A2 Biology Unit 4 page 1
AQA A2 Biology Unit 4 Contents
Specification 2
Ecology Populations 5 The ecological niche 8 Food chains and Pyramids 11
Energy Flow 14
Nutrient Cycles 16
Productivity and farming 21
Fertilisers 23 Pest control 25
Livestock rearing 28 Eutrophication 29
Global warming 30 Succession 34 Conservation 36
Human Populations 38
Metabolism Aerobic respiration 42 Anaerobic respiration 48
Photosynthesis 50 Limiting factors 54
Genetics Genetic crosses 55
Gene Frequencies 66 Natural Selection 69
Speciation 74
HGS Biology A-level notes NCM/7/09 A2 Biology Unit 4 page 2
Biology Unit 4 Specification
A population is all the organisms of one species in a
Populations and Ecosystems habitat. Populations of different species form a
,community. Within a habitat a species occupies a niche relating to evidence of global warming and its effects on
governed by adaptation to both biotic and abiotic the yield of crop plants; the lifecycles and numbers of
conditions. Population size may vary as a result of the insect pests; and the distribution and numbers of wild
effect of abiotic factors and interactions between animals and plants.
organisms: interspecific and intraspecific competition
and predation. Succession
Succession from pioneer species to climax community.
Food Chains At each stage in succession certain species may be
Photosynthesis is the main route by which energy recognised that change the environment so that it
enters an ecosystem. Energy is transferred through the becomes more suitable for other species. The changes
trophic levels in food chains and food webs and is in the abiotic environment result in a less hostile
dissipated. Quantitative consideration of the efficiency environment and changing diversity.
of energy transfer between trophic levels. Pyramids of
numbers, biomass and energy and their relationship to Conservation
their corresponding food chains and webs. Conservation of habitats frequently involves
management of succession. Present scientific
Nutrient Cycles arguments and ideas relating to the conservation of
The role of microorganisms in the carbon and nitrogen species and habitats. Evaluate evidence and data
cycles in sufficient detail to illustrate the processes of concerning issues relating to the conservation of
saprobiotic nutrition, ammonification, nitrification, species and habitats and consider conflicting evidence.
nitrogen fixation and denitrification. (The names of Explain how conservation relies on science to inform
individual species are not required.) decision making.
Productivity and Farming Human populations
Comparison of natural ecosystems and those based on Population size and structure, population growth rate,
modern intensive farming in terms of energy input and age population pyramids, survival rates and life
productivity. Net productivity as defined by the expectancy. Interpret growth curves, survival curves
expression and age pyramids. Calculate population growth rates
Net productivity = Gross productivity – Respiratory loss The from data on birth rate and death rate. Relate changes
ways in which productivity is affected by farming in the size and structure of human populations to
practices that increase the efficiency of energy different stages in demographic transition.
conversion. These include
• the use of natural and artificial fertilisers • the use of Fieldwork
chemical pesticides, biological agents and integrated Carry out fieldwork involving the use of frame quadrats
systems in controlling pests on agricultural crops • and line transects, and the measurement of a specific
intensive rearing of domestic livestock. abiotic factor. Collect quantitative data from at least
one habitat, including the use of percentage cover and
Apply understanding of biological principles to present frequency as measures of abundance. Apply
scientific arguments that explain how these and other elementary statistical analysis to the results.
farming practices affect productivity. Evaluate
economic and environmental issues involved with A critical appreciation of some of the ways in which the
farming practices that increase productivity. Consider numbers and distribution of organisms may be
ethical issues arising from enhancement of productivity. investigated.
• Random sampling with quadrats and counting along
Eutrophication transects to obtain quantitative data. • The use of
The environmental issues arising from the use of percentage cover and frequency as measures of
fertilisers. Leaching and eutrophication. Analyse, abundance.
interpret and evaluate data relating to eutrophication. • The use of mark-release-recapture for more mobile
species.
Greenhouse Effect Carry out experimental and investigative activities
investigating populations, including appropriate risk
The importance of respiration, photosynthesis and
management. Consider ethical issues arising when
human activity in giving rise to short-term fluctuation
carrying out field work, particularly those relating to the
and long-term change in global carbon dioxide
organisms involved and their environment. Analyse and
HGS Biology A-level notes NCM/7/09
concentration. The roles of carbon dioxide and methane interpret data relating to the distribution of organisms,
in enhancing the greenhouse effect and bringing about recognising correlations and causal
global warming. Analyse, interpret and evaluate data
,A2 Biology Unit 4 page 3
relationships. Appreciate the tentative nature of rate of photosynthesis. Candidates should be able to
conclusions that may be drawn from such data. explain how growers apply a knowledge of limiting
factors in enhancing temperature, carbon dioxide
concentration and light intensity in commercial
glasshouses. They should also be able to evaluate such
The synthesis of ATP from ADP and phosphate and its applications using appropriate data.
role as the immediate source of energy for biological
processes.
