IGCSE (Complete Biology) Chapter 1

1 1 Biology is the study of life and living organisms Objectives To understand that living things differ from non-living things To be able to list the characteristics of living things To understand that energy must be expended to maintain life

The dawn of life Scientists believe that the Earth was formed from an enormous cloud of gases about 5 billion years ago. Atmospheric conditions were harsh (there was no molecular oxygen, for example), the environment was very unstable, and conditions were unsuitable for life as we know it. Many scientists believe that the first and simplest living organisms appeared on Earth about 2.8 billion years ago. These organisms probably fed on molecules in a sort of ‘soup’ (called the primordial soup) which made up some of the shallow seas on the Earth at that time. A question that has always intrigued scientists, philosophers and religious leaders is: What distinguishes these first living organisms from the molecules in the primordial soup?

In other words, what is life?

Characteristics and classification of living organisms

Characteristics of living organisms

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You know that a horse is alive, but a steel girder is not. However, it is not always so obvious whether something is alive or not – is a dried-out seed or a virus particle living or non-living? To try to answer questions like this, biologists use a list of characteristics that living organisms show. Living organisms: Respire show Irritability (sensitivity to their environment) and movement Nourish themselves Grow and develop Excrete Reproduce. The opposite page gives more detail of the characteristics of life.

You may see other similar lists of these characteristics using slightly different words. You can remember this particular list using the word RINGER. It gives Ringer’s solution its name. This is a solution of ions and molecules that physiologists use to keep living tissues in – it keeps the cells alive. As well as the characteristics in the ‘ringer’ list, living things have a complex organisation that is not found in the non-living world. A snowflake or a crystal of quartz is an organised collection of identical molecules, but even the simplest living cell contains many different complex substances arranged in very specific structures. Living things also show variation – the offspring are often different from one another and from their parents. This is important in adaptation to the environment and in the process of evolution.

How the characteristics of life depend on each other Each of the characteristics of life is linked to the others – for example, organisms can only grow if they are nourished. As they take nourishment from their environment, they may also produce waste materials which they must then excrete. To respond to the environment they must organise their cells and tissues to carry out actions. Because of the random nature of reproduction, they are likely to show variation from generation to generation.

Depending on energy The organisation in living things and their ability to carry out their life processes depends on a supply of energy. Many biologists today define life as a set of processes that result from the organisation of matter and which depend on the expenditure of energy. In this book we shall see: how energy is liberated from food molecules and trapped in a usable form how molecules are organised into the structures of living organisms how living organisms use energy to drive their life processes.

R espiration is the process by which living cells release energy from organic molecules. The form of respiration that releases the most energy uses oxygen. Many organisms have gaseous exchange systems that supply their cells with oxygen from their environment.

I rritability (or sensitivity) is the ability to detect changes in the environment either inside or outside the organism, and to respond to them. These responses often involve movement.

N utrition supplies an organism with the food it needs for respiration, growth, repair and reproduction. Plants make their foods using the process of photosynthesis, whilst animals obtain their foods ‘ready-made’ by eating them. G rowth and development are the processes by which an organism changes in size and in form. For example, as a young animal increases in size (as it grows), the relative sizes of its body parts change (it develops).

R eproduction is the generation of offspring – new individuals. An organism may simply split into two, or reproduction may be a more complex process involving fertilisation. Reproduction makes new organisms of the same species as the parents. This depends on a set of chemical plans (the genetic information). contained within each living organism.

1 Approximately how many years passed between

the formation of the Earth and the appearance of the first living organisms? 2 What sort of molecules do you think might have

been present in the primordial soup? 3 RINGER is a word that helps people remember

the characteristics of living organisms. Think of your own word to help you remember these characteristics. 4 Suggest two ways in which reproduction is

essential to living organisms.

Characteristics and classification of living organisms

E xcretion removes the waste products of processes such as respiration and nutrition from the organism’s body.

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1 2 The variety of life Objectives put all living organisms into categories. The science of placing organisms into categories on the basis of their observable characteristics is called classification. There are so many different types of living organism (i.e. an enormous variety of life) that the study of these organisms would be impossible without an ordered way of describing them.

