A2 AQA Biology unit 4 revision cards

Revision cards for all of the topics covered in unit 4. I copied these out from the revision cards I made myself, which included diagrams not shown here. They're fairly important, so these are the diagrams I have that aren't here: population pyramids; demographic transition model; section of a leaf; mesophyll cell; chloroplast; Z scheme; Calvin cycle; respiration summary; glycolysis; link reaction and Krebs cycle; electron transport chain; carbon cycle; the greenhouse effect; nitrogen cycle; genetic diagrams; pedigree charts and selection graphs.

Also bear in mind that you need to know unit 1 and 2 pretty well for this exam, too. Other than that, I hope these cards are somewhat useful! Let me know if you find any mistakes.

  • Created by: Amy
  • Created on: 26-11-12 11:50

Populations and ecosystems

Ecology is the study of inter-relationships between organisms and their environment.

Abiotic factors - ecological factors that make up part of the non-biological environment of an organism e.g. temperature, pH, rainfall, humidity.

Biotic factors - ecological factors that make up part of the living environment of an organism, e.g. food availabilty, competition, predation.

Ecosystem - a more or less self-contained functional unit in ecology made up of all the interacting biotic and abiotic factors in a specific area.

Species - a group of similar organisms that can breed together to produce fertile offspring.

Population - all the individuals of the same species that occupy the same habitat at the same time.

Community - the organisms of all species that live in the same area.

Habitat - the place where an organism normally lives, which is characterised by physical conditions and the species of other organisms present.

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Populations and ecosystems 2

Ecological niche - all conditions and resources required for an organism to survive, reproduce and maintain a viable population.

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Investigating populations

Factors to consider when using quadrats:

  • Size of quadrat - larger species require larger quadrats.
  • Number of quadrats - results are more relaible when more quadrats are used.
  • Position of quadrats - random sampling must be used.

Random sampling:

  • Lay out two long tape measures at right angles along two sides of the study area.
  • Obtain co-ordinates from a table or computer.
  • Place a quadrat at the intersection of each pair of co-ordinates.
  • Record the species within it.

Systematic sampling:

If some form of transition in the community is taking place, then systematic sampling is better. This can be done with a transect.

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Investigating populations 2

Abundance - number of individuals of a species within a given space. Can be measured by frequency or percentage cover.


estimated population size = (total number of individuals in first sample x total number of individuals in second sample) / number of marked individuals recaptured

Assumptions of mark-release-recapture method:

  • Proportion of marked to unmarked in sample is the same as in the population.
  • Marked individuals distribute evenly in the population after release.
  • There is no immigration into/emigration out of the population.
  • There are few births/deaths in the population.
  • The mark is non-toxic and not visible to predators.
  • The mark is not lost or rubbed off during the investigation.
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Variation in population size

Population growth curves usually show:

  • Period of slow growth.
  • Period of rapid growth as they continue to reproduce.
  • Growth declines until it remains stable - the decline may be due to food supply limiting numbers, or increased predation.

Abiotic factors influencing the size of a population:

  • Temperature - Low = enzymes slow. High = enzymes denature.
  • Light - Increase = increase in photosynthesis.
  • pH - enzymes need opimum pH.
  • Water - Scarce = small, adpated populations.
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Symbiosis - term including a variety of close associations between species.

Mutualism - both species benefit from the association.

Commensalism - one species benefits from the association, the other is unaffected.

Amensalism - one species is harmed by the association, the other species is unaffected.

Exploitation - one species benefits from the association, the other species is harmed.

Antibiosis - one species produces a compound that inhibits the other species.

Competition - both species are harmed by the association. Animals compete for food, space and mates. Plants compete for light, water and nutrients.

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Competition 2

Intraspecific competition - between members of the same species.

Interspecific competition - between members of different species.

Intraspecific competition increases with population size.

Species with similar niche requirements may coexist through niche differentiation (developing different habitat/feeding preferences).

