Unit 4 Summary Notes A2 Biology
Abiotic factors: the non-living/physical components of the environment (temperature, light, soil pH) Light intensity: affects plants only Carbon dioxide concentration: affects plant populations only Mineral ions: affects plants only Water availability: affects both plants and animals Temperature: affects both plants and animals Abundance: counting the number of organisms in the sample. Usually the abundance of each species is recorded. If we divide abundance by size of the sampling area we get the density (number/m2) Autotroph: an organism that can trap an inorganic carbon source using energy from light or chemicals Biomes: parts of the atmosphere that have very different environmental conditions to each other. Biosphere: the parts of the earth that support life. Then organisms of the biosphere depend on one another and the earth’s physical environment which consists of the…. Biotic Factors: A living factor that affects a population or a process (predation, competition, parasitism, disease) Carrying capacity: The highest population that can be maintained for an indefinite period of time by a particular environment
Detritus: dead or decaying matter Ecology: the study of interrelationships between organisms and their environment. The environment includes both abiotic and biotic factors Ecosystems: An ecosystem is a self-supporting system made up of all the interacting biotic and abiotic features in a specific area. Ecological niche: the position an organism fills in its environment, comprising its habitat, the resources it uses and the time at which it occurs there Environmental resistance: conditions that reduce the growth rate of a population Food webs: a diagram showing all the feeding relationships in a single ecosystem or community Gross primary production: the rate at which chemical energy is stored in plants Habitat: the place where an organism is found Inorganic fertiliser: a fertiliser containing inorganic ions such as, nitrate, ammonium, potassium and phosphate ions. Intraspecific competition: between members of the same species
Climax community: the final community in succession Community: all the populations of different species that live and interact together in the same area at the same time Competitive exclusion principle: when two species are competing for limited resources the one using the resources most effectively will eliminate the other. Two species can’t occupy same niche indefinitely when resources are limiting Consumers: an organism that obtains energy by eating other living things
Interspecific competition: between members of different species Limiting factor: the one factor of many that affect a process, that is nearest its lowest value and hence is rate-limiting. Microhabitats: an area within a habitat that has specific conditions Net primary production: the energy that remains after the energy used in respiration has been subtracted from the gross primary production
Decomposers: live in the soil (generally) and feed on detritus, dead, decaying organic matter. There are two groups, the detritivores and the saprobionts/saprophytes.
Organic fertiliser: a fertiliser containing organic substances such as, urea.
Detritivores: organisms that feed on dead or decaying organic matter
Omnivores: animals that regularly feed at both primary and higher trophic level.
Ecosystems: an area within which the organisms interact with each other and their physical environment
Pioneers species: species which are first to colonise cleared or disturbed ground.
Primary succession: succession that occurs on previously uninhabited ground Population: a group of organisms of the same species that live together in the same area at the same time Producers: an organism that uses solar energy in photosynthesis to produce carbohydrates Pyramid of numbers: A diagram that shows the number of organisms at each trophic level in an ecosystem/food chain at a given moment irrespective of size. Pyramid of biomass: A diagram that shows the total biomass at each trophic level in an ecosystem/food chain, at a given moment, irrespective of the numbers Pyramid of energy: A diagram that shows the energy transferred to each trophic level of an ecosystem/food chain in a period of time irrespective of the numbers and biomass. Richness: number of different species found in the sample Saprophytes/saprobionts: microorganisms (fungi and bacteria0 that feed through extracellular digestion, secreting enzymes onto organic matter and absorbing the soluble products into their body to use in respiration (releases carbon dioxide to the environment again for use in photosynthesis) or to use in assimilation building new cells (biomass) Secondary succession: succession that occurs on in a place where there was some vegetation already present and the area has been disturbed by natural disaster or by deforestation etc. Succession: the process by which a community changes over time, a directional process where organisms affect the environment making it less suitable for themselves and more suitable for the next dominating species. Food chains: A very simple diagram showing how energy flows through an ecosystem Trophic level: the position in a food chain at which an organism feeds
Populations Describe how the mark-release-recapture method could be used to estimate the population of mice in the area being studied. Method of trapping (long worth traps) trap a large sample to be representative Method of marking; (in an unobtrusive way, does not make them more vulnerable to predation or less likely to reintegrate, shave a little piece of fur form underneath) Release and wait a period of time to allow reintegration into population Number marked are counted then released Number recaptured are counted Number of marked animals/in second sample are recorded; Correct method of calculating results; P=
N 1 xN 2 N 2 (M )
Ground beetles are large black insects. The mark-release-recapture method can be used to estimate the ground beetle population on a roundabout. Describe how. 1 Sample of ground beetles captured and counted (a); 2 Released and second sample captured; 3 Count total number of beetles (B) and number marked (b); 4 Total population (A) estimated from the relationship
a b ; A B
5 Detail of method e.g. pitfall trap/marking with tippex; 6 Refinement to ensure greater accuracy e.g. large number/marking in position such that does not affect survival; Give two conditions necessary for results from mark-release-recapture investigations to be valid. (2) No immigration/migration (Ignore references to emigration); No reproduction (Ignore references to death); Idea of mixing; Marking does not influence behaviour / increase vulnerability to predation; Sample/population large enough; Give two assumptions that must be made when using this technique. Mixes randomly/completely in population; marking does not have an effect/does not wear off; no migration/emigration/immigration; no change in population size between samples/life span longer than time between release and recapture; no births or deaths; not trap-happy/trap-shy; Describe the techniques you would use to obtain reliable data in order to compare the sizes of the populations of the snail in the muddy area and in the area covered by vegetation. Use of large numbers of quadrats in each area (if number stated then 10+); random sampling method (e.g. grid + random numbers)/systematic sampling method (allow regular sampling along a transect); counting.
