Subject Content - A-Levels Biology

Topic 1: Biological Molecules

This topic introduces the chemicals of life: the organic and inorganic molecules and ions that are fundamental to the structure and physiology of living organisms. The role of monomers in the synthesis of polymers and how the structure and properties of these relate to their functions are considered. An understanding of scientific methods is developed in the practical investigation of enzyme action.

Opportunities for developing mathematical skills within this topic include: recognising and making use of appropriate units in calculations; using ratios, fractions and percentages; constructing and interpreting frequency tables and diagrams; translating information between graphical, numerical and algebraic forms; understanding that y = mx + c represents a linear relationship; determining the intercept of a graph; calculating rate of change from a graph; drawing and using the slope of a tangent to a curve as a measure of rate of change. (Please see Appendix 6: Mathematical skills and exemplifications for further information.)

1.1 Carbohydrates

	Students should:
i	Know the difference between monosaccharides, disaccharides and polysaccharides.
ii	Know the structure of the hexose glucose (alpha and beta) and the pentose ribose.
iii	Understand how monosaccharides (glucose, fructose, galactose) join to form disaccharides (sucrose, lactose and maltose) and polysaccharides (starch formed from amylose and amylopectin; glycogen) through condensation reactions forming glycosidic bonds, and how these can be split through hydrolysis reactions.
iv	Understand how the structure of glucose, starch, glycogen and cellulose relates to their function.

1.2 Lipids

	Students should:
i	Understand how a triglyceride is synthesised, including the formation of ester bonds during condensation reactions between glycerol and three fatty acids.
ii	Know the differences between saturated and unsaturated lipids.
iii	Understand how the structure of lipids relates to their role in energy storage, waterproofing and insulation.
iv	Understand how the structure and properties of phospholipids relate to their function in cell membranes.

1.3 Proteins

	Students should:
i	Know the structure of an amino acid (structures of specific amino acids are not required).
ii	Understand the formation of polypeptides and proteins (as amino acid monomers linked by peptide bonds in condensation reactions).
iii	Understand the role of ionic, hydrogen and disulfide bonding in the structure of proteins.
iv	Understand the significance of the primary, secondary, tertiary and quaternary structure of a protein in determining the properties of fibrous and globular proteins, including collagen and haemoglobin.
v	Understand how the structure of collagen and haemoglobin are related to their function.

1.4 DNA and protein synthesis

	Students should:
i	Know the structure of DNA, including the structure of the nucleotides (purines and pyrimidines), base pairing, the two sugar-phosphate backbones, phosphodiester bonds and hydrogen bonds.
ii	Understand how DNA is replicated semi-conservatively, including the role of DNA helicase, polymerase and ligase.
iii	Know that a gene is a sequence of bases on a DNA molecule coding for a sequence of amino acids in a polypeptide chain.
iv	Know the structure of mRNA including nucleotides, the sugar phosphate backbone and the role of hydrogen bonds.
v	Know the structure of tRNA, including nucleotides, the role of hydrogen bonds and the anticodon.
vi	Understand the processes of transcription in the nucleus and translation at the ribosome, including the role of sense and anti-sense DNA, mRNA, tRNA and the ribosomes.
vii	Understand the nature of the genetic code, including triplets coding for amino acids, start and stop codons, degenerate and non-overlapping nature, and that not all the genome codes for proteins.
viii	Understand the term gene mutation as illustrated by base deletions, insertions and substitutions.
ix	Understand the effect of point mutations on amino acid sequences, as illustrated by sickle cell anaemia in humans.

1.5 Enzymes

	Students should:
i	Know the structure of enzymes as globular proteins.
ii	Understand the concepts of specificity and the induced fit hypothesis.
iii	Understand that enzymes are catalysts that reduce activation energy.
iv	Understand how temperature, pH, substrate and enzyme concentration affect the rate of enzyme activity. CORE PRACTICAL 1: Investigate a factor affecting the initial rate of an enzyme-controlled reaction.
v	Understand how the initial rate of enzyme activity can be measured and why this is important.
vi	Understand how enzymes can be affected by competitive, non-competitive and end-product inhibition.
vii	Know that enzymes catalyse a wide range of intracellular reactions as well as extracellular ones.