To appreciate why classification is necessary To understand the use of a key To be able to name the five kingdoms, and describe their distinguishing characteristics To understand the hierarchy of classification To know why a binomial system of nomenclature is valuable

The need to classify living things

Classification keys

Variation and natural selection lead to evolution. Evolution, and the isolation of populations, leads to the development of new species (see page 220). Each species has different characteristics, and some of these characteristics can be inherited by successive generations of this species. Observing these inherited characteristics allows scientists to

Taxonomists (people who study classification) place organisms into groups by asking questions about their characteristics, such as ‘Does the organism photosynthesise?’ or ‘Does the organism contain many cells?’. A series of questions like this is called a classification key. Examples of such keys are shown below.

This kind of key, with only two answers to each question (in this case, YES or NO), is called a dichotomous key (‘dichotomous’ means branching). It can be written as a branching or spider key, using the same questions:

All living organisms

1 Does the organism have cells with a definite nucleus? NO

YES

The five kingdoms Bacteria Protoctistans Plants Fungi Animals

2 Is the organism made up of many different cells?

Characteristics and classification of living organisms

NO

It is a bacterium

YES 3 Is the organism a heterotroph?

It is a protoctistan

NO

YES

4 Does the organism have cell walls and does it feed by external digestion? It is a plant YES

NO

It is a fungus 1

Does the organism have cells with a definite nucleus?

2

Is the organism made up of many different cells?

3

Is the organism a heterotroph?

4

Does the organism have cell walls and does it feed by external digestion?

It is an animal YES NO YES NO YES NO YES NO

A key may be used to place an organism in one of the five kingdoms.

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Go to question 2 It is a bacterium Go to question 3 It is a protoctistan Go to question 4 It is a plant It is a fungus It is an animal

Branching keys are easy to use, but take up a lot of space when fully drawn out. For this reason the listed form of a dichotomous key is usually used for identification of organisms outside the laboratory.

Five Kingdoms Using the key opposite, it is possible to place any living organism into one of five very large groups. These groups, distinguished from one another by major and obvious characteristics, are called the five Kingdoms. Each of these kingdoms contains an enormous number of different species, and keys can be used within a kingdom to place any individual species into further groups. The diagram below shows the names of these groups, and how the lion is classified within the Animal Kingdom. The sequence of kingdom, phylum, class, order, family, genus and species is called a hierarchy of classification. Notice that each classification group is given a name. Lions belong to the class Mammalia and the order Carnivora, for example. The final two group names are written in italics – this is a worldwide

convention amongst scientists. The lion is called simba in Swahili, león in Spanish and leu in Romanian but is known as Panthera leo to scientists in each of these countries. This convention of giving organisms a two-part name made up of their genus and species was introduced by the Swedish biologist Carolus Linnaeus. He gave every organism known to science a two-part name based entirely on the body structure of the organism. This binomial system of nomenclature is still in use today (binomial = ‘two name’). New species today may be given names based on characteristics such as chromosome number or gene sequence, which Linnaeus knew nothing about. The pages that follow describe the characteristics that distinguish living organisms in some of the most important phyla and classes. Kingdom (Animalia)

Phylum (Chordata) Class (Mammalia)

More similarities and fewer differences between the members

Order (Carnivora) Family (Felidae)

Genus (Panthera) Species Panthera leo

Other classes

Other orders

Other phyla

All animals are ingestive heterotrophs. All chordates have a notochord (becomes the backbone). All mammals have fur and mammary glands. All carnivores have well developed carnassial (flesh-cutting) teeth.

Other families

Other genera

All Felidae have retractable claws.

Other species

All Panthera (big cats) can roar but cannot purr. All lions can mate and produce fertile offspring with other lions. The English scientist John Ray used this feature to define the term ‘species’.

The hierarchical classification of the lion

1 The scientific names for the weasel and mink are

Mustela nivalis and Mustela vison, respectively. Both of these animals belong to the order Carnivora, as do the fox (Vulpes vulpes) and otter (Lutra lutra). The otter, mink and weasel all belong to the family Mustelidae. a Which feature must they have in common to belong to the order Carnivora?

b Which two animals are most closely related? c Which animal is the most different from the other three?

d Suggest one feature that places all of these organisms in the Animal Kingdom. 2 The scientific name for the human is Homo sapiens.