Describe how you would use quadrats in an investigation to determine whether or not there is a difference in the number of clover plants in two large areas of equal size. Large (and equal) number of quadrats in each area; (reject several) random sampling method, described; (accept described ‘systematic’ method) percentage cover/point hits per quadrat/count plants; mean/average value for each area; statistics test to see if differences significant. Describe how you would use a half meter by half meter quadrat frame and a 30-metre tape measure to produce a transect along a stream to investigate the distribution of species growing there Use of tape measure to produce transect placing of quadrats transect placed across stream score presence of each plant species use quadrat at regular intervals along tape Repeat transect several times ( 3) Along stream At random or regular intervals
Explain what limits the size of populations in a climax community. 1. Named nutrient availability; 2. Numbers of producers providing energy (for a food chain); 3. Light intensity affecting the rate of photosynthesis; 4. Disease killing (weaker) members of species; 5. Space for nest building / niches; 6. Reproductive rate balancing death rate; 7. Competition for a named limited resource; 8. (Intra and interspecific) competition explained; 9. Predation described; Using one example of each to illustrate your answer, explain the difference between density dependent and density independent factors. Example of density dependent factor (factors whose effct change as population size changes), e.g. food, space, disease; Example of density independent factor, e.g. light, temperature (reject: weather, climate); Density dependent factors depend on / affected by size of population; Density independent factors affect organisms whatever the population size, or, examples used to explain, e.g. increasing competition for food Suggest how predation by weasels acts as a density-dependent factor controlling great tit population size. (4) At low densities / high distance between nests few are killed by weasels; so more great tits survive; Great tit population increases; so greater percentage taken by weasels;
or At high densities / low distance between nests more are killed by weasels; so fewer great tits survive; Great tit population decreases; so smaller percentage taken by weasels;
ATP Give three uses of energy from ATP in a liver cell. Active transport; Phagocytosis; Synthesis of glycogen; Protein / enzyme; DNA / RNA; Lipid / cholesterol; Urea in glycolysis; Bile production; Cell division; ATP is useful in many biological processes. Explain why.(5) 1. Releases energy in small / manageable amounts; 2. (Broken down) in a one-step / single bond broken; 3. Immediate energy compound/makes energy available rapidly; 4. Phosphorylates/adds phosphate; 5. Makes (phosphorylated substances) more reactive / lowers activation energy; 6. Reformed/made again; ATP is sometimes described as an immediate source of energy. Explain why. (Energy release) only involves a single reaction/one-step/ (energy released) in ATP ADP (+Pi)/ energy transfer direct to reaction requiring energy; Explain why humans make more than their body mass of ATP each day 1. ATP is unstable; 2. ATP cannot be stored / is an immediate source of energy; 3. Named process uses ATP ; 4. ATP only releases a small amount of energy at a time;
Photosynthesis Describe how the leaf is adapted to allow this process to occur effectively.(5) Large surface area to collect solar energy; transparent nature of cuticle to allow light penetration; position of chlorophyll to trap light; stomata to allow exchange of gases; thin / max. surface area to volume ratio for diffusion of gases; spongy mesophyll / air spaces for carbon dioxide store; xylem for input of water; phloem for removal of end products; Describe how the structure of a chloroplast is adapted to its function in photosynthesis.(5) Membranes / (disc) shape provides large surface for light absorption; layering of membrane allows a lot of pigment; (permeable) membrane allows diffusion of gases / carbon dioxide; membranes provide surface for attachment of electron / hydrogenacceptors; stroma / matrix containing enzymes for Calvin cycle /light– independent reactions; Contains chlorophyll / pigments for light absorption; Different pigments to absorb different wavelengths; Stacking / arrangement of grana/thylakoids maximises light catchment; Stroma contains enzymes for photosynthesis; Outer membrane keeps enzymes in chloroplast; Starch grains / lipid droplets store products of photosynthesis; Ribosomes / DNA for enzyme/protein synthesis; Shape of chloroplast gives large surface area for CO2, absorption. Describe how plants absorb light energy from the sun and use this energy to produce useful substances in the light-dependent stage of photosynthesis. (5) Light absorbed by/strikes,chlorophyll/photosystem/PSI/PSII; electrons excited; pass down chain of carriers; energy released/transferred; producing ATP from ADP and phosphate; reduced NADP/formed with electrons; photolysis of water /allow light splits water; + (water) supplies protons/H ions to reduce NADP;
Describe the light-independent reactions of photosynthesis and explain how they allow the continued synthesis of hexose sugars.(6)
1. 2. 3. 4. 5. 6. 7.
