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AQA A2 BIOLOGY UNIT 4A2 Biology Unit 4 page 1
AQACONTENTS
A2 Biology Unit 4
Contents
Specification 2
Ecology Fieldwork 4
Statistics 13
Populations 15
The ecological Niche 18
Ecological Succession and Conservation 20
Food Chains 24
Nutrient Cycles 27
Energy Flow and Pyramids 32
Productivity 36
Intensive Farming 37
Greenhouse Effect and Global Warming 47
Human Populations 52
Metabolism Aerobic respiration 56
Anaerobic respiration 62
Photosynthesis 64
Factors affecting Photosynthesis 69
Genetics Genetic crosses 71
Population Genetics and the Gene Pool 82
Natural Selection 85
Speciation 91
Appendices 1 – Biological Principles 94
2 – The Unit 4 Exam 96
These notes may be used freely by A level biology students and
teachers, and they may be copied and edited.
Please do not use these materials for commercial purposes.
I would be interested to hear of any comments and corrections.
Neil C Millar (nmillar@ntlworld.co.uk)
Head of Biology, Heckmondwike Grammar School
High Street, Heckmondwike, WF16 0AH
May 2011
HGS Biology A-level notes NCM/5/11
,A2 Biology Unit 4 page 2
Biology Unit 4 Specification
Ecology processes of saprobiotic nutrition, ammonification,
Fieldwork nitrification, nitrogen fixation and denitrification. (The
A critical appreciation of some of the ways in which names of individual species are not required.)
the numbers and distribution of organisms may be
investigated. Energy Flow
• Random sampling with quadrats and counting along Photosynthesis is the main route by which energy
transects to obtain quantitative data. enters an ecosystem. Energy is transferred through
• The use of percentage cover and frequency as the trophic levels in food chains and food webs and is
measures of abundance. dissipated. Quantitative consideration of the efficiency
of energy transfer between trophic levels. Pyramids of
• The use of mark-release-recapture for more
numbers, biomass and energy and their relationship to
mobile species.
their corresponding food chains and webs. Net
Carry out fieldwork involving the use of frame
productivity as defined by the expression
quadrats and line transects, and the measurement of a
Net productivity = Gross productivity – Respiratory loss
specific abiotic factor. Collect quantitative data
investigating populations from at least one habitat,
Intensive Farming
including appropriate risk management. Consider
Comparison of natural ecosystems and those based on
ethical issues arising when carrying out field work,
modern intensive farming in terms of energy input and
particularly those relating to the organisms involved
productivity. The ways in which productivity is
and their environment.
affected by farming practices that increase the
efficiency of energy conversion. These include
Statistics
• the use of natural and artificial fertilisers
Analyse and interpret data relating to the distribution
• The environmental issues arising from the use of
of organisms, recognising correlations and causal
relationships. Apply elementary statistical analysis to fertilisers. Leaching and eutrophication. Analyse,
the results. Appreciate the tentative nature of interpret and evaluate data relating to
conclusions that may be drawn from such data. eutrophication.
• the use of chemical pesticides, biological agents and
Populations and the Niche integrated systems in controlling pests on
A population is all the organisms of one species in a agricultural crops
habitat. Populations of different species form a • intensive rearing of domestic livestock.
community. Population size may vary as a result of the
effect of abiotic factors and interactions between Apply understanding of biological principles to present
organisms: interspecific and intraspecific competition scientific arguments that explain how these and other
and predation. Within a habitat a species occupies a farming practices affect productivity. Evaluate
niche governed by adaptation to both biotic and economic and environmental issues involved with
abiotic conditions. farming practices that increase productivity. Consider
ethical issues arising from enhancement of
Succession productivity.
Succession from pioneer species to climax community.
At each stage in succession certain species may be Greenhouse Effect
recognised that change the environment so that it The importance of respiration, photosynthesis and
becomes more suitable for other species. The changes human activity in giving rise to short-term fluctuation
in the abiotic environment result in a less hostile and long-term change in global carbon dioxide
environment and changing diversity. concentration. The roles of carbon dioxide and
methane in enhancing the greenhouse effect and
Conservation bringing about global warming. Analyse, interpret and
Conservation of habitats frequently involves evaluate data relating to evidence of global warming
management of succession. Present scientific and its effects on the yield of crop plants; the life-
arguments and ideas relating to the conservation of cycles and numbers of insect pests; and the
species and habitats. Evaluate evidence and data distribution and numbers of wild animals and plants.
concerning issues relating to the conservation of
species and habitats and consider conflicting evidence. Human populations
Explain how conservation relies on science to inform Population size and structure, population growth rate,
decision making. age population pyramids, survival rates and life
expectancy. Interpret growth curves, survival curves
Nutrient Cycles and age pyramids. Calculate population growth rates
The role of microorganisms in the carbon and from data on birth rate and death rate. Relate changes
nitrogen cycles in sufficient detail to illustrate the in the size and structure of human populations to
different stages in demographic transition.