Try to find out the meaning of this name.

Characteristics and classification of living organisms

On moving down the hierarchy of groups, note that there are ...

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1 3 Bacteria and viruses Objectives To know the structure of a typical bacterial cell and a typical virus To know the requirements for bacterial growth To know how bacteria reproduce To understand some of the ways in which bacteria and viruses affect human activities To understand why viruses do not fit into the five kingdoms of living organisms

Staphylococcus (causes boils)

Pneumococcus (pneumonia)

Bacilli (singular bacillus) are rodshaped bacteria

Bacterial structure Bacteria (singular: bacterium) are single-celled organisms that have no true nucleus. Bacterial cells do not contain organelles like those found in typical animal and plant cells (see page 23), but are able to carry out all of their life processes without them. A few can photosynthesise, but most feed off other organisms. They may be parasites, feeding off living organisms, or saprotrophs, feeding off dead organisms. Bacteria are very small, usually about 1–2 ␮m in length, and so are only visible using a highpowered microscope. The structure of a typical bacterium is shown in the diagram below.

Characteristics and classification of living organisms

Bacteria exist in a number of different shapes, some of which are shown below. Shape can be used to classify bacteria. Glycogen grains may be a food store Cytoplasm contains enzymes and food molecules but no organelles.

Slime coat is present in some species.

0.1 ␮m Cell wall does not contain cellulose.

Cell membrane

Flagella – may be one (flagellum) or several, which beat to move bacterium

Main DNA strand is not contained inside a nucleus.

Plasmids are small rings of DNA, carrying only a few genes.

Bacteria have a cell wall but do not have a nucleus or organelles.

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Cocci (singular coccus) are spherical bacteria

Lactobacillus (sours milk)

Salmonella typhi (typhoid)

Spirilli (singular spirillum) are spiral bacteria Treponema (syphilis)

Vibrio (cholera)

Bacterial shapes.

Requirements of bacteria Bacteria have certain requirements that their environment must provide. An understanding of these requirements (see page 272) has been important in biotechnology and in the control of disease. If the environment supplies these needs, the bacteria can multiply rapidly by binary fission (see page 164). In this process each bacterium divides into two, then each of the two divides again and so on, until very large populations are built up. A bacterial colony can quickly dominate its environment, making great demands on food and oxygen and perhaps producing large quantities of waste materials.

The generation time (time taken for each cell to divide into two) can be as little as 20 minutes under ideal conditions. One E. coli cell in the human gut could theoretically become 272 cells in 24 hours – this number of cells weighs about 8000 kg!

The importance of bacteria Bacteria are important to humans in many ways. Some are pathogenic – they cause disease (see page 258). All pathogenic bacteria are parasites. Some are involved in nutrient cycles (see pages 240–3). Some are exploited by humans in food production and in biotechnology (see page 48). Bacteria are probably the organisms that carry out the largest number of different activities, and are the most numerous organisms on Earth. There may be as 3 many as 5000 undiscovered bacterial species in 1 m of woodland soil, and more bacteria live on or in your body than all of the humans that ever existed!

Virus and bacteria compared Covered by Cell membrane Cytoplasm Genetic material Living or non-living?

Virus

Bacteria

Protein coat No No DNA or RNA – only a few genes Non-living unless in host

Cell wall Yes Yes DNA – enough for several hundred genes Living

Viruses and bacteria also differ in the method of nutrition: all viruses are parasitic, but some bacteria feed as parasites, some as photosynthesisers and some feed like animals (as heterotrophs).

Coat of spiral protein

Viruses When the five-kingdom system of classification was devised, no one was able to find a place for the group of organisms called the viruses. This is because viruses do not show the typical features of living things – respiration, nutrition and reproduction, for example – unless they are inside the cells of another living organism. In other words, all viruses are parasites and therefore cause harm to their host. Some taxonomists have suggested that viruses belong in a sixth kingdom. There is great variation in the structure of viruses, but they all have certain common features. The structure of a typical virus is shown below.

RNA

cut-away view TMV (tobacco mosaic virus) infects the leaves of tobacco plants.