5C/RuBP combines with CO2; to form 3C compound / TP / GP; using ATP; and reduced NADP / eq; 2 molecules of 3C compound/ TP / GP form hexose; all RuBP is regenerated; 10 molecules of 3C/TP/GP form 6 molecules of 5C/RuBP;
Describe how the products of the lightdependent stage of photosynthesis are used in the Calvin cycle and how carbohydrate is synthesised as a result of the cycle. (6) RuBP converted to GP; RuBP as carbon dioxide acceptor/combines with carbon dioxide; GP converted to triose phosphate/TP/GALP; this reaction is a reduction; reduced NADP provides hydrogen; ATP provides energy; some triose phosphate/TP/GALP converted to glucose/carbohydrate; some triose phosphate used to produce RuBP ATP supplies phosphate for this reaction;
Explain the roles of water, light and ribulose bisphosphate in the process of photosynthesis. (6) water: provides hydrogen; to reduce NADP; provides electron; to stabilise / reduce chlorophyll; light : excites / oxidises / removes an electron from chlorophyll / photosystem; photophosphorylation / ATP produced; electron used in reduction of NADP; Ribulose bisphosphate: carbon dioxide acceptor; forms GP; Explain why an increase in temperature will increase the rate of photosynthesis. Enzymes are involved; extra kinetic energy / molecules move faster; molecules collide more often / more enzyme - substrate complexes formed; increased rate of diffusion of raw materials; The carbon dioxide concentration was monitored at ground level in the centre of a small roundabout. The measurements were made on a summer day. Describe and explain how you would expect the concentration of carbon dioxide to fluctuate over the period of 24 hours. 1 Higher carbon dioxide concentration at night/during darkness; 2 Photosynthesis only takes place during light; 3 Photosynthesis removes carbon dioxide and respiration adds carbon dioxide; 4 Respiration taking place throughout 24 hours; 5 Quantitative considerations such as that in plants overall photosynthetic rate greater than respiration rate; 6 Human effects such as additional carbon dioxide from heavy daytime traffic/street lighting could prolong photosynthesis;
Respiration Describe how ATP is made in mitochondria.(5) 1. Substrate level phosphorylation / ATP produced in Krebs cycle; 2. Krebs cycle/link reaction produces reduced coenzyme/reduced NAD/reduced FAD; 3. Electrons released from reduced /coenzymes/ NAD/FAD; 4. (Electrons) pass along carriers/through electron transport chain/through series of redox reactions; 5. Energy released; 6. ADP/ADP + Pi; 7. Protons move into intermembrane space; 8. ATP synthase; In the presence of oxygen, respiration yields more ATP per molecule of glucose than it does in the absence of oxygen. Explain why. (3) Oxygen as terminal hydrogen/electron acceptor; Operation of electron transport chain/ oxidative phosphorylation; Fate of pyruvate; Krebs cycle; Significance of ATP formed in glycolysis; Describe how oxidation takes place in glycolysis and in the Krebs cycle. (3) Removal of hydrogen/dehydrogenation; by enzymes/dehydrogenases; H accepted by NAD/reduced NAD formed; in Krebs cycle, FAD (used as well); Describe the roles of the coenzymes and carrier proteins in the synthesis of ATP. NAD/FAD reduced / hydrogen attached to NAD/FAD; + H ions/electrons transferred from coenzyme to coenzyme/carrier to carrier / series of redox reactions; energy made available as electrons passed on; energy used to synthesise ATP from ADP and phosphate / using ATPase; + H / protons passed into intermembrane space; + H / protons flow back through stalked particles/enzyme;
Explain why oxygen is needed for the production of ATP on the cristae of the mitochondrion. ATP formed as electrons pass along transport chain; oxygen is terminal electron acceptor / accepts electrons from electron transport chain; electrons cannot be passed along electron transport chain if no O2 to accept them; + forms H2O / accepts H from reduced NAD/FAD / oxidises reduced NAD/FAD; Mitochondria in muscle cells have more cristae than mitochondria in skin cells. Explain the advantage of mitochondria in muscle cells having more cristae. (2) (more cristae / larger surface area) for electron transport chain / more enzymes for ATP production/oxidative phosphorylation; muscle cells use more ATP (than skin cells)(not just more respiration); Human skeletal muscle can respire both aerobically and anaerobically. Describe what happens to pyruvate in anaerobic conditions and explain why anaerobic respiration is advantageous to human skeletal muscle. Any four from: Forms lactate; [extras – C2H5OH / CO2 – CANCEL] Use of reduced NAD / NADH;
P y r u v a te N A D H
L a c ta te N A D
= 3 m a rk s
Regenerates NAD; NAD can be re-used to oxidise more respiratory substrate / correct e.g./ allows glycolysis to continue; Can still release energy/form ATP when oxygen in short supply/when no oxygen; Describe the effect of lactate production on muscles. (2) Decrease in acidity / pH; Increase in acidity / pH; Muscle fatigue; Denaturation / alteration of proteins / enzymes; Give two ways in which anaerobic respiration of glucose in yeast is (i) Similar to anaerobic respiration of glucose in a muscle cell;(2) ATP formed/used; pyruvate formed/reduced; NAD/reduced NAD; glycolysis involved/two stage process; (ii) different from anaerobic respiration of glucose in a muscle cell.(2) ethanol/alcohol formed by yeast, lactate (allow lactic acid) by muscle cell; CO2 released by yeast but not by muscle cell; Describe the similarities between photosynthesis and respiration. (6) Both processes involve: Transfer of energy/conversion of energy from one form to another; Use and produce ATP; chain of electron carriers; located on membranes; detail of process (eg ref to chemiosmotic theory); involve cycle of reactions; oxidation and reduction/redox reactions involved; and coenzymes; processes are controlled by enzymes; some common intermediates/GALP is common to both;