1.6 Inorganic ions

	Students should:
i	Understand the role in plants of: • nitrate ions – to make DNA and amino acids • calcium ions – to form calcium pectate for the middle lamellae • magnesium ions – to produce chlorophyll • phosphate ions – to make ADP and ATP.

1.7 Water

	Students should:
i	Understand the importance of the dipole nature of water leading to hydrogen bonding and the significance of the following to organisms: • high specific heat capacity • polar solvent • surface tension • incompressibility • maximum density at 4 °C.

Topic 2: Cells, Viruses and Reproduction of Living Things

This topic considers the ultrastructure of prokaryotes, eukaryotes and viruses. Details of the types of nuclear division are included and how these are involved in animal and plant reproduction. Microscopy and observational skills are developed through the preparation of stained plant tissue.

Opportunities for developing mathematical skills within this topic include: recognising and using expressions in decimal and standard form; making order of magnitude calculations; changing the subject of an equation; plotting two variables from experimental or other data. (Please see Appendix 6: Mathematical skills and exemplifications for further information.)

2.1 Eukaryotic and prokaryotic cell structure and function

	Students should:
i	Understand that cell theory is a unifying concept that states that cells are a fundamental unit of structure, function and organisation in all living organisms.
ii	Understand that in complex organisms, cells are organised into tissues, organs, and organ systems.
iii	Know the ultrastructure of prokaryotic cells and the structure of organelles, including: nucleoid, plasmids, 70S ribosomes and cell wall.
iv	Be able to distinguish between Gram positive and Gram negative bacterial cell walls and understand why each type reacts differently to some antibiotics.
v	Know the ultrastructure of eukaryotic cells and the functions of organelles, including: nucleus, nucleolus, 80S ribosomes, rough and smooth endoplasmic reticulum, mitochondria, centrioles, lysosomes, Golgi apparatus, cell wall, chloroplasts, vacuole and tonoplast.
vi	Know how magnification and resolution can be achieved using light and electron microscopy.
vii	Understand the importance of staining specimens in microscopy. CORE PRACTICAL 2: Use of the light microscope, including simple stage and eyepiece micrometers and drawing small numbers of cells from a specialised tissue.

2.2 Viruses

	Students should:
i	Understand that the classification of viruses is based on structure and nucleic acid types as illustrated by λ (lambda) phage (DNA), tobacco mosaic virus and Ebola (RNA) and human immunodeficiency virus (RNA retrovirus).
ii	Know the lytic cycle of a virus and latency.
iii	Know that viruses are not living cells and so antivirals must work by inhibiting virus replication.
iv	Know that as viruses can be difficult to treat once infection has occurred, the focus of disease control should be on preventing the spread, as exemplified by the 2014 Ebola outbreak in West Africa.
v	Be able to evaluate the ethical implications of using untested drugs during epidemics.

2.3 Eukaryotic cell cycle and division

	Students should:
i	Know that the cell cycle is a regulated process in which cells divide into two identical daughter cells, and that this process consists of three main stages: interphase, mitosis and cytokinesis.
ii	Understand what happens to genetic material during the cell cycle, including the stages of mitosis.
iii	Understand how mitosis contributes to growth, repair and asexual reproduction. CORE PRACTICAL 3: Make a temporary squash preparation of a root tip to show stages of mitosis in the meristem under the light microscope.
iv	Understand how meiosis results in haploid gametes, including the stages of meiosis.
v	Understand that meiosis results in genetic variation through recombination of alleles, including independent assortment and crossing over.
vi	Understand what chromosome mutations are, as illustrated by translocations.
vii	Understand how non-disjunction can lead to polysomy, including Down's syndrome, and monosomy, including Turner's syndrome.