When two similarly adapted species without niche differentiation are brought into competition, one usually benefits at the expense of the other e.g. grey squirrel and red squirrel.

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Predator - feeds on other organisms.

Prey - fed on by other organisms.

Predators need to be able to move quickly, be camouflaged, and detect prey. Prey need to be camouflaged, have protective features, and have concealment behaviours.

Predators reduce the prey population. They compete more for remaining prey, reducing their population. Fewer prey are eaten, so the prey population increases. This means there is more prey, so the predator population increases.

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Human populations

Percentage population growth rate = population change during this period / population at the start of this period

Birth rate

Birth rate = (number of births per year / total population in the same year) x 1000

Factors affecting birth rate:

  • Economic conditions
  • Cultural/religious backgrounds
  • Social pressures/conditions
  • Birth control
  • Political factors
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Human populations 2

Death rate

Death rate = (number of deaths per year / total population in the same year) x 1000

Factors affecting death rate:

  • Age profile
  • Life expectancy at birth
  • Food supply
  • Safe drinking water/effective sanitation
  • Medical care
  • Natural disasters
  • War
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Energy and ATP

Energy - the ability to do work. Forms include: light, heat, sound, electrical, magnetic, mechanical, chemical and atomic. It can't be created or destroyed, but can be changed from one form to another. Measured in joules (J).

Energy used for:

  • Metabolism
  • Movement
  • Active transport
  • Maintenance, repair and the division of cells and organelles
  • Production of substances
  • Maintenance of body temperature

ATP has 3 phosphate groups with unstable bonds, that have a low activation energy. They are easily broken and release lots of energy when broken. Usually only phosphate removed:

ATP + H2O  ----> ADP + Pi + energy

(adenosine triphosphate) + (water) ----> (adenosine diphosphate) + (inorganic phosphate)

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Energy and ATP 2

This is a hydrolysis reaction.

Synthesis of ATP from ADP needs phosphate. Occurs in three ways:

  • Photophosphorylation - in chlorophyll-containing plant cells during photosynthesis
  • Oxidative phosphorylation - in mitochondria during electron transport
  • Substrate-level phosphorylation - when phosphate groups are transferred from donor molecules to ADP to make ATP e.g. in formation of pyruvate at the end of glycolysis.

In the first two processes, ATP is synthesised using the energy released by the transfer of electrons along a chain of electron carrier molecules in chloroplasts/mitochondria.

ATP is a good immediate energy source because it is rapidly re-formed from ADP and Pi. It is a bad long-term energy store because of its instable phosphate bonds. ATP is a better immediate energy source than glucose because:

  • ATP releases manageable quantities of energy - not too much.
  • The hydrolysis of ATP to ADP is a single reaction, so happens quickly.
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Energy and ATP 3

ATP must be continuously made by the mitochondria.

It is an energy source for:

  • Metabolic processes
  • Movement
  • Active transport
  • Secretion of cell products
  • Activation of molecules
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Overview of photosynthesis

Adaptations of leaves:

  • Large surface area collects lots of light.
  • Arrangement of leaves that prevents overlapping.
  • Thin - keeps diffusion pathway short.
  • Transparent cuticle and epidermis let light through to mesophyll cells beneath.
  • Long, narrow upper mesophyll cells packed with chloroplasts.
  • Numerous stomata for gas exchange.
  • Stomata that open and close in response to light changes.
  • Air spaces in the lower mesophyll layer to allow diffusion.
  • Network of xylem that brings water to leaf cells, and phloem that carries away sugars produced in photosynthesis.

6CO2 + 6H2O ----> C6H12O6 + 6O2

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Overview of photosynthesis 2

Structure of chloroplast:

  • Grana - stacks of discs called thylakoids where the light-dependent stage of photosynthesis takes place. They contain chlorophyll.
  • Inter-granal lamellae - tubular extensions that join up with thylakoids in adjacent grana.
  • Stroma - fluid-filled matrix where the light-independent stage takes place. Contain starch grains.
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The light-dependent reaction

Provides energy and hydrogen.