Energy flow and farming to maximise productivity Explain how energy is transferred into biomass by producers. Light / solar energy used for photosynthesis; Synthesis of materials used in growth / storage; Chemical energy stored / energy in biomass; Suggest two reasons why not all of the solar energy can be used in photosynthesis. Light missed plant / leaf/ chloroplast / reflected; wrong wavelength of light / inefficiency of photosynthesis /other limiting factors Some light reflected/ not absorbed/refracted (if qualified) back into atmosphere; some light misses chloroplasts/chlorophyll; only certain wavelengths of light used (in photosynthesis); Energy lost as heat/by respiration/metabolic processes; qualified comment on the inefficiency of photosynthesis e.g. 25% efficient/energy lost as electrons passed on; carbon dioxide/temperature limiting; Explain why a food chain rarely contains more than four trophic levels. Energy losses (at each trophic level) / energy use; In named process – e.g. excretion / egestion / movement / respiration / … / as heat; (NOT ‘growth’ – CANCEL, ignore ‘waste’) Not available / (too) little left to sustain higher trophic levels /to be passed on; Give two ways in which energy is lost between trophic levels (2) energy is lost in respiration; (small amount is) lost as heat; lost to decomposers/lost in excretion/leaf fall/death and decay; part of oak tree not eaten/not digested; Explain why only a small percentage of the energy in the heather biomass is transferred to the biomass in the next trophic level. (3) Only a proportion of heather eaten/not all plants eaten/energy lost indecay; not all food eaten is digested/energy lost in faeces; heat/energy lost due to respiration;
Explain, in terms of energy, why food chains with the fewest steps are most efficient. Energy lost at each trophic level/step; due to respiration/heat loss/other valid reason; fewest steps means least energy loss Explain how the intensive rearing of domestic livestock increases net productivity. 1 Slaughtered when still growing/before maturity/while young so more energy transferred to biomass/tissue/production; 2 Fed on concentrate /controlled diet /controlled conditions/so higher proportion of (digested) food absorbed/lower proportion lost in faeces / valid reason for addition; 3 Movement restricted so less respiratory loss / less energy used; 4 Kept inside/heating/shelter / confined so less heat loss / no predators; 5 Genetically selected for high productivity; Explain how farming practices increase the productivity of agricultural crops. 1. Fertilisers/minerals/named ion (added to soil); 2. Role of named nutrient or element e.g. nitrate/nitrogen for proteins / phosphate/phosphorus for ATP/DNA; 3. Pesticides/biological control prevents damage/consumption of crop; 4. Pesticides/weed killers /herbicides/weeding remove competition; 5. Selective breeding / genetic modification (of crops); 6. Glass/greenhouses enhance temp/CO2/ light; 7. Ploughing aerates soil/improves drainage; 8. Ploughing/aeration allows nitrification/decreases denitrification; 9. Benefit of crop rotation in terms of soil nutrients/fertility/pest reduction; 10. Irrigation/watering to remove limiting factor; 11. Protection of crops from birds/pests/frost by covers/netting etc.; Explain two ways in which a shortage of nitrogen-containing compounds could limit plant growth. (max 2 marks for each consequence of shortage and its effect on growth) reduced/lack of/unable to synthesise protein/amino acids; lack of enzymes for metabolism / named metabolic process; reduced/lack of/unable to synthesise DNA/nucleic acids/organic bases; mitosis/cell division reduced; reduced NADP/ less chlorophyll; reduced photosynthesis; reduced levels / less NAD; reduced respiration; Explain why plants may fail to grow if high concentrations of nitrate are applied to the soil. Water potential of soil reduced/more negative/ reduced water potential gradient; less water moves into roots/water moves out of roots by osmosis; Fertilisers are added to soils to replace the nutrients lost when crops are harvested. Give two advantages of using An organic fertiliser such as farmyard manure; (2) More micronutrients / greater range of nutrients; Nutrients released slowly; Improves soil quality / adds humus / adds microbes / improves soil structure; Improves water-holding capacity of soil / reduces leaching/eutrophication; Improves soil aeration; Already available; An inorganic fertiliser. (2) Known nutrient content; Nutrients available immediately/fast acting; Nutrients distributed evenly; Doesn’t contain pests; Better to handle / easy to use / easy to store/transport;
Concentrated in nutrients / needed in smaller amounts; Applied using light machinery so avoids soil compaction; Describe why phosphates are needed by a growing plant. (4) Production of phospholipids; in cell membranes; synthesis of ATP; production of DNA; production of RNA; production of NADP; Explain two advantages of growing cereal crops in rotation with clover instead of growing them every year in the same field and applying fertiliser. Clover is a natural/green fertiliser; adds organic material/humus to the soil; clover adds nitrogen compounds/nitrates; needed by crop for protein production; clover releases minerals slowly; less run-off/less pollution; clover cheaper than fertiliser; therefore more profitable/fertilizer applied several times; Describe and explain the effects of monoculture on the environment. Loss of hedgerows; since small fields impracticable for large machines; soil more exposed to wind; resultant increase in soil erosion (once); reduction in diversity; since smaller variety of niches/habitats; since smaller variety of producers/plants deeper rooted plants removed; resultant increased soil erosion (once); increased risk of large-scale crop failure/increased disease/increased number of pest; since large numbers of same crop species grown close to each other; increased use of fertilisers result in eutrophication/damage to soil structure; reduction of gene pool Explain how each of the following activities associated with modern farming might reduce the number of species of invertebrate animal. The use of herbicides Loss of food/habitat/shelter reduces numbers of invertebrates; and so less food for carnivorous invertebrates/effect further down the food chain described; Using large areas for the growth of single crops Fewer habitats; limited range of food sources; unstable ecosystem; Explain what is meant by ‘biological control’, and describe one example of how biological control has been used to control a specific pest. Using a predator / parasite / pathogen to control (the numbers of) a pest organism; name of control organism and pest; explanation of control method; Describe two features that a predator must have if it is to be a successful biological control agent. Only feeds on pest species/does not affect non-target population; can live in environment of the host/ establish/maintain its population/ can reproduce under conditions of use/active during the season; (ignore references to effect on crop)