2.4 Sexual reproduction in mammals

	Students should:
i	Understand the processes of oogenesis and spermatogenesis.
ii	Understand the events of fertilisation from the first contact between the gametes to the fusion of nuclei.
iii	Understand the early development of the embryo to blastocyst stage.

2.5 Sexual reproduction in plants

	Students should:
i	Understand how a pollen grain forms in the anther and the embryo sac forms in the ovule.
ii	Understand how the male nuclei formed by division of the generative nucleus in the pollen grain reach the embryo sac, including the roles of the tube nucleus, pollen tube and enzymes. CORE PRACTICAL 4: Investigate the effect of sucrose concentrations on pollen tube growth or germination.
iii	Understand the process of double fertilisation inside the embryo sac to form a triploid endosperm and a zygote.

Topic 3: Classification and Biodiversity

This topic considers the evidence used in the development of models for the classification of organisms. It also considers the limitations of these models. The topic includes the principles underlying natural selection and how this can lead to speciation.

Opportunities for developing mathematical skills within this topic include the principles of sampling as applied to scientific data, and assessment of species diversity using a formula to calculate an index of diversity. (Please see Appendix 6: Mathematical skills and exemplifications for further information.)

3.1 Classification

	Students should:
i	Know that the classification system consists of a hierarchy of domain, kingdom, phylum, class, order, family, genus and species.
ii	Understand the limitations of the definition of a species as a group of organisms with similar characteristics that interbreed to produce fertile offspring.
iii	Understand why it is often difficult to assign organisms to any one species or to identify new species.
iv	Understand how gel electrophoresis can be used to distinguish between species and determine evolutionary relationships.
v	Know that DNA sequencing and bioinformatics can be used to distinguish between species and determine evolutionary relationships.
vi	Understand the role of scientific journals, the peer review process and scientific conferences in validating new evidence supporting the accepted scientific theory of evolution.
vii	Understand the evidence for the three-domain model of classification as an alternative to the five-kingdom model and the role of the scientific community in validating this evidence.

3.2 Natural selection

	Students should:
i	Understand how evolution can come about through natural selection acting on variation bringing about adaptations.
ii	Understand how organisms occupy niches according to physiological, behavioural and anatomical adaptations.
iii	Understand how reproductive isolation can lead to allopatric and sympatric speciation.
iv	Understand that there is an evolutionary race between pathogens and the development of medicines to treat the diseases they cause.

3.3 Biodiversity

	Students should:
i	Know that biodiversity can be assessed at different scales: • within a habitat at the species level using a formula to calculate an index of diversity: $D = \frac{N(N-1)}{\sum n(n-1)}$ • within a species at the genetic level by looking at the variety of alleles in the gene pool of a population.
ii	Understand the ethical and economic reasons (ecosystem services) for the maintenance of biodiversity.
iii	Understand the principles of ex-situ (zoos and seed banks) and in-situ conservation (protected habitats), and the issues surrounding each method.

Topic 4: Exchange and Transport

This topic considers the requirements for transport mechanisms in cells and mass flow systems in organisms. The roles of the components of the mammalian circulatory system and the vascular system in plants are studied. Practical skills are developed through the investigation of factors that affect membrane permeability and water potential of plant tissues.

Opportunities for developing mathematical skills within this topic include: recognising and making use of appropriate units in calculations; recognising and using expressions in decimal and standard form; using ratios, fractions and percentages; finding arithmetic means; solving algebraic equations; translating information between graphical, numerical and algebraic forms; plotting two variables from experimental data; understanding that y = mx + c represents a linear relationship; determining the intercept of a graph; calculating rate of change from a graph showing a linear relationship; calculating the circumferences, surface areas and volumes of regular shapes. (Please see Appendix 6: Mathematical skills and exemplifications for further information.)

4.1 Surface area to volume ratio

	Students should:
i	Understand how surface area to volume ratio affects transport of molecules in living organisms.
ii	Understand why organisms need a mass transport system and specialised gas exchange surfaces as they increase in size.