Photosynthetic pigment molecules are arranged in photosystems with accessory pigments and a primary pigment.

PSII - Larger. In membranes of grana. Primary pigment is chlorophyll a. Peak absorption 680nm.

PSI - Smaller. In stromal thylakoid membranes. Primary pigment is chlorophyll a. Peak absorption 700nm.

Acessory pigments = chlorophyll a and b, and carotenoids.

Photophosphorylation = ADP + Pi ----> ATP

Photolysis - light energy is used to split water molecules and release electrons, due to raised energy levels of electrons.

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The light-dependent reaction 2

  • Primary pigment excited and emits electrons.
  • High energy electrons received by electron acceptor molecule.
  • Electron Transport Chain.
  • PSI is final electron acceptor after ETC.
  • PSI excited and emits electrons, received by an electron acceptor.
  • Electrons pass down a chain of carriers to reduce NAD+. This reduction also requires 2H+ from the hydrolysis of water.
  • Non-cyclic photophosphorylation - can only continue if hydrolysis of water provides electrons to replace those lost from PSII. Produces ATP, reduced NADP and oxygen.
  • Cyclic photophosphorylation - only involves PSI. When water unavailable. Electrons cycle from PSI to acceptor and drop back to PSI via ETC. Provides ATP not NADP.
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The light independent reaction

Key molecules in the Calvin Cycle (light independent stage):

  • RuBP is ribulose 1,5-biphosphate [5C]
  • CO2 [1C]
  • GP is glycerate-3-phosphate [3C]
  • TP is triose phosphate [3C]
  • RuP is ribulose phosphate [5C]

Calvin cycle:

RuBP is the CO2 acceptor. The 5C molecule is carboxylated, catalysed by the enzyme RuBisCo. The molecule is split into 2 GP (3C molecule). Each GP is reduced using NADPH + H+ and energy from ATP ----> ADP + Pi. Triose phosphate is the main product. For every 6 TP produced, 5 are recycled back to RuBP and 1TP is used to make glucose.

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Factors affecting photosynthesis

At any given moment, the rate of a physiological process is limited by the factor that is at its least favourable value.

Light intensity:

Increases in light intensity cause an increase in the rate of photosynthesis. A point will be reached at which further light increases will have no effect. Light affects the light-dependent reaction.

CO2 concentration:

Affects enzyme activity. The higher the CO2 concentration, the higher the rate of photosynthesis.


Between 0-25C, the rate of photosynthesis roughly doubles for every 10C increase. Above this, photosynthesis declines due to enzyme denaturation. Temperature affects the light-independent reaction.

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The splitting of 6-carbon glucose molecule into two 3-carbon pyruvate molecules. The first stage of respiration.

Stage one:

Activation of glucose by phosphorylation.

Stage two:

Splitting of the phosphorylated glucose.

Stage three:

Oxidation of triose phosphate.

Stage four:

Production of ATP.

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Glycolysis 2

Energy yield from one glucose molecule through glycolysis:

  • Two molecules of ATP
  • Two molecules of reduced NAD
  • Two molecules of pyruvate
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The link reaction and Krebs cycle

Link reaction:

Pyruvate + NAD + CoA ----> acetyl coA + reduced NAD + CO2

Krebs cycle:

ATP produced as a result of substrate-level phosphorylation.

Products of link reaction and Krebs cycle from one pyruvate molecule:

  • Reduced coenzymes such as NAD and FAD
  • One molecule of ATP
  • Three molecules of CO2

Because there are two pyruvate molecules produced for each glucose molecule in Glycolysis, the actual products are double these quantities.

