A2 Unit 4: Populations and Environment Student checklist
Unit 4 focuses on how living organisms form communities within ecosystems. In ecosystems, energy is transferred and chemical elements are cycled.
Humans are part of the ecological balance and human activities affect it both directly and indirectly. Sustainability of resources depends on effective management of the conflict between human needs and conservation.
– at the end of this unit, you will be expected to have an understanding of the following:
 Random sampling results in the collection of data which is unbiased and suitable for statistical analysis
 ATP is the immediate source of energy for biological processes
 Limiting factors
 Energy is transferred through ecosystems
 Chemical elements are recycled in ecosystems
 Genetic variation occurs within a species and geographic isolation leads to the accumulation of genetic difference in populations.
1

A2 Unit 4: Populations and Environment
Student checklist
At the end of this topic you will be able to:
a) define a population as all the organisms of one species in a habitat. b) define a community as all the populations of different species.

 c) recognise that within a habitat a species occupies a niche governed by adaptation to both biotic and abiotic conditions.

d) critically appreciate some of the ways in which the numbers and distribution of organisms may be investigated. e) obtain quantitative data by carrying out random sampling with quadrats and counting along transects. f) measure abundance by using percentage cover and frequency. g) measure abundance by using the mark–release–recapture method for more mobile species. h) carry out experimental and investigative activities, including appropriate risk management. i) consider ethical issues arising when carrying out fieldwork. j) analyse and interpret data relating to the distribution of organisms, recognising correlations and causal relationships. k) appreciate the tentative nature of conclusions that may be drawn from such data.








l) recognise that population size may vary as a result of:
 the effect of abiotic factors.
 interactions between organisms: interspecific and intraspecific competition and predation.


2

A2 Unit 4: Populations and Environment
Student checklist m) describe population size and structure, population growth rate, age-population pyramids, survival rates and life expectancy. n) interpret growth curves, survival curves and age-population pyramids. o) calculate population growth rates from data on birth rate and death rate. p) relate changes in the size and structure of human populations to different stages in demographic transition.




3

A2 Unit 4: Populations and Environment
At the end of this topic you will be able to: a) describe the synthesis of ATP from ADP and phosphate. b) describe the role of ATP as the immediate source of energy for biological processes.
4
Student checklist



A2 Unit 4: Populations and Environment
At the end of this topic you will be able to:
a) describe the light-independent and light-dependent reactions in a typical C3 plant.
b) describe how light energy excites electrons in chlorophyll. c) describe how energy from these excited electrons generates ATP and reduced NADP.
Student checklist d) describe how the production of ATP involves electron transfer and the electron transfer chain in chloroplast membranes. e) describe how the photolysis of water produces protons, electrons and oxygen.
f) describe how carbon dioxide reacts with ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP). g) describe how ATP and reduced NADP are required for the reduction of GP to triose phosphate. h) describe how RuBP is regenerated in the Calvin cycle. i) describe how triose phosphate is converted to useful organic substances.
j) describe the effects of limiting factors ( temperature, carbon dioxide conc. and light intensity) on the rate of photosynthesis. k) explain how growers apply a knowledge of limiting factors in controlling conditions in commercial glasshouses l) evaluate such applications using appropriate data.











5


A2 Unit 4: Populations and Environment
At the end of this topic you will be able to:
Student checklist
a) state that glycolysis occurs in the cytoplasm and involves the oxidation of glucose to pyruvate with a
net gain of ATP and reduced NAD.
 b) describe how pyruvate combines with coenzyme A in the link reaction to produce acetylcoenzyme A
 c) describe that in a series of oxidation-reduction reactions the Krebs cycle generates reduced coenzymes and ATP by substrate-level phosphorylation, and carbon dioxide is lost.
 d) describe acetylcoenzyme A as a two carbon molecule that combines with a four carbon molecule to produce a six carbon molecule which enters the Krebs cycle.
 e) explain how synthesis of ATP by oxidative phosphorylation is associated with the transfer of electrons down the electron transport chain and passage of protons across mitochondrial membranes.

f) Glycolysis followed by the production of ethanol or lactate and the regeneration of NAD in anaerobic respiration.