4.2 Cell transport mechanisms

	Students should:
i	Know the structure of the cell surface membrane with reference to the fluid mosaic model.
ii	Understand how passive transport is brought about by: • diffusion • facilitated diffusion (through carrier proteins and protein channels) • osmosis.
iii	Understand how the properties of molecules affects how they are transported, including solubility, size and charge.
iv	Know that large molecules can be transported into and out of cells through the formation of vesicles, in the processes of endocytosis and exocytosis. CORE PRACTICAL 5: Investigate the effect of temperature on beetroot membrane permeability. CORE PRACTICAL 6: Determine the water potential of plant cells. Water potential = turgor pressure + osmotic potential $\Psi = P + \pi$
v	Understand the process of active transport, including the role of ATP.
vi	Know that phosphorylation of ADP requires energy and that hydrolysis of ATP provides an accessible supply of energy for biological processes.

4.3 Gas exchange

	Students should:
i	Understand how insects, fish and mammals are adapted for gas exchange. CORE PRACTICAL 7: Dissect an insect to show the structure of the gas exchange system, taking into account the safe and ethical use of organisms.
ii	Understand gas exchange in flowering plants, including the role of stomata, gas exchange surfaces in the leaf and lenticels.

4.4 Circulation

	Students should:
i	Know the structure of the heart, arteries, veins and capillaries.
ii	Understand the advantages of a double circulatory system in mammals over the single circulatory systems in bony fish, including the facility for blood to be pumped to the body at higher pressure and the splitting of oxygenated and deoxygenated blood.
iii	Know the sequence of events of the cardiac cycle.
iv	Understand myogenic stimulation of the heart, including the roles of the sinoatrial node (SAN), atrioventricular node (AVN) and bundle of His.
v	Be able to interpret data showing ECG traces and pressure changes during the cardiac cycle.
vi	Know the structure of blood as plasma and blood cells, to include erythrocytes and leucocytes (neutrophils, eosinophils, monocytes and lymphocytes).
vii	Know the function of blood as transport, defence, and formation of lymph and tissue fluid.
viii	Understand the role of platelets and plasma proteins in the sequence of events leading to blood clotting, including: • platelets form a plug and release clotting factors, including thromboplastin • prothrombin changes to its active form, thrombin • soluble fibrinogen forms insoluble fibrin to cover the wound.
ix	Understand the stages that lead to atherosclerosis, its effect on health and the factors that increase the risk of its development.

4.5 Transport of gases in the blood

	Students should:
i	Understand the structure of haemoglobin in relation to its role in the transport of respiratory gases, including the Bohr effect.
ii	Understand the oxygen dissociation curve of haemoglobin.
iii	Understand the similarities and differences between the structures and functions of haemoglobin and myoglobin.
iv	Understand the significance of the oxygen affinity of fetal haemoglobin as compared to adult haemoglobin.

4.6 Transfer of materials between the circulatory system and cells

	Students should:
i	Understand how the interchange of substances occurs through the formation and reabsorption of tissue fluid, including the effects of hydrostatic pressure and oncotic pressure.
ii	Know that tissue fluid that is not reabsorbed is returned to the blood via the lymph system.

4.7 Transport in plants

	Students should:
i	Understand the structure of xylem and phloem tissues in relation to their role in transport.
ii	Understand how water can be moved through plant cells by the apoplastic and symplastic pathways.
iii	Understand how the cohesion-tension model explains the transport of water from plant roots to shoots.
iv	Understand how temperature, light, humidity and movement of air affect the rate of transpiration.
v	Understand the strengths and weaknesses of the mass-flow hypothesis in explaining the movement of sugars through phloem tissue. CORE PRACTICAL 8: Investigate factors affecting water uptake by plant shoots using a potometer.