RNA Influenza virus, cut-away view Protein outer coat

Most viruses cause disease – they may infect humans, domestic animals or plants. 1 Look at the diagram of bacterial shapes. Make a

dichotomous key that would enable a biologist to distinguish the different bacterial types on the basis of their structure. 2 Make a table to show which of the following

structures are present in:

a an animal cell b a plant cell c a bacterial cell.

Protein coat – protects the single strand of nucleic acid. Made of many subunits.

Write ⫹ if the structure is present and ⫺ if it is absent. Cell wall, slime capsule, cell membrane, nucleus, chloroplast, mitochondrion, DNA, cytoplasm 3 Why is it difficult to classify viruses into one of

the five kingdoms of living organisms? 4 If ten bacteria landed on a bowl of soup, and Spikes – may contact and recognise the cell to be infected.

A typical virus has genetic material and a protein coat, but cannot carry out its life processes. It has no cytoplasm.

reproduced every 30 minutes, how many would be present after 5 hours?

Characteristics and classification of living organisms

Genetic material – may be DNA or RNA, but always a single strand.

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1 4 Fungi Objectives To know the structure of a fungus To understand the methods of nutrition used by fungi To appreciate the impact of fungi on the lives of humans

Spores Sporangium

Vertical hypha

Fungal cells have a common structure The fungi are a very large group of organisms. They range in size from single-celled yeasts to enormous fungi whose underground parts may occupy an area greater than a football or hockey field.

Hyphae of the mycelium

Spores are dispersed when sporangium bursts open.

Fungal cells have a cell wall made of a mixture of substances including chitin. The cytoplasm contains many organelles, since the fungus manufactures digestive enzymes. It feeds by saprotrophic (‘deadfeeding’) nutrition, as illustrated below.

Reproduction in fungi Single-celled yeasts reproduce asexually by binary fission (see page 164), but all other fungi reproduce by the production of spores, as shown above right.

Spore lands on food source – it can germinate and a new mycelium can develop.

Characteristics and classification of living organisms

Spore formation in a pin mould fungus

Cross wall

Hypha The mycelium is made up of branching hyphae, and penetrates throughout the food source. The food may be the remains of an animal or plant.

Saprotrophic nutrition involves external digestion by enzymes.

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Vacuole

One section of the hypha may contain several nuclei Cell wall Cytoplasm may release digestive enzymes

Insoluble Soluble compounds Digestive products of in food enzymes digestion source absorbed by fungus by diffusion or active transport

Requirements of fungi

Single cell of a fungus such as yeast

Fungi have very similar requirements to those of bacteria, that is: a moist environment, so that they can absorb the soluble products of digestion of their food source in solution

Cell wall containing chitin.

a warm environment, so that enzymes can work at their optimum temperature

Cell membrane Cytoplasm, containing organelles and many vacuoles with digestive enzymes

a nutrient source to provide the raw materials and energy required for growth Fungi do not require light since they do not rely on photosynthesis for the production of food compounds. This means that fungi are rarely found in light environments, since such environments are usually too warm and dry for fungal growth.

Parasitic fungi Fungi may also feed by parasitic methods. These fungi produce digestive enzymes, but only once they have killed the host and can no longer obtain soluble foods directly from its tissues. An example of a parasitic fungus is shown right.

Nucleus

Yeast is a single-celled fungus.

Parasitic fungus Feeding mycelium penetrates to vascular bundles of the host tree.

The importance of fungi Fungi have a number of effects on the lives of humans, for example: They are decomposers, and play a vital role in nutrient cycles (see page 240). Their decomposing action may destroy materials. Wooden buildings, in particular, are at risk from wet and dry rot.

Only the reproductive hyphae are visible on the outside of the tree clumped together to form a bracket.

Fungi may be agents of disease, as in athlete’s foot for example. They may themselves be a source of food, for example mushrooms. Fungi are used in biotechnology – the brewing and baking industries (see page 50) are entirely dependent on the activities of yeast, for example.

The best of both worlds! A bracket fungus eventually weakens and kills its host. It then feeds as a saprotroph on the dead remains of the tree.

1 Suggest why toadstools are likely to be found in dark areas of woodland where green plants are very limited. 2 Some fungi are saprotrophs, and secrete enzymes onto their food source. Suggest three such enzymes and name

their substrates and products. 3 List, with examples, four ways in which fungi affect the lives of humans.