Describe the principles involved in biological control. Control organism a parasite/ predator; specific to pest; population varies with population of pest; controls size of pest population but does not kill all; keeps pest population low enough to prevent significant (economic) damage Explain the advantages and disadvantages of using biological rather than chemical pests control Advantages (if well-screened) only attacks the pest; forms self-perpetuating population (only one application required); cheaper (qualified) e.g. saves cost of repeatedly using chemicals; safer because does not leave chemical residue; organisms do not become resistant to biological control; Disadvantages doesn’t completely eradicate pest; cost of researching / setting up a biological control system; biological control agent may become a nuisance itself/must be well screened; slower to get rid of pest than chemicals; more subject to environmental factors; Advantages: Specific to one pest/ chemicals may kill pollinators/useful insects Application linked to life cycle of pest; Number of applications depends on survival of control organism/ self sustaining; No residues harmful to health left on crop; Does not result in resistant varieties of pest; Disadvantages:. Can only be used for glasshouse crops; May create an imbalance in natural ecosystem; May be labour intensive/costly to maintain; Have to retain some of the pest to maintain the control organism; Explain the benefits of an integrated pest management scheme. If one method fails, other still partially effective; reduced amounts of pesticides needed; increased yield / less chance of resistant species developing /less effect on food webs; Chemical controls initial surges in pest numbers / less chemicals used; biological gives longer term control of pests; Explain one advantage of using a combination of chemical and biological approaches to pest control. Chemical controls initial surges in pest numbers / less chemicals used; biological gives longer term control of pests; Explain how the use of pesticides can result in resistant strains of insect pests. 1. Variation/variety in pest population; 2. Due to mutation; 3. Allele for resistance; 4. Reference to selection; 5. Pests with resistance (survive and) breed / differential reproductive success; 6. Increase in frequency of allele
Cultural control Practices that reduce pest problems without using chemicals or biological agents. Provide suitable habitats close to crop for natural predators of the pest Weeding: removal of weeds and diseased crops Crop rotation: breaks the life cycle of host specific pests Intercropping: planting two crops in the same field rye grass and wheat encourages ladybirds to feed on aphids on wheat. Tilling: ploughing to turn soil burying weeds and expose insects to predatory birds Insect barriers: sticky bands on fruit trees to catch crawling insects Beetle banks: strips of uncultivated land around and within fields. This allows invertebrates to thrive that may predate a pest. Regularly monitor the crops for early signs of pest problems
Integrated Pest Management (IPM) Brings together all forms of pest management, aim to reduce effect of pesticides on the environment without compromising the maximisation of crop yields. There are 4 stages 1. Identify pests and population density at which they cause economic harm (economic threshold) only act when population exceeds threshold 2. Use suitable cultural methods to avoid population reaching threshold 3. If population exceeds threshold use biological control to reduce it 4. If biological control fails to reduce population use chemical control at low and controlled levels and at times of year to minimise impact on the environment Evaluate the effectiveness of each stage before proceeding to next Benefits If one method fails others are still partially effective Reduced amount of pesticide needed Increase yield Reduced chances of resistant species developing Less impact on food webs Fewer chemicals used Long term effect rather than the initial improvement seen by chemical methods alone, but loss in effectiveness over time and the need to reapply chemicals
Cycles Describe how detritivores are involved in the recycling of nutrients. (Larger detritivores/named example) break up larger pieces (by feeding); Excrete nitrogenous wastes/faeces/droppings; Increases surface area available to bacteria/microorganisms/ fungi/decomposers; Decomposition by microorganisms releases minerals/nutrients to soil; Explain two ways in which the presence of detritivores may increase the activity of microbial decomposers. Break down larger pieces of dead organic matter; providing more surface for microbial activity; Add products of excretion More nutrients/nitrogen / higher nitrogen carbon ratio; Aeration by e.g. tunnelling; increases oxygen content for respiration of microorganisms What is the difference between the ways in which microbial decomposers and detritivores obtain their nutrients? Decomposers secrete enzymes / onto organic matter/ food/ extracellular breakdown; Detritivores ingest / eat/ take in organic matter/food first; Explain how microorganisms obtain the carbon compounds from cell walls. Secrete enzymes/cellulase/carbohydrase; extracellular digestion; absorption of soluble/digested products/sugars; Explain how the carbon in the dead insects is made available to the plant. Hydrolysis/breakdown/digestion of