Topic 5: Energy for Biological Processes

This topic builds on knowledge of carbohydrates and enzymes. It considers the sources of energy in living organisms and how energy transfers take place. Details of the stages in respiration and photosynthesis, the roles of co-enzymes and electrons along with the uses of intermediates are included. Practical skills are developed in the investigations of photosynthetic pigments and the factors that affect rates of respiration and photosynthesis.

Opportunities for developing mathematical skills within this topic include: translating information between graphical, numerical and algebraic forms; plotting variables from experimental data; understanding that y = mx + c represents a linear relationship; determining the intercept of a graph; calculating rate of change from a graph showing a linear relationship; determining the intercept of a graph; drawing and using the slope of a tangent to a curve as a measure of rate of change. (Please see Appendix 6: Mathematical skills and exemplifications for further information.)

5.1 Aerobic respiration

	Students should:
i	Know that cellular respiration yields ATP which is used as a source of energy for metabolic reactions, and the process also generates heat.
ii	Know the different stages in aerobic respiration, including: • glycolysis in the cytoplasm • link reaction • Krebs cycle • oxidative phosphorylation in mitochondria.

5.2 Glycolysis

	Students should:
i	Understand the conversion of monosaccharides to pyruvate during glycolysis in the cytoplasm, including: • the phosphorylation of hexose molecules by ATP • breakdown to glycerate 3-phosphate (GP) • production of reduced coenzyme (NADH) and ATP (details of intermediate compounds and other reactions are not required).

5.3 Link reaction and Krebs cycle

	Students should:
i	Know that the link reaction and Krebs cycle take place in the mitochondrial matrix.
ii	Understand that during the complete oxidation of pyruvate the events of the link reaction and the Krebs cycle result in the removal of carbon atoms to produce: • carbon dioxide • reduced coenzyme (NADH) • ATP (detailed knowledge of the intermediate compounds in the Krebs cycle is not required).

5.4 Oxidative phosphorylation

	Students should:
i	Know that the electron transport chain takes place in the inner mitochondrial membrane.
ii	Understand the role of the electron transport chain in generating ATP (oxidative phosphorylation).
iii	Understand the role of oxygen as a terminal electron acceptor forming water.
iv	Understand how ATP is synthesised by chemiosmosis.
v	Understand the importance of mitochondrial membranes in this process.

5.5 Anaerobic respiration

	Students should:
i	Know that anaerobic respiration is the partial breakdown of hexoses to produce a limited yield of ATP in the absence of oxygen.
ii	Understand the difference in ATP yields from one molecule of hexose sugar in aerobic conditions compared with anaerobic conditions.
iii	Understand how lactate as a by-product of anaerobic respiration affects mammalian muscle contraction.
iv	Understand how anaerobic respiration in plants results in ethanol formation. CORE PRACTICAL 9: Investigate factors affecting the rate of aerobic or anaerobic respiration using a respirometer, taking into account the safe and ethical use of organisms.

5.6 Photosynthetic pigments

	Students should:
i	Understand what is meant by absorption and action spectra. CORE PRACTICAL 10: Investigate the effects of different wavelengths of light on the rate of photosynthesis.
ii	Understand why many plants have a variety of different photosynthetic pigments. CORE PRACTICAL 11: Investigate the presence of different chloroplast pigments using chromatography.

5.7 Photosynthesis

	Students should:
i	Know the structure of chloroplasts, including: envelope, stroma, grana and lamellar structure.
ii	Light-dependent stage: Understand the role of the thylakoid membranes in the light-dependent stage of photosynthesis.
iii	Understand that the processes of cyclic and non-cyclic photophosphorylation result in the production of reduced NADP, ATP and oxygen.
iv	Light-independent stage: Understand the role of the stroma in the light-independent stage of photosynthesis.
v	Understand how carbon dioxide is fixed by combination with 5C ribulose bisphosphate (RuBP) to form glycerate 3-phosphate (GP) using the enzyme ribulose bisphosphate carboxylase (RUBISCO).
vi	Understand how reduced NADP and ATP from the light-dependent stage are used: • to synthesise glyceraldehyde phosphate (GALP) from GP • to regenerate 5C ribulose bisphosphate in the Calvin cycle (details of intermediate compounds are not required).
vii	Understand how GALP is used as a raw material in the production of monosaccharides, amino acids and other molecules.
viii	Understand the factors that limit photosynthesis including carbon dioxide, light intensity and temperature.