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The link reaction and Krebs cycle 2

Role of the Krebs cycle

  • Breaks down macromolecules into smaller ones.
  • Produces hydrogen atoms carried by NAD to ETC for oxidative phosphorylation. Leads to production of ATP, providing energy.
  • Regenerates 4-carbon molecule that combines with acetyl coenzyme A.
  • Source of intermediate compounds used by cells in the maufacture of other important substances.
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Electron Transport Chain

  • Takes place in the mitochondrial cristae.
  • H2 is oxidised to water using oxygen. Energy is released through ATP.

ATP is synthesised by ETC as follows:

  • Hydrogen atoms produced during glycolysis and Krebs cycle combine with NAD and FAD that are attached to cristae of mitochondria.
  • Reduced NAD and FAD donate electrons of H atoms to first molecule in ETC.
  • This releases protons from H atoms, which are actively transported across the inner mitochondrial membrane.
  • Electrons pass through chain of electron transport carrier molecules in a series of oxidation-reduction reactions. They lose energy as they pass down the chain, some of which is used to combine ADP and Pi to make ATP. The remaining energy is released as heat.
  • Protons accumulate in the space between two mitochondrial membranes, then diffuse back into the mitochondrial matrix through special protein channels.
  • End of chain - electrons combine with protons and oxygen (final electron acceptor) to form water.
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Anaerobic respiration

Under anaerobic conditions there is only glycolysis to yield ATP.

In plants:

pyruvate + reduced NAD ----> ethanol + carbon dioxide + NAD

The NAD is regenerated when reduced NAD is used to provide H atoms for reduction of ethanal to ethanol.

In animals:

pyruvate + reduced NAD ----> lactate + NAD

The NAD is regenerated when reduced NAD is used to provide H atoms for reduction of pyruvate to lactate.

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Food chains and food webs

Producers - organisms that maufacture organic substances using light energy, water and CO2.

Consumers - organisms that feed on other organisms.

Decomposers - break down dead producers and consumers, releasing minerals and elements to be absorbed by plants.

Producers --> Primary consumers --> Secondary consumers --> Tertiary consumers

Trophic level - the stage in a food chain.

Food chains are overly simplistic because animals don't generally rely on a single food source. In a single habitat, many food chains link together to form a food web. These are very complex.

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Energy transfer between trophic levels

Most of the Sun's energy isn't converted to organic matter because:

  • Most is reflected back into space.
  • Not all wavelengths of light are absorbed.
  • Light may not hit a chlorophyll molecule.
  • There might be limiting factors on photosynthesis.

Gross production - total quantity of energy plants in a community convert to organic matter.

Net production - rate at which plants in a community store energy.

Net production = gross production - respiratory losses

Not much energy is transferred at each stage because:

  • Some of the organism isn't eaten.
  • Some parts can't be digested.
  • Some energy is lost in excretory materials.
  • Energy is lost as heat from respiration.
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Energy transfer between trophic levels 2

Inefficiency of energy transfer explains why:

  • Most food chains have a maximum of 4/5 trophic levels.
  • Total mass of organisms in particular place is less at higher trophic levels.
  • Total energy stored is less at each level as one moves up in a food chain.

Energy transfer = (energy available after the transfer / energy available before the transfer) x 100

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Ecological pyramids


- Size/mass not taken into account.

- Can be difficult to represent all individuals accurately on the same scale.

Biomass - total mass in a particular place

+More reliable than numbers.

+ Fresh mass easy to assess.

- Fresh mass unreliable.

- Dry mass uses small samples.

- Only organisms present at fixed point in time are shown.

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Ecological pyramids 2


+ Most reliable

- Data collecting is difficult and complex

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Agricultural ecosystems

Made up of domesticated animals/plants to produce food for humans. Tries to reduce lost energy to increase productivity of human food chain.

Productivity - rate at which something is produced.