Genetics
Genetic Crosses
Aerobic respiration
The genotype is the genetic constitution of an organism.
Aerobic respiration in such detail as to show that
The phenotype is the expression of this genetic
• Glycolysis takes place in the cytoplasm and involves
constitution and its interaction with the environment.
the oxidation of glucose to pyruvate with a net gain
The alleles at a specific locus may be either homozygous
of ATP and reduced NAD • Pyruvate combines with or heterozygous. Alleles may be dominant, recessive or
coenzyme A in the link reaction to produce acetyl codominant. There may be multiple alleles of a single
coenzyme A • Acetyl coenzyme A is effectively a two gene. Use fully labelled genetic diagrams to predict the
carbon molecule that combines with a four carbon results of • monohybrid crosses involving dominant,
molecule to produce a six carbon molecule which recessive and codominant alleles
enters the Krebs cycle. In a series of oxidation • crosses involving multiple alleles and sex-linked
reduction reactions the Krebs cycle generates characteristics.
reduced coenzymes and ATP by substrate-level
phosphorylation, and carbon dioxide is lost. •
The Hardy-Weinberg principle
Synthesis of ATP is associated with the transfer of
electrons down the electron transport chain and Species exist as one or more populations. The concepts
passage of protons across mitochondrial of gene pool and allele frequency. The Hardy-Weinberg
membranes. principle. The conditions under which the principle
Investigate the effect of a specific variable such as applies. Calculate allele, genotype and phenotype
substrate or temperature on the rate of respiration of a frequencies from appropriate data and from the Hardy-
suitable organism. Weinberg equation, p2 + 2pq + q2 = 1 where p is the
frequency of the dominant allele and q is the frequency
of the recessive allele. Understand that the Hardy-
Anaerobic respiration Weinberg principle provides a mathematical model that
Glycolysis followed by the production of ethanol or predicts that allele frequencies will not change from
lactate and the regeneration of NAD in anaerobic generation to generation.
respiration.
Natural Selection
Photosynthesis Differential reproductive success and its effect on the
The light-independent and light-dependent reactions in allele frequency within a gene pool. Directional and
a typical C3 plant. stabilising selection. Use both specific examples and
• The light-dependent reaction in such detail as to unfamiliar information to explain how selection
show that: light energy excites electrons in produces changes within a species. Interpret data
chlorophyll; energy from these excited electrons relating to the effect of selection in producing change
generates ATP and reduced NADP; the production of within populations.
ATP involves electron transfer associated with the
electron transfer chain in chloroplast membranes;
Speciation
photolysis of water produces protons, electrons and
oxygen. Geographic separation of populations of a species can
• The light-independent reaction in such detail as to result in the accumulation of difference in the gene
show that: carbon dioxide is accepted by ribulose pools. The importance of geographic isolation in the
bisphosphate (RuBP) to form two molecules of formation of new species.
glycerate 3-phosphate (GP); ATP and reduced NADP
are required for the reduction of GP to triose
phosphate; RuBP is regenerated in the Calvin cycle;
Triose phosphate is converted to useful organic
substances.
Limiting Factors
The principle of limiting factors as applied to the effects
of temperature, carbon dioxide concentration and light
intensity on the rate of photosynthesis. Investigate the
effect of a specific limiting factor such as light intensity,
carbon dioxide concentration or temperature on the
HGS Biology A-level notes NCM/7/09
, A2 Biology Unit 4 page 4
Ecology
Ecology (or environmental biology) is the study of living organisms and their environment. Its aim it to explain
why organisms live where they do. To do this ecologist’s study ecosystems: areas that can vary in size from a
pond to the whole planet.
Biosphere The part of the planet Earth where life occurs, including land, sea and air.
Ecosystem A reasonably self-contained area together with all its living organisms, e.g. oak forest,
deep sea, sand dune, rocky shore, moorland, hedgerow, garden pond, etc.
Habitat The physical or abiotic part of an ecosystem, i.e. a defined area with specific
characteristics where the organisms live. Most ecosystems have several habitats.
Microhabitat A localised specific habitat within a larger habitat e.g. under a rotting log, in a rock pool,
etc.
Community The living or biotic part of an ecosystem, i.e. all the organisms of all the different species
living in one habitat.
Biotic Any living or biological factor.
Abiotic Any non-living or physical factor.
Population The members of the same species living in one habitat.