carbon compounds; respiration (by bacteria); releasing carbon dioxide; taken up by the plant during photosynthesis; Describe how the carbohydrates in the dead leaves in the beech wood would be recycled by the activity of detritivores and microorganisms. Detritivores break leaves into small pieces / increase surface area; Deposit faeces; Increases rate of microbial action; Bacterial fungi decompose / break down leaves or organic matter; Secretion of enzymes for digestion; Absorption of sugars; Respiration by detritivores/ microorganisms; Release of carbon dioxide; Carbon dioxide used in photosynthesis; Explain the role of bacteria in making carbon in dead plant remains available to plants. Decomposers/ saprotrophs; release enzymes and digest detritus/ substances found in detritus/ eq.; absorb products of digestion/ suitable e.g. that relates to nd candidates 2 point; respired and CO2 released; used by plants in photosynthesis/ enters leaves;
The carbon dioxide concentration was monitored at ground level in the centre of a small roundabout. The measurements were made on a summer day. Describe and explain how you would expect the concentration of carbon dioxide to fluctuate over the period of 24 hours. 1 Higher carbon dioxide concentration at night/during darkness; 2 Photosynthesis only takes place during light; 3 Photosynthesis removes carbon dioxide and respiration adds carbon dioxide; 4 Respiration taking place throughout 24 hours; 5 Quantitative considerations such as that in plants overall photosynthetic rate greater than respiration rate; 6 Human effects such as additional carbon dioxide from heavy daytime traffic/street lighting could prolong photosynthesis; The concentration of carbon dioxide in the air at different heights above the ground changes over a 24 hour period. Use your knowledge of photosynthesis to describe how and why these changes occur High CO2 at night No photosynthesis in absence of light In dark plants and other organism’s respire In light net uptake of CO2 as rate of photosynthesis is greater than rate of respiration in plants CO2 decreases with height above ground At lower levels there are less leaves and a lower light intensity so less photosynthesis, more animals respiring Explain how a reduction in the amount of ploughing would lead to more carbon being stored in the soil Less oxygen can enter the soil (from the air); For saprobionts / soil microorganisms / bacteria / fungi /decomposers / correctly named soil organisms; For use in aerobic respiration; Less breakdown of organic matter / humus / dead plants /dead animals / other e.g.;Less carbon dioxide released / formed; Clearing the forests and burning the vegetation affects the carbon dioxide concentration in the atmosphere. Describe how and explain why. 1. Carbon dioxide concentration increases; Clearing 2. No/Less vegetation so no/less photosynthesis / photosynthetic organisms; 3. No/Less carbon dioxide removed (from the atmosphere); Burning 4. Burning/combustion releases / produces carbon dioxide; Describe the part played by soil bacteria in making the nitrogen in compounds in the dead spruce seeds available to pine seedlings. Release ammonia / ammonium / ammonification; BY Decomposers / putrefying / saprotrophic / ammonifying bacteria; Ammonia nitrite nitrate / nitrification; BY Nitrifying bacteria / named bacteria; Nitrogen compounds in the detritus are broken down by bacteria to ammonium ions ( NH 4 ) . Describe how ammonium ions are converted into a form that can be readily absorbed by the producers. (Ammonium) nitrite; Nitrite nitrate; OR Ammonium nitrate; (1 mark only) If symbols: correct symbols e.g. ammonium ( nitrate (NO3) = NO MARKS By nitrifying bacteria / Nitrosomonas / Nitrobacter / nitrification; By oxidation / using oxygen / aerobic;
The processes which naturally form part of the nitrogen cycle can make nitrogen contained in urine and faeces available to crop plants. Describe how these processes occur (6) Organic compounds of nitrogen / named example; converted to ammonium compounds / ammonia; by saprophytes / saprobionts / decomposers / equivalent; to nitrites; to nitrates; by nitrifying bacteria / named bacteria; uptake by roots; Fertiliser, such as manure, contains ammonium compounds. Explain how soil bacteria and the use of manure improve crop yield. 1 Ammonium compounds from proteins / amino acids urea / N-containing; 2 Converted into nitrite; 3 Into nitrate; [Reject: Incorrect sequence once] 4 By nitrifying bacteria / correctly named; 5 Nitrogen-fixing bacteria; 6 Fix nitrogen from atmosphere / air; 7 Nitrate taken up by plants; 8 Nitrogen needed for protein synthesis / plant growth; Substances found in fallen leaves contain the elements carbon and nitrogen. Explain how the activities of decomposers and nitrifying bacteria recycle the substances in fallen leaves for re-use by the trees. (Decomposers):Secretaion/release of enzymes; [REJECT ‘excrete] Digest/hydrolyse organic matter; Absorption /’taken in’ – by named process e.g. diffusion/active transport; Respiration Release carbon dioxide; Carbon dioxide used in photosynthesis; Release ammonia/ammonium salts/ions/mineral salts/nutrients; (Nitrifying bacteria):Ammonia/ammonium to nitrate; Nitrate to nitrate;OR ammonia nitrate = 1mk Aerobic/use of oxygen/by oxidation; [ALLOW correct symbols] Nitrates/nitrites/ammonium used in synthesis of amino acids/protein/nucleic acids/other correct organic –N; Explain how environmental damage may arise from leaching of fertiliser.(5) Nitrate/phosphate enters into the surrounding rivers /ponds; possible eutrophication/ excessive plant growth/algal blooms; high phosphate causing blue - green blooms/ high nitrate giving blue - green blooms; excess plant growth exceeds supply of mineral salts; death and decay of plants by microorganisms/decay increases BOD; oxygen depletion causes death of fish/fresh water animals; Fertilisers may leach out of farmland into freshwater streams and lakes. Explain how this can be harmful to the environment. (6) 1. More growth of algae/ surface plants; 2. Blocks light; 3. Plants lower down unable to photosynthesise; 4. Less oxygen produced
5. 6. 7. 8. 9.