Natural ecosystem:

  • Solar energy only.
  • Lower productivity.
  • More species diversity.
  • More genetic diversity in species.
  • Nutrients recycled naturally.
  • Populations controlled naturally.
  • Natural climax community.
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Agricultural ecosystems 2

Agricultural ecosystem:

  • Solar energy plus food (labour) and fossil fuels.
  • Higher productivity.
  • Less species diversity.
  • Less genetic diversity in species.
  • Limited natural recycling, supplemented by artificial fertilisers.
  • Populations controlled naturally and artificially.
  • Artificial community prevented from reaching natural climax.
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Chemical and biological control of agricultural pe

Pest - competes with humans for food/space or pose a danger to health.

Pesticides - chemicals that kill pests.

Effective pesticides are:

  • Specific to the organism.
  • Biodegradeable.
  • Chemically stable.
  • Cost-effective.
  • Not accumulative.


  • Specific.
  • Reproduces itself.
  • Pests don't become resistant to it.
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Chemical and biological control of agricultural pe


  • Affects non-target species.
  • Must be reapplied at intervals.
  • Pests develop resistance.

Integrated pest-control systems:

  • Choose animal/plant varieties that are pest-resistant.
  • Provide suitable habitats for natural predators.
  • Monitoring for pests.
  • Remove pests mechanically.
  • Use biological agents if necessary.
  • Use pesticides as a last resort.
  • Decide acceptable level of pest rather than trying to kill them all.
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Intensive rearing of domestic livestock

- Animals kept in confined spaces:

  • Movement restricted, which reduces energy loss.
  • Environment kept warm.
  • Feeding controlled.
  • Predators excluded.

- Selective breeding for highest efficiency.

- Hormones to increase growth rate.

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Intensive rearing of domestic livestock 2

Features of intensive rearing:

  • Efficient energy conversion.
  • Cheaper.
  • Lower quality.
  • Less space used.
  • Safer.
  • Disease spreads quickly.
  • Overuse of drugs.
  • Animal welfare issues.
  • Pollution.
  • Reduced genetic diversity.
  • More fossil fuels used.
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The carbon cycle

Global level of CO2 has increased over the past few hundred years because:

  • Combustion of fossil fuels - has released CO2 locked within fuels.
  • Deforestation - has removed photosynthesising biomass, so less CO2 being removed from atmosphere.

If there is excess CO2 in atmosphere, some dissolves in oceans. The reverse happens when atmospheric levels are low. Aquatic photosynthetic organisms use this to form macromolecules making up their bodies. This passes along food chain, until decomposers break down dead animals, secreting enzymes onto them that break complex molecules down into soluble ones. The decomposers absorb these by diffusion. CO2 released during respiration by decomposer.

Organisms fossilised if decay is prevented. Carbon eventually returned to atmosphere as rocks are weathered.

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The greenhouse effect and global warming

CO2 most important greenhouse gas - remains in atmosphere for longest and there is more of it. Concentration increasing due to human activities.

Methane - another greenhouse gas. Produced when microorganisms break down organic molecules of organisms - inside intestines digesting food, or breaking down dead organisms.

Global warming - global temperature has increased. May be due to human activity, though this isn't certain. Expected to change temperature, precipitation, timing of seasons and frequency of extreme events. Climate change will affect niches available in a community. Affects compisition of communities. Some species may migrate to new areas and compete with native species, or adapt in their current locations.

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The greenhouse effect and global warming 2

If global warming continues:

  • Melting of polar ice caps could cause extinction of wild plants and animals and cause sea levels to rise, due to thermal expansion of oceans.
  • This could flood low-lying land. Salt water would extend further up rivers, making cultivation of crop plants difficult.
  • Higher temperatures and less rainfall lead to failure of crops. In some areas only xerophytes would survive. This would affect distribution of animals, and drought-resistant crops would have to be grown.
  • Greater rainfall and intense storms in some areas due to disturbance of climate patterns. Distribution of plants and animals would change.
  • Life cycles and populations of insect pests would alter. Tropical diseases could spread towards the poles in this way. Crop-damaging species would also spread this way.
  • Increased rainfall would fill reservoirs.
  • Warmer temperatures - crops could be grown where it is currently too cold. Rate of photosynthesis could increase, so harvests could be twice a year instead of once.
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The Nitrogen cycle

Organisms require nitrogen to manufacture proteins, nucleic acids and other nitrogen-containing compounds. Plants take up nitrate ions from soil by active transport. Nitrate ions are soluble and leach through soil beyond plant roots. Nitrate levels in natural ecosystems restored through recycling of nitrogen-containing compounds - in agricultural ecosystems, by the addition of fertilisers.