Topic 6: Microbiology and Pathogens

This topic builds on knowledge of prokaryotes, eukaryotes, viruses and transport systems. It considers how some microorganisms act as pathogens. Details of how the human body responds to infection are included. The social, economic and ethical implications of the methods of treatment and control of the spread of infection are discussed. Microbial techniques are used in the isolation of bacteria and the investigation of the factors that affect their rate of growth.

Opportunities for developing mathematical skills within this topic include: recognising and making use of appropriate units in calculations; recognising and using expressions in decimal and standard form; using ratios, fractions and percentages; estimating results; finding and using power, exponential and logarithmic functions; using an appropriate number of significant figures; finding arithmetic means; using logarithms in relation to quantities that range over several orders of magnitude; translating information between graphical, numerical and algebraic forms; plotting two variables from experimental data; determining the intercept of a graph; drawing and using the slope of a tangent to a curve as a measure of rate of change. (Please see Appendix 6: Mathematical skills and exemplifications for further information.)

6.1 Microbial techniques

	Students should:
i	Understand the basic aseptic techniques used in culturing organisms.
ii	Understand the principles and techniques involved in culturing microorganisms.
iii	Understand the use of different media (broth cultures, agar and selective media).
iv	Understand the different methods of measuring the growth of a bacterial culture as illustrated by cell counts, dilution plating, mass and optical methods (turbidity).
v	Understand the different phases of a bacterial growth curve (lag phase, log phase, stationary phase and death phase) and calculate exponential growth rate constants. CORE PRACTICAL 12: Investigate the rate of growth of bacteria in liquid culture taking into account the safe and ethical use of organisms. CORE PRACTICAL 13: Isolate individual species from a mixed culture of bacteria using streak plating taking into account the safe and ethical use of organisms.

6.2 Bacteria as pathogens

	Students should:
i	Understand that bacteria can be agents of infection, invading and destroying host tissues and producing toxins.
ii	Understand that pathogenic effects can be produced by exotoxins (Staphylococcus spp.), endotoxins (Salmonella spp.) and invasion of host tissue (Mycobacterium tuberculosis).

6.3 Action of antibiotics

	Students should:
i	Understand the action of bactericidal and bacteriostatic antibiotics, as illustrated by penicillin and tetracycline.

6.4 Antibiotic resistance

	Students should:
i	Understand the development and spread of antibiotic resistance in bacteria.
ii	Understand the methods and difficulties of controlling the spread of antibiotic resistance in bacteria.

6.5 Other pathogenic agents

	Students should:
i	Understand the transmission, mode of infection and pathogenic effect of the following: • stem rust fungus on cereal crops (Puccinia graminis on wheat) • influenza virus • the malarial parasite (Plasmodium spp.). (Detailed life cycles are not required.)

6.6 Problems of controlling endemic diseases

	Students should:
i	Understand the social and economic and ethical implications of different control methods for endemic malaria and the role of the scientific community in validating these methods.

6.7 Response to infection

	Students should:
i	Know the mode of action of macrophages, neutrophils and lymphocytes.
ii	Understand the development of the humoral immune response, including the role of: • antigen presenting T cells • T helper cells and cytokines • B cells • clonal selection • plasma cells • antibodies.
iii	Understand the development of the cell-mediated immune response including the role of: • antigen presenting cells • T helper cells and cytokines • T killer cells.
iv	Understand the role of T and B memory cells in the secondary immune response.
v	Understand how immunity can be natural or artificial, and active or passive.
vi	Understand how vaccination can be used in the control of disease and the development of herd immunity.
vii	Understand the potential issues in populations where a proportion choose not to vaccinate.