HGS Biology A-level notes NCM/5/11
,A2 Biology Unit 4 page 3
and light intensity on the rate of photosynthesis.
Metabolism Investigate the effect of a specific limiting factor such
The synthesis of ATP from ADP and phosphate and its as light intensity, carbon dioxide concentration or
role as the immediate source of energy for biological temperature on the rate of photosynthesis.
processes. Candidates should be able to explain how growers
apply a knowledge of limiting factors in enhancing
Aerobic respiration temperature, carbon dioxide concentration and light
Aerobic respiration in such detail as to show that intensity in commercial glasshouses. They should also
• Glycolysis takes place in the cytoplasm and be able to evaluate such applications using appropriate
involves the oxidation of glucose to pyruvate with data.
a net gain of ATP and reduced NAD
• Pyruvate combines with coenzyme A in the link Genetics
reaction to produce acetylcoenzyme A Genetic Crosses
• Acetylcoenzyme A is effectively a two carbon The genotype is the genetic constitution of an
molecule that combines with a four carbon organism. The phenotype is the expression of this
molecule to produce a six carbon molecule which genetic constitution and its interaction with the
enters the Krebs cycle. In a series of oxidation- environment. The alleles at a specific locus may be
reduction reactions the Krebs cycle generates either homozygous or heterozygous. Alleles may be
reduced coenzymes and ATP by substrate-level dominant, recessive or codominant. There may be
phosphorylation, and carbon dioxide is lost. multiple alleles of a single gene. Use fully labelled
• Synthesis of ATP is associated with the transfer of genetic diagrams to predict the results of
electrons down the electron transport chain and • monohybrid crosses involving dominant, recessive
passage of protons across mitochondrial and codominant alleles
membranes. • crosses involving multiple alleles and sex-linked
Investigate the effect of a specific variable such as characteristics.
substrate or temperature on the rate of respiration of
a suitable organism. The Hardy-Weinberg Principle
Species exist as one or more populations. The
Anaerobic respiration concepts of gene pool and allele frequency.
Glycolysis followed by the production of ethanol or • Calculate allele, genotype and phenotype
lactate and the regeneration of NAD in anaerobic frequencies from appropriate data and from the
respiration. Hardy-Weinberg equation, p2 + 2pq + q2 = 1
where p is the frequency of the dominant allele and
Photosynthesis q is the frequency of the recessive allele.
The light-independent and light-dependent reactions in • The Hardy-Weinberg principle. The conditions
a typical C3 plant. under which the principle applies. Understand that
• The light-dependent reaction in such detail as to the Hardy-Weinberg principle provides a
show that: light energy excites electrons in mathematical model that predicts that allele
chlorophyll; energy from these excited electrons frequencies will not change from generation to
generates ATP and reduced NADP; the production generation.
of ATP involves electron transfer associated with
the electron transfer chain in chloroplast Natural Selection
membranes; photolysis of water produces protons, Differential reproductive success and its effect on the
electrons and oxygen. allele frequency within a gene pool. Directional and
• The light-independent reaction in such detail as to stabilising selection. Use both specific examples and
show that: carbon dioxide is accepted by ribulose unfamiliar information to explain how selection
bisphosphate (RuBP) to form two molecules of produces changes within a species. Interpret data
glycerate 3-phosphate (GP); ATP and reduced relating to the effect of selection in producing change
NADP are required for the reduction of GP to within populations.
triose phosphate; RuBP is regenerated in the
Calvin cycle; Triose phosphate is converted to Speciation
useful organic substances. Geographic separation of populations of a species can
result in the accumulation of difference in the gene
Limiting Factors pools. The importance of geographic isolation in the
The principle of limiting factors as applied to the formation of new species.
effects of temperature, carbon dioxide concentration
HGS Biology A-level notes NCM/5/11
, 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 ecologists 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.
Terrestrial An ecosystem on dry land
Aquatic An ecosystem in water
Marine An ecosystem in the sea
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
Fieldwork
Ecology is best studied in the organisms’ natural habitat, but working in “the field” presents particular
practical difficulties: the habitats can be very large; there can be a very large number of different organisms
present; many of the organisms move about or are difficult to find; the organisms can be difficult to identify;
some organisms may eat other organisms; and confounding variables, like the weather, can be impossible to
control. To deal with these problems there are a number of specific fieldwork techniques.
Sampling
In unit 2 we came across the idea of sampling a population, in other words looking at a small sample of the
biota in an ecosystem, rather than studying every living thing, which would be impossible. There are two
strategies for sampling an ecosystem, depending on your objective.
HGS Biology A-level notes NCM/5/11
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