. Enzymes usually speed up the rates of reactions that would otherwise have taken place much more slowly. Their action is specific: they catalyse only one type of reaction. epidemiology The study of diseases in populations.

Glossary 393

epidermis Outermost layer of cells in plants and animals. erythrocyte Red blood cell. estuary Region of a river or creek close to its mouth, where saline water from the sea mixes with fresh water from upstream. eubacteria Procaryotic cells sensitive to common antibiotics, with murein in cell wall and unbranched lipids in cell membrane. eucaryote An organism with cells containing Golgi apparatus and mitochondria, and in which the nucleus is surrounded by a membrane. evolution Change over time. In living organisms this occurs through natural selection. excretion The removal of the waste products of metabolism. extant organism a living organism with a long fossil history, e.g. the Wollemi pine. external fertilisation Fertilisation that takes place outside the body, in the external environment. facilitated diffusion A type of passive diffusion across cell membranes involving carrier proteins. feedback system A system in which the response alters the stimulus; for example, the control of hormone levels in the body, in which an increase in the level of the hormone in the blood decreases the output by the gland. fertilisation Penetration of egg by sperm and fusion of the egg and sperm nuclei. fibres Long narrow plant cells, with thick lignified cell walls but no cytoplasm or nucleus, that provide strength and support. filament Stalk of the stamen which bears the anther. filtration The passive removal of wastes from blood and other bodily fluids by passage through a selectively permeable membrane. flagellum (pl. flagella) Relatively long, fine, motile organelle; often only one or two per cell. In eucaryotic cells composed of a ‘9 + 2’ arrangement of microtubules enclosed by an extension of the cell membrane. In procaryotic cells, composed of three protein fibrils. flower In many plants, the structure that contains the reproductive organs. food vacuoles Granular reserves of food (starch, oils, etc.) in the cytoplasm of cells. food web A series of interacting food chains link up to form a food web. fruit The mature fertilised ovary, containing the seeds. fungi (sing. fungus) Heterotrophic organisms with eucaryotic cells. The cells have cell walls but never contain chlorophyll, e.g. mushrooms, mould, yeast. They help bring about the decay of organic matter. gamete A haploid sex cell; egg or sperm. genes Parts of DNA molecules that contain the instructions to make proteins. Particular genes