Ammonification - production of ammonia from organic ammonium-containing compounds e.g. urea, proteins, nucleic acids and vitamins. Decomposers feed on these and release ammonia, forming ammonium ions in soil.

Nitrification - conversion of ammonium ions to nitrate ions. Carried out by nitrifying bacteria. Oxidation reaction.

Bacteria need soil with lots of air spaces for oxygen. Farmers should keep soil light and well aerated by ploughing. Good drainage prevents spaces being filled with water.

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The Nitrogen cycle

Nitrogen fixation - nitrogen gas converted to nitrogen-containing compounds. Two main types of microorganism:

  • Free-living nitrogen-fixing bacteria: reduce nitrogen to ammonia, which they use to make amino acids. Nitrogen-rich compounds released when they die.
  • Mutualistic nitrogen-fixing bacteria: live on roots of plants. Obtain carbohydrates from plants, which obtain amino acids from the bacteria.

Denitrification - conversion of soil nitrates into gaseous nitrogen. These bacteria are anaerobic, so are present when soil becomes waterlogged. This reduces the availability of nitrogen-containing compounds for plants. Soil must be kept aerated to prevent the build-up of denitrifying bacteria.

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Use of natural and artificial fertilisers

In natural ecosystems, minerals removed from soil by plants are returned when dead plant is broken down.

In agricultural ecosystems, mineral ions must be added to the soil in the form of fertilisers, or they would become a limiting factor on growth. There are two types:

  • Natural (organic) fertilisers: dead and decaying plant and animal remains and animal waste.
  • Artificial (inorganic) fertilisers: rocks and deposits converted into different forms and blended together to give the appropriate balance of minerals.

Always contain nitrogen, phosphorus and potassium.

Combination of natural and artificial fertilisers give the greatest long-term increase in productivity. There is a certain point where further increases no longer result in increased productivity.

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Environmental consequences of using nitrogen ferti

Effects of nitrogen fertilisers:

  • Reduced species diversity - nitrogen-rich soil favours rapidly growing species (e.g. grasses and nettles).
  • Leaching - the process by which nutrients are removed from the soil. This can lead to pollution of watercourses.
  • Eutrophication - process by which nutrients build up in bodies of water. Caused by leaching. Nitrate concentration ceases to be a limiting factor for the growth of plants and algae. The upper layers of water become populated with algae, absorbing light. Light becomes a limiting factor for plant growth, so they die. Saprobiotic algae uses dead organisms as food. They need oxygen for respiration, which they use up. Nitrates are released from decaying organisms. Oxygen becomes a limiting factor, so aerobic organisms (e.g. fish) die. Anaerobic organism populations rise, decomposing more dead material. This releases more nitrate and toxic wastes (e.g. hydrogen sulphide), making water putrid.
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Describes changes that ecosystems undergo over time, in species occupying a particular area.

Colonisation: glacier retreating and depositing rock; sand piled into dunes by wind/sea; volcanoes depositing lava; lakes/ponds created by subsiding land; silt/mud deposited at river estuaries.

Features that suit pioneer species to colonisation:

  • Production of wind-dispersing seeds/spores.
  • Rapid germination of seeds.
  • Ability to photosynthesise.
  • Tolerance to extreme conditions.
  • Ability to fix nitrogen from atmosphere.

Climax community - stable state comprising balanced equilibrium of species with few species replacing those established. Many species flourish, with a dominant plant and animal species.