Characteristics and classification of living organisms

Mould fungi consume food which might otherwise be eaten by humans.

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1 5 Plants Objectives To recall that all plants are autotrophs, and are able to absorb light energy to drive photosynthesis To understand some of the steps in the adaptation of plants to life on dry land To recall the characteristics of the four main plant groups

Plants are autotrophs

Movement Chlorophyll present Method of nutrition

Plant

Yes Yes, but not made of cellulose No No As saprotroph or parasite

Yes Yes, made of cellulose No Yes By photosynthesis

to absorb water from cold soil). They do not rely on insects for pollination (flying insects are uncommon in cold areas) – conifers reproduce using structures called cones.

Adaptations to life on land

Angiosperms

The first plants lived in water, but as living organisms evolved, plant forms developed that could live on land. The classification of plants into groups follows this sequence of evolution.

The angiosperms or flowering plants are the most successful of plants – they have evolved into many species and have colonised almost every available habitat. More than 80% of all plants are angiosperms (plants with enclosed seeds). Many features of the lives of flowering plants are covered elsewhere in this book (see pages 60–73, 86–93, 160 and 186–199, for example). The diagram at the top of the opposite page summarises these features, and emphasises the adaptations of flowering plants to a successful life on land, including warmer habitats.

Seed plants

Characteristics and classification of living organisms

Nucleus in cell Cell wall

Fungus

As autotrophs, plants manufacture food molecules from simple, inorganic sources by the process of photosynthesis using light as a source of energy. Plants all contain the light-absorbing pigment chlorophyll (or similar molecules which perform the same function) inside cells which have a definite cellulose cell wall.

The Plant Kingdom may be divided into four main groups (phyla): algae, mosses, ferns and seed plants.

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Fungus or plant?

Algae, mosses and ferns all depend to some extent on a moist environment, especially for the transfer of gametes. The seed plants have proper roots, stems, leaves, vascular tissues and waterproof coverings, and their gametes can be transferred without a film of moisture. As a result the seed plants are able to live and reproduce in dry environments. The two groups of plants that dominate our fields, woods and gardens are the conifers (cone bearers) and the angiosperms (flowering plants).

Conifers The conifers dominate the landscape in colder climates where flowering plants cannot compete. Conifers have needle-like leaves with a very thick cuticle which prevents water loss (it is very difficult

Two groups of angiosperms There are two major subgroups within the angiosperms. In one group there is a single cotyledon in the seed (see page 195) – these are the monocotyledons. In the other group there are two cotyledons – these are the dicotyledons. There are other differences between monocotyledons and dicotyledons, as shown in the diagram on the next page.

Angiosperm adaptations Growth, particularly of reproductive structures, can be very rapid. For example, pollination, pollen tube formation and fertilisation may take only one hour (compared with one year in some conifers), and some bamboos can grow 1m per day. The ovary protects the ovules and developing embryo, particularly from drying out. (‘Angiosperm’ means ‘enclosed seed’)

Flowers – the colour, pattern, shape, scent or nectar of the flower may attract insects, birds or mammals. These animals transfer pollen from male to female flower parts much more efficiently than wind or water.

Endosperm is a store of nutrients for the developing plant embryo. It does not develop until after fertilisation so that food stores are not wasted, as they might be in conifers.

Large leaf surface allows high rate of photosynthesis to supply energy for growth and fruit production. However, water losses by evaporation and diffusion through stomata are high.

Fruits are formed from ripened ovaries. Their specialised shapes, colours, smells and textures aid seed dispersal by wind, water and animals.

Stomata with guard cells regulate loss of water vapour and exchange of oxygen and carbon dioxide between plant and atmosphere

Xylem vessels have no end walls and conduct water efficiently.

Waterproof cuticle reduces water loss to atmosphere Vascular system transports water, ions and organic solutes

Roots form beneficial associations with other organisms. For example, legumes such as peas and beans form root nodules with nitrogenfixing Rhizobium bacteria. See page 242.