Dead (plant) material present; Broken down by bacteria/decomposers; Respiration; Depletes oxygen in water; Other organisms unable to live/grow;
Succession Explain succession and climax community Change in community over time; either due to change environmental/abiotic factors / change is due to species present; Stable community/no further succession/final community; Explain ecological succession. 1. Colonisation/pioneering; 2. Microscopic plants at start; 3. Death / decomposition; 4. Named change in environment e.g. increase in organic matter/ stabilisation; 5. New species colonise once there is a change; 6. Increase in number of species/diversity; 7. Increase in total amount of living material/biomass/ more niches; 8. Increase in nutrient availability; 9. Change from more extreme conditions / more stability; Describe what will happen to an area of land which is set aside and not returned to agriculture. (4) Colonisation by pioneer plants/colonisation by herbaceous plants/change in herbaceous community already present; colonisation by woody plants; reference to succession/climax community in correct context; specified change in the animal community; specified change in the soil structure/composition; Describe those features of a succession that would bring about an increase in the index of diversity. (3) Initial environment hostile / few organisms adapted; These organisms change the environment / suitable example; More niches / more habitats; Allowing other organisms to become established Under natural and suitable conditions, bare soil would eventually become covered by a woodland community. Explain how farming practices prevent this from happening. (4) e.g. crops are planted (not native plants); these compete with native plants; Ploughing returns to bare soil; destroys herbaceous plants/tree/shrub seedlings; Grazing by farm animals; destroys herbaceous/shrub seedlings/communities The species that are present change during succession. Explain why. 1. Species/plants/animals change the environment/conditions/add humus/nutrients etc.; 2. Less hostile (habitat); 3. Species/plants better competitors;
Conservation What is meant by conservation? Concept of preservation/maintenance – e.g. sustainable management/sustainable use of resources/management to maintain diversity/maintain forest; (Allow ref. To ‘keeping’ / ‘saving’ / ‘non-destruction’) Give two aims of biological conservation. To maintain diversity; to maintain organisms’ habitats/ecosystem; Explain two environmental problems that are normally associated with large-scale deforestation. Soil erosion / mud slides / flooding / leaching of minerals – trees no longer protect soil from rain / from wind / roots no longer hold soil; Increased CO2 (in air) OR “greenhouse effect” – trees remove CO2 / trees photosynthesise / burning releases CO2; Less diversity / loss of (forest) species / fewer individuals – loss of food / loss of habitat / niches / ecosystem; Changed rainfall patterns / drought – less transpiration from trees; Give three reasons why tropical rainforests should be conserved. To avoid: Any three from: Loss of species / decrease in diversity / loss of habitat / loss of niche / disruption of food chain; Loss of pharmaceuticals / ‘medicines’ / timber / ‘wood’; CO2 build-up in atmosphere / global warming / trees take in CO2 / trees = carbon sink (described) / to maintain CO2 in air; (NOT just ‘carbon’ in air) Leaching of ions / mud slides / flooding / desertification; [ALLOW converse of above – e.g. ‘Rainforest is a habitat for (various) species’] Explain the advantages of conserving a forest ecosystem. Trees available as a sustainable resource; Maintain habitats / niches / shelter; Maintain diversity / avoid loss of species / protect endangered species. Maintain stability (of ecosystem); Maintain food chains / webs / supply of food; Reduced loss of soil / erosion; Reduced flooding; Act as carbon sink / maintain O2and C02 balance reduce greenhouse effect Reduce global warming; Source of medicines; Describe ways in which the endemic species (those characteristic of a particular habitat) could be conserved and suggest reasons for protecting them from extinction. 1. Protection of habitat; 2. Legal measures, e.g. quotas, hunting bans; 3. Capture/culling of non-native species; 4. Captive breeding; 5. Surrogacy / artificial insemination / genetic manipulation techniques; 6. Ethical / aesthetic reasons for conservation / tourism; 7. Possible undiscovered benefits, e.g. crop plants, drug sources; 8. Maintaining genetic diversity for future breeding programmes; 9. Avoid damage to food webs / control local pests;
Selection and speciation Explain how natural selection produces changes within a species. variation between members of population/species; predation/disease/competition results in differential survival; some have adaptations that favour survival; survive to reproduce/have more offspring/ pass on their alleles/genes; produces changes in frequency of allele /gene pool/genotypes/phenotypes; Explain how resistance to an antibiotic could become widespread in a bacterial population following a gene mutation conferring resistance in just one bacterium. 1. Frequent use of antibiotic creates selection pressure/ antibiotic kills bacteria; 2. Bacteria with mutation/ resistance have (selective) advantage over others / described; 3. (Survive to) reproduce more than other types; 4. Pass on advantageous allele/ mutated allele in greater numbers; 5. Frequency of (advantageous) allele increases in subsequent generations; Explain how selection can result in an insect population which is resistant to a particular insecticide. Insecticide resistance already in population; (resulting) from mutation; resistant insects are not killed (by insecticide)/survive; (And are able to) reproduce/breed; passing on the relevant allele/gene to the next generation/offspring; resulting in increasing frequency of resistance allele in population Describe how stabilising selection will affect the mean and standard deviation. Give the reason for your answer. Mean – no change; Standard deviation – decreases; Reason – selects against/removes (both) extremes/extremes die/better survival of middle nos.; Explain what is meant by stabilising selection and describe the circumstances under which it takes place. 1. Occurs in an unchanging environment; 1 2. (Initial range of values in which) mean is best adapted; 3. Selection against extremes / selection for the mean; 4. Mean/median/mode unaltered 5. Range/S.D is reduced; 6. Repeated over many generations; 7. Increasing proportion of populations becomes well adapted to environment;