Species A group of organisms that can successfully interbreed
HGS Biology A-level notes NCM/7/09
AQA A2 Biology Unit 4 Contents
Specification 2
Ecology Populations 5 The ecological niche 8 Food chains and Pyramids 11
Energy Flow 14
Nutrient Cycles 16
Productivity and farming 21
Fertilisers 23 Pest control 25
Livestock rearing 28 Eutrophication 29
Global warming 30 Succession 34 Conservation 36
Human Populations 38
Metabolism Aerobic respiration 42 Anaerobic respiration 48
Photosynthesis 50 Limiting factors 54
Genetics Genetic crosses 55
Gene Frequencies 66 Natural Selection 69
Speciation 74
HGS Biology A-level notes NCM/7/09 A2 Biology Unit 4 page 2
Biology Unit 4 Specification
A population is all the organisms of one species in a
Populations and Ecosystems habitat. Populations of different species form a
,community. Within a habitat a species occupies a niche relating to evidence of global warming and its effects on
governed by adaptation to both biotic and abiotic the yield of crop plants; the lifecycles and numbers of
conditions. Population size may vary as a result of the insect pests; and the distribution and numbers of wild
effect of abiotic factors and interactions between animals and plants.
organisms: interspecific and intraspecific competition
and predation. Succession
Succession from pioneer species to climax community.
Food Chains At each stage in succession certain species may be
Photosynthesis is the main route by which energy recognised that change the environment so that it
enters an ecosystem. Energy is transferred through the becomes more suitable for other species. The changes
trophic levels in food chains and food webs and is in the abiotic environment result in a less hostile
dissipated. Quantitative consideration of the efficiency environment and changing diversity.
of energy transfer between trophic levels. Pyramids of
numbers, biomass and energy and their relationship to Conservation
their corresponding food chains and webs. Conservation of habitats frequently involves
management of succession. Present scientific
Nutrient Cycles arguments and ideas relating to the conservation of
The role of microorganisms in the carbon and nitrogen species and habitats. Evaluate evidence and data
cycles in sufficient detail to illustrate the processes of concerning issues relating to the conservation of
saprobiotic nutrition, ammonification, nitrification, species and habitats and consider conflicting evidence.
nitrogen fixation and denitrification. (The names of Explain how conservation relies on science to inform
individual species are not required.) decision making.
Productivity and Farming Human populations
Comparison of natural ecosystems and those based on Population size and structure, population growth rate,
modern intensive farming in terms of energy input and age population pyramids, survival rates and life
productivity. Net productivity as defined by the expectancy. Interpret growth curves, survival curves
expression and age pyramids. Calculate population growth rates
Net productivity = Gross productivity – Respiratory loss The from data on birth rate and death rate. Relate changes
ways in which productivity is affected by farming in the size and structure of human populations to
practices that increase the efficiency of energy different stages in demographic transition.
conversion. These include
• the use of natural and artificial fertilisers • the use of Fieldwork
chemical pesticides, biological agents and integrated Carry out fieldwork involving the use of frame quadrats
systems in controlling pests on agricultural crops • and line transects, and the measurement of a specific
intensive rearing of domestic livestock. abiotic factor. Collect quantitative data from at least
one habitat, including the use of percentage cover and
Apply understanding of biological principles to present frequency as measures of abundance. Apply
scientific arguments that explain how these and other elementary statistical analysis to the results.
farming practices affect productivity. Evaluate
economic and environmental issues involved with A critical appreciation of some of the ways in which the
farming practices that increase productivity. Consider numbers and distribution of organisms may be
ethical issues arising from enhancement of productivity. investigated.
• Random sampling with quadrats and counting along
Eutrophication transects to obtain quantitative data. • The use of
The environmental issues arising from the use of percentage cover and frequency as measures of
fertilisers. Leaching and eutrophication. Analyse, abundance.
interpret and evaluate data relating to eutrophication. • The use of mark-release-recapture for more mobile
species.
Greenhouse Effect Carry out experimental and investigative activities
investigating populations, including appropriate risk
The importance of respiration, photosynthesis and
management. Consider ethical issues arising when
human activity in giving rise to short-term fluctuation
carrying out field work, particularly those relating to the
and long-term change in global carbon dioxide
organisms involved and their environment. Analyse and
HGS Biology A-level notes NCM/7/09
concentration. The roles of carbon dioxide and methane interpret data relating to the distribution of organisms,
in enhancing the greenhouse effect and bringing about recognising correlations and causal
global warming. Analyse, interpret and evaluate data
,A2 Biology Unit 4 page 3
relationships. Appreciate the tentative nature of rate of photosynthesis. Candidates should be able to
conclusions that may be drawn from such data. explain how growers apply a knowledge of limiting
factors in enhancing temperature, carbon dioxide
concentration and light intensity in commercial
glasshouses. They should also be able to evaluate such
The synthesis of ATP from ADP and phosphate and its applications using appropriate data.
role as the immediate source of energy for biological
processes.