Specialised supporting tissues (air has a low density and does not offer support as water does) Extensive root systems anchor the shoot systems and absorb water and ions

Monocotyledons and dicotyledons – two groups of angiosperms (flowering plants)

Seed

Mature leaf

Seedling One seed leaf One cotyledon

Dicotyledon e.g. pea

Two cotyledons

Parallel veins

Two seed leaves

Branched veins

1 Name the four different plant groups. Why are algae

2 Seed plants are well adapted to live and to reproduce

confined to water, whilst ferns are well adapted to life on land?

in dry environments. What major adaptation allows reproduction on dry land?

Characteristics and classification of living organisms

Monocotyledon e.g. maize

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1 6 Invertebrate animals Objectives To know the difference between a vertebrate animal and an invertebrate animal To be able to describe the main characteristics of four invertebrate groups – annelids, nematodes, molluscs and arthropods To be able to distinguish between different classes of arthropods To understand the importance of metamorphosis in insects

Annelids Annelids such as the earthworm have a long segmented body and chaetae. Clitellum – used to bind two worms together during sexual reproduction.

Single segment Mouth can bite off pieces of fallen leaves

Anus

Vertebrates and invertebrates All animals share one characteristic – they feed on organic molecules (see page 76). Members of the Animal Kingdom can be divided into two large groups based on whether they have a backbone as part of a bony skeleton. Animals with a backbone are called vertebrates and those without a backbone are called invertebrates. Four groups of invertebrates are described here:

Nematodes Hookworms are nematodes with bodies that are specialised for feeding and reproducing; often they are parasites inside the gut of another animal.

Chaetae (bristles) are found on each segment – used during movement. Cross-section of worm, showing chaetae

Long, cylindrical body covered with a mucus layer helps to stop drying out

Molluscs Molluscs have a hard shell protecting a soft body with no limbs. Snails have a single, coiled shell.

Shell - hardened by calcium carbonate. Protects the soft body of the snail from predators and from drying out. The whole body can be withdrawn into the shell.

Characteristics and classification of living organisms

Eye on tentacle

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Mouth with sucking mouthparts for drinking blood

Has long muscles for wriggling movements

Mouthparts to scrape at vegetation Foot – muscle with a slimy covering so that snail can move by creeping over the surface.

Arachnids are arthropods with four pairs of legs and no wings.

Annelid, nematode or mollusc? Long, thin body so not washed out of host’s intestine

Body covered with mucus to resist attack by host's digestive juices

Body covering Segments visible Movement

Large sex organs to produce many sperm and eggs

Anus

Feeding method

Annelid

Nematode

Mollusc

Hard, slightly waterproof Yes

Soft, not waterproof No

Soft – shell helps to save water No

Uses chaetae (bristles) to move from place to place Herbivores

Wriggles but lives in one place

Creeps on foot from place to place

Mainly parasites

Mainly herbivores – some carnivores

Arthropods The arthropods are the most numerous of all animals, both in terms of the number of different species and the number of individuals in any one species. The insects are arthropods that show an interesting adaptation in their life cycle called metamorphosis that allows them to use the resources of their habitat to the maximum. Apart from insects, the arthropod phylum includes three other classes:

Crustacea Crabs are slightly unusual because many of their segments are tucked under their body. Hardened, serrated edge to claw: can hold onto slippery food, and break open shells of molluscs Jointed limbs: flexible Eyes: important sensors to help with feeding in aquatic environment

Many body segments: body is flexible for movement and wriggling around and under leaves and stones

The body is segmented, with the segments grouped into head thorax and abdomen. Compound eyes detect food and avoid predators Antennae: sense chemicals in the environment

Two pairs of wings for flying (rear set may be absent or not used for flying). The adult blowfly is highly adapted for locomotion (aids dispersal) and reproduction.

Mouthparts: very specialised for feeding e.g. piercing by mosquitoes Three pairs of legs, attached to the thorax. Allow walking movements, over food or to avoid predators.

Metamorphosis (means ‘change of body form’) allows different stages which: do not compete for the same food sources can be highly specialised for different functions. The larva is adapted for feeding and growth, and the adult for locomotion and reproduction.