6

A2 Unit 4: Populations and Environment
At the end of this topic you will be able to:
Student checklist
a) outline the role of microorganisms in the carbon and nitrogen cycles. b) illustrate the processes of saprobiotic nutrition, ammonification, nitrification, nitrogen fixation and denitrification
(the names of individual bacterial species are not required).


c) the importance of respiration, photosynthesis and human activity in giving rise to short-term fluctuation and long-term change in global carbon dioxide concentration.
 d) describe the roles of carbon dioxide and methane in enhancing the greenhouse effect and bringing about global warming.
 e) analyse, interpret and evaluate data relating to evidence of global warming and its effects on:
 the yield of crop plants

 the life-cycles and numbers of insect pests
 the distribution and numbers of wild animals and plants.


f) describe the environmental issues arising from the use of fertilisers, including leaching and eutrophication. g) analyse, interpret and evaluate data relating to eutrophication.


7

A2 Unit 4: Populations and Environment
At the end of this topic you will be able to:
 intensive rearing of domestic livestock.
Student checklist a) state that photosynthesis is the main route by which energy enters an ecosystem. b) describe how energy is transferred through the trophic levels in food chains and food webs and is dissipated. c) calculate the efficiency of energy transfer between trophic levels. d) construct pyramids of numbers, biomass and energy for corresponding food chains and webs.
e) compare natural ecosystems with those based on modern intensive farming in terms of energy input and productivity. f) calculate net productivity using the expression: Net productivity = Gross productivity – Respiratory loss. g) describe how productivity is affected by farming practices that increase the efficiency of energy conversion, including:
 the use of natural and artificial fertilisers
 the use of chemical pesticides, biological agents and integrated systems in controlling pests on agricultural crops








 h) present scientific arguments that explain how these and other farming practices affect productivity.
 i) evaluate economic and environmental issues involved with farming practices that increase productivity.
 j) consider ethical issues arising from increasing productivity.

8

A2 Unit 4: Populations and Environment
At the end of this topic you will be able to: a) describe succession from pioneer species to climax community. b) recognise certain species at each stage in succession which change the environment so that it becomes more suitable for other species.
Student checklist c) describe how changes in the abiotic environment result in a less hostile environment and affect diversity. d) describe how conservation of habitats frequently involves management of succession.




 e) present scientific arguments and ideas relating to the conservation of species and habitats. f) evaluate evidence and data concerning issues relating to the conservation of species and habitats and consider conflicting evidence. g) explain how conservation relies on science to inform decision-making.


9

A2 Unit 4: Populations and Environment
At the end of this topic you will be able to:
a) define genotype as the genetic constitution of an organism. b) define phenotype as the expression of this genetic constitution and its interaction with the environment. c) recognise that alleles are one or more alternative versions of the same gene. the alleles at a specific locus may be either homozygous or heterozygous. d) recognise that alleles may be dominant, recessive or codominant. e) recognise that there may be multiple alleles of a single gene. f) using fully labelled genetic diagrams, predict the results of:
 monohybrid crosses involving dominant, recessive and codominant alleles.
Student checklist







 crosses involving multiple alleles and sex-linked characteristics.
g) recognise that species exist as one or more populations. h) explain the concepts of gene pool and allele frequency. i) describe the conditions under which the Hardy-Weinberg principle applies. j) calculate allele, genotype and phenotype frequencies from appropriate data using the Hardy-Weinberg equation, p2 + 2pq + q2 = 1, where p is the frequency of the dominant allele and q is the frequency of the recessive allele. k) understand that the Hardy-Weinberg principle provides a mathematical model which predicts that allele frequencies will not change from generation to generation.





10

A2 Unit 4: Populations and Environment
l) describe differential reproductive success and its effect on the allele frequency within a gene pool.
Student checklist m) describe directional and stabilising selection. n) explain how selection produces changes within a species.



 o) interpret data relating to the effect of selection in producing change within populations.
p) explain how geographic separation of populations of a species can result in the accumulation of difference
in the gene pools. q) explain the importance of geographic isolation in the formation of new species.


11

A2 Unit 4: Populations and Environment
Student checklist
These will be assessed using specific experiments set by the exam board as part of Unit 6.
The investigations outlined below will be carried out during Unit 4 to develop your practical skills:
• the effect of a specific limiting factor such as light intensity, carbon dioxide concentration or temperature on the rate of photosynthesis
• the effect of a specific variable such as substrate or temperature on the rate of respiration of a suitable organism.
• fieldwork involving the use of frame quadrats and line transects, and the measurement of a specific abiotic factor; collection of quantitative
data from at least one habitat, and the application of elementary statistical analysis to the results; the use of percentage cover and frequency
as measures of abundance.
While carrying out the above investigations, you will be required to:
 use knowledge and understanding to pose scientific questions and define scientific problems
 carry out investigative activities, including appropriate risk management
 analyse and interpret data you have collected to provide evidence
 evaluate your method, evidence and data
 explain conflicting evidence
12