Topic 7: Modern Genetics

This topic builds on the knowledge of nucleic acids and proteins and how they are involved in gene expression. Knowledge of epigenetics, the use of stem cells and how these are contributing to medical advances are considered. The ethical implications of the use of gene technology in scientific developments are discussed.

Opportunities for developing mathematical skills within this topic include: recognising and using expressions in decimal and standard form; using an appropriate number of significant figures. (Please see Appendix 6: Mathematical skills and exemplifications for further information.)

7.1 Using gene sequencing

	Students should:
i	Understand what is meant by the term genome.
ii	Understand how PCR can be used to amplify DNA samples, and how these samples can be used: • to predict the amino acid sequence of proteins and possible links to genetically determined conditions, using gene sequencing • in forensic science, to identify criminals and to test paternity, using DNA profiling.

7.2 Factors affecting gene expression

	Students should:
i	Know that transcription factors are proteins that bind to DNA.
ii	Understand the role of transcription factors in regulating gene expression.
iii	Understand how post-transcriptional modification of mRNA in eukaryotic cells (RNA splicing) can result in different products from a single gene.
iv	Understand that gene expression can be changed by epigenetic modification, including non-coding RNA, histone modification and DNA methylation.
v	Know that epigenetic modification is important in ensuring cell differentiation.

7.3 Stem cells

	Students should:
i	Understand what is meant by a stem cell, including the differences between totipotent, pluripotent and multipotent stem cells.
ii	Understand that pluripotent stem cells from embryos provide opportunities to develop new medical advances although there are ethical considerations.
iii	Understand how epigenetic modifications can result in totipotent stem cells in the embryo developing into pluripotent cells in the blastocyst and finally into fully differentiated somatic cells.
iv	Understand how differentiated fibroblasts can be reprogrammed to form induced pluripotent stem cells (iPS cells) by the artificial introduction of named genes.
v	Understand why the use of iPS stem cells may be less problematic than the use of embryonic stem cells.

7.4 Gene technology

	Students should:
i	Understand how recombinant DNA can be produced, including the role of restriction endonucleases and DNA ligase.
ii	Understand how recombinant DNA can be inserted into other cells, and the use of various vectors such as viruses and gene guns.
iii	Understand how antibiotic resistance marker genes and replica plating are used to identify recombinant cells.
iv	Understand how 'knockout' mice can be used as a valuable animal model to investigate gene function.
v	Understand the process of genetic modification of soya beans and how it has been used to improve production, including altering the balance of fatty acids to prevent oxidation of soya products.
vi	Understand why the widespread use of genetic modification of major commercial crops and other transgenic processes have caused public debate of their advantages and disadvantages.

Topic 8: Origins of Genetic Variation

This topic builds on the knowledge of meiosis and natural selection. It considers the dihybrid inheritance of alleles and genes. The inheritance of unlinked and linked genes is studied. The effect of selection pressures on the allele frequencies in gene pools and their impact on speciation are discussed.

Opportunities for developing mathematical skills within this topic include: using ratios, fractions and percentages; understanding simple probability; using Chi squared tests; understanding measures of dispersion, including standard deviation and range; solving algebraic equations; the analysis of allele frequencies using the Hardy-Weinberg equation. (Please see Appendix 6: Mathematical skills and exemplifications for further information.)

8.1 Origins of genetic variation

	Students should:
i	Understand that mutations are the source of new variations and that the processes of random assortment and crossing over during meiosis give rise to new combinations of alleles in gametes.
ii	Understand how random fertilisation during sexual reproduction brings about genetic variation.

8.2 Transfer of genetic information

	Students should:
i	Understand the terms 'genotype and phenotype', 'homozygote and heterozygote', 'dominance', 'recessive', 'codominance' and 'multiple alleles'.
ii	Be able to construct genetic crosses and pedigree diagrams.
iii	Understand the inheritance of two non-interacting unlinked genes.
iv	Understand that autosomal linkage results from the presence of alleles on the same chromosome and that the results of crosses can be explained by the events of meiosis, including black/grey body and long/vestigial wing in Drosophila.
v	Understand sex linkage on the X chromosome, including haemophilia in humans.
vi	Be able to use chi squared tests to test the significance of the difference between observed and expected results.