Carapace: a thick, hardened shell for protection against predators

Arachnids

Myriapods Antennae: these are important sense organs in the millipedes' dark, damp environment. They can 'sense' chemicals and the level of moisture in the environment

e.g. housefly, mosquito (page 274)

Mouthparts: these are modified legs! Have hard edges for biting vegetation (millipedes) or other animals (centipedes) Hard exoskeleton: protection against predators Walking legs: have many joints and internal muscles, and allow movement over rough surfaces Body covering: waterproof, helping animal to live on land

Head and thorax are combined into one body part, the cephalothorax.

Powerful, piercing jaws – all spiders are predators

Simple eyes (but more than one pair) help to detect prey Four pairs of legs attached towards the rear of the cephalothorax. Allow chasing prey or holding onto web Abdomen Spinneret which produces long, thin strands of silk. (The word arachnid comes from the Greek goddess Arachne who was skilled at spinning.)

Spider silk is so strong and thin that it has been used to make the cross hairs in the telescopic sights of rifles.

1 Copy and complete the following paragraph.

All animals have one common characteristic – . The invertebrates are animals that do not have . Examples of important invertebrate groups are , which are very clearly segmented with bristles called to aid in movement; , which have a soft body inside one or two hard shells; and , which are the most numerous of all. 2 The arthropods include four classes – insects, arachnids, crustaceans and myriapods.

a List three features that all of these classes possess. b List three features that only insects possess. c Compare insects and spiders under the headings ‘Number of legs’, ‘Number of body sections’, Number of wings’ and ‘Type of eyes’. 3 Insects are the most abundant of all animals on land. Many of them show an adaptation called complete

metamorphosis. What does this term mean, and how does it help to explain why there are so many insect species?

Characteristics and classification of living organisms

Gills under shell: allow uptake of oxygen from water

Insects

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1 7 Vertebrate animals: five classes Objectives To know the characteristics of the vertebrates To understand how different classes of vertebrates show increasing adaptation to dry land To know the five classes of vertebrate, and to provide examples of each

If asked to name an animal, most people would probably name a mammal because these are the most familiar animals to us. Mammals are just one class of the phylum Chordata. The chordates are often called the vertebrates, although strictly speaking there are a few chordates that aren’t vertebrates. Vertebrates have a hard, usually bony, internal skeleton with a backbone. The backbone is made up of separate bones called vertebrae which allow these animals to move with great ease. There are five classes of vertebrates, which, like the members of the Plant Kingdom, show gradual adaptations to life on land. The classes are fish, amphibians, reptiles, birds and mammals.

Fish Scales covered in mucus help streamlining for swimming

Operculum covering gills: gills have a large surface area for gas exchange

Lateral line contains sense organs to detect vibration

Fins for movement and stability

Amphibians Nostrils leading to lungs which are used for gas exchange

Moist skin (also usedfor gas exchange)

Wide mouth as adult amphibians are all carnivorous

Four limbs, with hind limbs webbed: walking and swimming

Vertebrates All have a backbone

Characteristics and classification of living organisms

Do they have smooth skin?

YES

NO Fish, Reptiles, Birds, Mammals

Scales on skin? Amphibians

YES

Vertebrates can be separated into classes based on their skin!

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Fish, Reptiles

Birds, Mammals

Are scales dry?

Feathers on skin?

YES

Reptile

NO

YES

NO

Fish

Birds

NO

Mammals

Reptiles Dry, scaly skin limits water loss

Tail can be used for swimming, as in crocodiles Limbs efficient for crawling and climbing Mouth has teeth or bony ridges to aid feeding

The crocodile has the typical dry scaly skin of reptiles, and the eyes on the top of its head and its sharp, pointed teeth adapt it for catching prey in water.

Birds Forelimbs are modified as wings.

Nostril, leading to lungs which are the organs of gas exchange

The heron has typical bird features of feathers and a beak. It is well adapted to capture fish and frogs as it has large eyes to spot its prey, a long pointed beak to grab its prey and large feet for walking over soft, muddy ground.

Mammals are endothermic vertebrates that have the characteristics shown in the diagram below. Beak: very light since no bone or teeth

Feathers, vital for flight and for endothermy (they give excellent insulation against heat loss)

A wide range of adaptations has allowed mammals to colonise many habitats as diverse as the polar wastes and the Arabian desert.