Genetics
Genetic Crosses
Aerobic respiration
The genotype is the genetic constitution of an organism.
Aerobic respiration in such detail as to show that
The phenotype is the expression of this genetic
• Glycolysis takes place in the cytoplasm and involves
constitution and its interaction with the environment.
the oxidation of glucose to pyruvate with a net gain
The alleles at a specific locus may be either homozygous
of ATP and reduced NAD • Pyruvate combines with or heterozygous. Alleles may be dominant, recessive or
coenzyme A in the link reaction to produce acetyl codominant. There may be multiple alleles of a single
coenzyme A • Acetyl coenzyme A is effectively a two gene. Use fully labelled genetic diagrams to predict the
carbon molecule that combines with a four carbon results of • monohybrid crosses involving dominant,
molecule to produce a six carbon molecule which recessive and codominant alleles
enters the Krebs cycle. In a series of oxidation • crosses involving multiple alleles and sex-linked
reduction reactions the Krebs cycle generates characteristics.
reduced coenzymes and ATP by substrate-level
phosphorylation, and carbon dioxide is lost. •
The Hardy-Weinberg principle
Synthesis of ATP is associated with the transfer of
electrons down the electron transport chain and Species exist as one or more populations. The concepts
passage of protons across mitochondrial of gene pool and allele frequency. The Hardy-Weinberg
membranes. principle. The conditions under which the principle
Investigate the effect of a specific variable such as applies. Calculate allele, genotype and phenotype
substrate or temperature on the rate of respiration of a frequencies from appropriate data and from the Hardy-
suitable organism. Weinberg equation, p2 + 2pq + q2 = 1 where p is the
frequency of the dominant allele and q is the frequency
of the recessive allele. Understand that the Hardy-
Anaerobic respiration Weinberg principle provides a mathematical model that
Glycolysis followed by the production of ethanol or predicts that allele frequencies will not change from
lactate and the regeneration of NAD in anaerobic generation to generation.
respiration.
Natural Selection
Photosynthesis Differential reproductive success and its effect on the
The light-independent and light-dependent reactions in allele frequency within a gene pool. Directional and
a typical C3 plant. stabilising selection. Use both specific examples and
• The light-dependent reaction in such detail as to unfamiliar information to explain how selection
show that: light energy excites electrons in produces changes within a species. Interpret data
chlorophyll; energy from these excited electrons relating to the effect of selection in producing change
generates ATP and reduced NADP; the production of within populations.
ATP involves electron transfer associated with the
electron transfer chain in chloroplast membranes;
Speciation
photolysis of water produces protons, electrons and
oxygen. Geographic separation of populations of a species can
• The light-independent reaction in such detail as to result in the accumulation of difference in the gene
show that: carbon dioxide is accepted by ribulose pools. The importance of geographic isolation in the
bisphosphate (RuBP) to form two molecules of formation of new species.
glycerate 3-phosphate (GP); ATP and reduced NADP
are required for the reduction of GP to triose
phosphate; RuBP is regenerated in the Calvin cycle;
Triose phosphate is converted to useful organic
substances.
Limiting Factors
The principle of limiting factors as applied to the effects
of temperature, carbon dioxide concentration and light
intensity on the rate of photosynthesis. Investigate the
effect of a specific limiting factor such as light intensity,
carbon dioxide concentration or temperature on the
HGS Biology A-level notes NCM/7/09
, A2 Biology Unit 4 page 4
Ecology
Ecology (or environmental biology) is the study of living organisms and their environment. Its aim it to explain
why organisms live where they do. To do this ecologist’s study ecosystems: areas that can vary in size from a
pond to the whole planet.
Biosphere The part of the planet Earth where life occurs, including land, sea and air.
Ecosystem A reasonably self-contained area together with all its living organisms, e.g. oak forest,
deep sea, sand dune, rocky shore, moorland, hedgerow, garden pond, etc.
Habitat The physical or abiotic part of an ecosystem, i.e. a defined area with specific
characteristics where the organisms live. Most ecosystems have several habitats.
Microhabitat A localised specific habitat within a larger habitat e.g. under a rotting log, in a rock pool,
etc.
Community The living or biotic part of an ecosystem, i.e. all the organisms of all the different species
living in one habitat.
Biotic Any living or biological factor.
Abiotic Any non-living or physical factor.
Population The members of the same species living in one habitat.
Species A group of organisms that can successfully interbreed
HGS Biology A-level notes NCM/7/09