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Succession 2

Common features of succession:

  • Environment less hostile.
  • Increased biodiversity.
  • Increased biomass.
  • Greater number and variety of habitats.
  • More complex food webs.
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Conservation of habitats

Conservation - management of natural resources so that maximum use of them can be made in the future. Involves active intervention by humans to maintain ecosystems and biodiversity.

Reasons for conservation:

  • Ethical (respect for other species).
  • Economic (organisms have genes that can make substances).
  • Cultural/aesthetic (habitats/organisms enrich our lives).
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Studying inheritance

Genotype - genetic constitution of organism.

Phenotype - characteristics of organism.

Gene - section of DNA that determines characteristic of organism by coding for particular polypeptides.

Locus - position of gene on chromosome.

Allele - different form of a gene.

Homozygous - both alleles identical.

Heterozygous - alleles different.

Dominant allele - allele always expressed in phenotype of organism.

Recessive allele - allele only expressed in phenotype of organism in prescence of identical allele.

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Studying inheritance 2

Homozygous dominant - homozygous organism with 2 dominant alleles.

Homozygous recessive - homozygous organism with 2 recessive alleles.

Co-dominant - two alleles both contribute to the phenotype.

Multiple alleles - gene with more than two allelic forms.

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Monohybrid inheritance

The inheritance of a single gene e.g. pod colour of pea plants.

If pea plants with green pods are bred repeatedly for this characteristic, they are pure breeding for the character of green pods. They are homozygous for the gene.

If these are bred with pure-breeding yellow pod plants, the offspring have green pods because this allele is dominant. When these are crossed together, there's a ratio of three green to one yellow in the offspring.

Test cross - tests if an unknown genotype is homozygous dominant or heterozygous.

Mendel's laws

  • Each trait is determined by a specific element.
  • Each trait is inherited separately.
  • Each trait is determined by the intersection (one from each parent).
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Sex inheritance and sex linkage

Sex is determined by chromosomes. Females = **. Males = XY.

Haemophilia - a condition where blood doesn't clot normally. Caused by a recessive allele with altered DNA nucleotides that don't code for the right protein. Linked to X chromosome. If the only one males inherit is defective, there is no 'correct' allele to override it. Females can also inherit this faulty allele from their father or mother.

Pedigree charts - trace inheritance of sex-linked characteristics.

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Co-dominance and multiple alleles


Snapdragon plants can be pink by inheriting a red allele and a white allele.

Someone can have AB type blood if they inherit group A and group B.

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Allelic frequencies and population genetics

Gene pool - all alleles of all genes of all individuals in the population.

Allelic frequency - number of times an allele occurs in a gene pool.

Hardy-Weinberg principle

Calculates frequencies of alleles of gene in population.

The proportion of dominant and recessive alleles of a gene in the population stays the same in each generation provided that:

  • No mutations arise.
  • The population is isolated.
  • There is no selection.
  • The population is large.
  • Mating within the population is random.
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Differences between reproductive success of individuals affects allele frequency in populations.

Selection - process by which organisms that are better adapted survive and breed, while others fail to do so.

Types of selection:

  • Directional selection - favours individuals varying in one direction from the mean. This changes the characteristics of the population.
  • Stabilising selection - favours average individuals. Preserves the characteristics of the population.
  • Disruptive selection - favours two different characteristics. Causes two main phenotypes in the population.
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Evolution of new species from existing species.

There is generally a single gene pool within each population in a species. Two separated populations may experience different environments, so selection affects them in different ways and they evolve along separate lines.

Geographical isolation - when a physical barrier prevents two populations from breeding.


  • Species X occupies a forest. Individuals form a single gene pool.
  • Climatic changes reduce forest size to two isolated regions. Groups from each do not meet each other.
  • Climatic changes make one region cold and wet and the other warm and dry. The groups adapt and evolve into different species.
  • The forest regrows and the two groups meet, but can't interbreed. Each species has its own gene pool.
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