Mammals Pinna on ear – can be moved for maximum efficiency in sound detection.

Whiskers are sensitive to touch and vibration.

Fur – body covering which allows endothermy as a means of keeping a constant body temperature. Mouth: contains teeth which allow mammals to use a wide variety of foods (page 42)

Penis – an organ of the male that enables efficient internal fertilisation.

Humans show the typical mammalian characteristics of hair, mammary glands and a diaphragm, for example. Humans, though, are unique amongst all animals in that the adaptations they show allow them to modify their environment so that it is suitable for human occupation. As a result humans have been able to live and work in many habitats – no animal has a wider range. Human adaptation has allowed advanced development of the brain, and of all the complex activities that the brain can coordinate. The human brain is extremely sensitive to changes in temperature. Human adaptations include many that are concerned with the fine regulation of blood temperature (see page 130). Other features that make humans very special mammals include an upright posture, freeing the hands for complex movements including the use of tools.

Characteristics and classification of living organisms

Humans are mammals Scales on legs: feet ideal for perching

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Questions on characteristics and classification

1 South Uist is a small island which provides one of the few remaining summer

habitats for a bird called the Corncrake (Crex crex). It lives in grass fields, which are cut in the late summer for hay, where it feeds on insects, worms and seeds. South Uist provides a good habitat because there are plenty of grass fields where the Corncrake can nest and there are few predators. However, a small mammal called the Hedgehog (Erinaceus europaeus) was released onto the island. The Hedgehog also has few natural predators and will feed on the eggs of Corncrakes, as well as on insects and worms. The number of Hedgehogs on South Uist has risen rapidly to 10 000 while Corncrakes are becoming endangered as their numbers worldwide are falling.

a State two features which birds and mammals have in common. b State two features which distinguish birds from mammals. (IGCSE 2004 [part])

2 Use the key to identify the five fish shown in the drawings. Write down the

letter of each fish and its name.

Characteristics and classification of living organisms

(SEG June 1992)

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SPIDERS e.g. tarantula

Characteristics and classification of living organisms

CRUSTACEANS e.g. crab

INSECTS e.g. mosquito

Two body segments (head– thorax, abdomen). Four pairs of legs. No wings

MOLLUSCS e.g. snail

ANNELIDS e.g. earthworm

Two pairs of antennae

7 Soft body with no limbs – covered by shell

4

Cells have no cell wall ANIMALS*

3

Made up of many cells

6 Segmented body with chaetae (bristles)

PROTOCTISTANS* e.g. Plasmodium

Cells have an obvious nucleus

Three body segments (head, thorax, abdomen). Three pairs of legs. Two pairs of wings. Antennae

5

Organism has hard exoskeleton, jointed limbs, segmented body ARTHROPODS

BACTERIA* e.g. Salmonella

Cells have no obvious nucleus

2

Made up of single cells

1

LIVING ORGANISMS

AMPHIBIANS e.g. frog

No covering on skin (smooth, moist skin)

8

REPTILE e.g. lizard

Dry skin

Organism has internal skeleton, non-segmented body VERTEBRATES

10

Feathers Beak Wings BIRD e.g. thrush

FISH e.g. stickleback

11

MAMMAL e.g. human

Fur/hair Mammary glands

SEED PLANTS e.g. oak tree

FERNS

No scales on skin

Seeds produced

14

Stem, leaves and roots

Spores produced

MOSSES

Moist (mucus-covered skin) Gills Fins

9

Skin covering

ALGAE

Stem, leaves but no roots

13

FUNGI* e.g. bread mould

Cells do not contain chlorophyll (so organism feeds by absorption)

Cells have an obvious cell wall 12

A key enables identification of an organism by observation of its characteristics. Close observation allows a series of questions (the branch points in this key) to be answered, eventually leading to the organism being studied.

Cells contain chlorophyll in chloroplasts (so organism feeds by photosynthesis) PLANTS*

No separate root, stem and leaves

Scales on skin

*the FIVE KINGDOMS

Keys and classification

3 a What are you? Follow the branch points at 1, 3, 4, 8, 9 and 11 to identify

b Name two additional characteristics of mammals. c In what way are humans special mammals?

yourself as a mammal.

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