What is meant by reproductive isolation? (1) Organisms cannot interbreed/ breed or mate or reproduce with another group/ incompatible gametes/ wrong courtship behaviour/ other valid; Explain how geographical isolation can lead to the formation of new species.4) 1 Populations separated by physical barrier/ example; 2 No mixing of gene pools; 3 Different selection pressures; 4 Become adapted to local environment; 5 Survive and reproduce; 6 Mutation in one group (different from other group); 7 Change in allele frequencies; [Reject: Gene] 8 Isolated populations/ new species cannot interbreed; Suggest how speciation may be occurring in salamanders separated by a geographical barrier. (4) (Populations) isolated/in different areas; no interbreeding (between populations)/gene exchange/flow; variation in each (population); (accept example of variation) due to mutation/meiosis; (accept reference to types of mutation) each population adapting to its own/different environment; through natural selection; producing differential survival; producing changes in allele/phenotype frequencies; producing reproductive isolation; These two species are thought to have evolved as a result of sympatric speciation. Suggest how this might have occurred. (4) Original population living in one area / 2 species evolved in the area; Idea of genetic variability; Concept of reproductive isolation; Possible mechanism; Gene pools become increasingly different; Until interbreeding does not produce fertile offspring; Explain how two species of lemming evolved from the original species after the melting of a glacier created a dividing lake. geographical isolation of populations; variation present in population(s); different environmental conditions; different selection pressures/different phenotypes selected; change in genetic constitution of populations/gene pools/allele frequency; (two populations) so unable (to breed) to produce fertile offspring;
Genetics Genetics definitions: you may wish to add more to this Alleles: alternative forms of the same gene Autosome: a chromosome not involved in sex determination (human genome has 22 pair’s autosomes and 1 pair of sex chromosomes) Chromosomes: the self-replicating genetic structures of the cell containing the DNA Co-dominant alleles: alleles whose effects both show in the phenotype of a heterozygote (Remember incomplete dominance is where we get a blending of characteristics, breeding a red and white flower gives pink flowers, co-dominance both characteristics are expressed so we would get flowers with red and white spots) Dominant allele: an allele whose effect always shows in the phenotype when it is present/ allele expressed in the heterozygous genotype Gene pool: all the alleles of all the genes in a population of organism, which results in variation Genotype: the alleles of a gene (genetic constitution) an individual inherits Haploid: a nucleus with only a single set of chromosomes Heterosomes: chromosomes involved in sex determination which are different in appearance. In humans that Y chromosome determining male sex characteristics is much shorter than the X. Heterozygous: Possessing different alleles of genes at one or more loci on homologous chromosomes Heterogametic sex: the sex that produces gametes containing sex chromosomes of two types. Males produce gametes with either an X or Y in them Homogametic sex: the sex that produces gametes containing sex chromosomes of the same type. Female gametes all have X chromosomes Homologous chromosomes: a pair of chromosomes containing the same gene sequences each derived from one parent Homozygous: possessing the same alleles of genes at one or more loci on homologous chromosomes Locus: the position on a chromosome of a gene or other chromosome marker Multiple alleles: genes that have more than two different alleles Phenotype: the features of an individual that result from the expression of the genes and their interaction with the environment Recessive allele: an allele whose effects only show when there are no dominant alleles present. A recessive phenotype is always homozygous. Allele not expressed in the heterozygous genotype. Sex chromosomes: the X and Y chromosomes in human beings which determine the sex of an individual Sex linkage: genes, other than those that determine sexual features, which occupy a locus on one sex chromosome but not the other Test-cross: cross fertilisation carried out between an unknown genotype showing the dominant phenotype and an individual showing the recessive phenotype.
Monohybrid crosses: A simple breeding experiment involving one characteristic. You can see an individual’s phenotype, but not the genotype. If an organism shows the recessive trait (white flowers in the example) then they must be homozygous recessive If they show the dominant trait then they could be homozygous dominant or heterozygous. You can find out which by performing a test cross with a pure-breeding homozygous recessive. This gives two possible results:
If the offspring all show the dominant trait then the parent is likely be homozygous dominant. However, the parent could be heterozygous and that is pure chance that the dominant allele has been passed on in reproduction, so a large number of off-spring would help confirm the genotypes. If the offspring are a mixture of phenotypes in a 1:1 ratio, then the parent must be heterozygous. Sex determination Sex chromosomes are X and Y, these are not homologous and are called heterosomes. The other homologous chromosomes are called autosomes. Females are XX and are said to be homogametic Males are XY and are said to be heterogametic
Sex linked traits Any gene on the X or Y chromosomes is sex linked The X chromosome is larger and so for the most part there is no equivalent Y portion Males get the X chromosome form their mother and as such most sex linked conditions in males are inherited form the mother. If she herself does not display the characteristic then she is a carrier and will be heterozygous for the characteristic. Haemophilia, slow blood clotting, is a recessive disorder and is sex linked, colour blindness in humans is sex linked.
One useful way of tracing sex linked inheritance is through pedigree charts, in these….. Males are represented by squares A female is represented by a circle Shading means the presence of the character in the phenotype A dot in the circle means that a woman is a carrier, has the recessive and dominant allele You can often spot if a disease is sex linked when it is only occurring in males. And you can often spot when it is recessive when the parents don’t have the disease but the offspring do In humans it is the Y chromosome that actually determines sex: all embryos start developing as females, but if the sex-determining “SRY” gene on the Y chromosome is expressed, male hormones are produced in the embryo, causing the development of male characteristics. In the absence of male hormones, the embryo continues to develop as a female. The X chromosome is not
involved in sex determination. Dominance of alleles Incomplete dominance: the action of one allele is not completely masked by the other, thus the heterozygous offspring is an intermediate phenotype of the two homozygous phenotypes. So for example, red and white snapdragon flowers when cross bred give rise to a pink heterozygous intermediate. Snapdragon flowers: Red, White or the hybrid Pink CRCR, CWCW, CRCW this is incomplete dominance as the heterozygote is a blend
Codominance: both alleles contribute to the phenotype and are independently equally expressed. IN human Blood group AB both alleles A and B are equally expressed. Colour in cattle. Roan coloured ctalle have both the red and white allele expressed Cattle hair colour: Red (CRCR), White (CWCW) or Roan (CWCR) red and White hairs on the cattle meaning that this is co-dominance as both traits are expressed separately
Multiple alleles More than two alleles exist for a gene of which only two can be present at the loci of homologous chromosomes. Sometimes there may be more than three allele’s present and they are arranged in a hierarchy with each allele being dominant to those below it