8.3 Gene pools

	Students should:
i	Understand that selection pressures acting on the gene pool change allele frequencies in the population, including: • stabilising selection maintaining continuity in a population • disruptive selection leading to changes or speciation.
ii	Understand that sometimes changes in allele frequencies can be the result of chance and not selection, including genetic drift.
iii	Understand that allele frequencies can be influenced by: • population bottlenecks • founder effect.
iv	Understand how the Hardy-Weinberg equation can be used to monitor changes in the allele frequencies in a population.

Topic 9: Control Systems

This topic builds on knowledge of transport mechanisms and considers the processes of chemical and nervous coordination. Details of the role of plant growth substances and hormonal control in mammals are included. Osmoregulation in mammals adapted to dry environments is considered. Details of the effect of drugs on the transmission of nerve impulses are studied. Practical skills are developed in the investigation of the effect of gibberellin on germination.

Opportunities for developing mathematical skills within this topic include: making use of appropriate units in calculations; using expressions in decimal and standard form; using ratios, fractions and percentages; interpreting frequency tables and diagrams, bar charts and histograms; understanding simple probability; translating information between graphical, numerical and algebraic forms; plotting variables from experimental data; understanding that y = mx + c represents a linear relationship; determining the intercept of a graph; calculating rate of change from a graph; using the slope of a tangent to a curve as a measure of rate of change. (Please see Appendix 6: Mathematical skills and exemplifications for further information.)

9.1 Homeostasis

	Students should:
i	Know that homeostasis is the maintenance of a state of dynamic equilibrium.
ii	Understand the importance of maintaining pH, temperature and water potential in the body.
iii	Understand what is meant by negative feedback and positive feedback control.

9.2 Chemical control in mammals

	Students should:
i	Understand the principles of mammalian hormone production by endocrine glands and their mode of action involving receptors on target cells.
ii	Know that there are two main modes of action in hormones: • hormones attach to receptor sites and trigger the release of a second messenger that activates specific enzymes in the cell, including adrenaline • hormones enter cells and bind directly to transcription factors, including oestrogen.

9.3 Chemical control in plants

	Students should:
i	Understand that chemical control in plants is brought about by plant growth substances such as auxins, cytokinins and gibberellins. CORE PRACTICAL 14: Investigate the effect of gibberellin on the production of amylase in germinating cereals using a starch agar assay.
ii	Know that auxin has several effects, including cell elongation, suppression of lateral buds (apical dominance) and promoting root growth.
iii	Understand that plant growth substances often interact with each other as shown by the antagonistic actions of cytokinin and auxin on apical dominance.
iv	Understand how phytochrome controls flowering and photomorphogenesis.

9.4 Structure and function of the mammalian nervous system

	Students should:
i	Know that the mammalian nervous system is composed of the central and peripheral nervous systems.
ii	Know the structure of the spinal cord.
iii	Know the location and main functions of: • the medulla oblongata – controls breathing and heart rate • cerebellum – controls balance and coordination of movement • cerebrum – initiates movement • hypothalamus – temperature regulation and osmoregulation.
iv	Know that the peripheral nervous system is divided into autonomic and voluntary systems.
v	Understand why the autonomic nervous system is divided into sympathetic and parasympathetic systems, which act antagonistically.

9.5 Nervous transmission

	Students should:
i	Understand how the properties of the axon membrane and the transport of Na⁺ ions and K⁺ ions result in a resting potential.
ii	Understand how an action potential is formed and how it is propagated along an axon.
iii	Understand why the speed of transmission along myelinated axons is greater than along non-myelinated axons, including the role of saltatory conduction.
iv	Understand the structure and function of a synapse, including the role of transmitter substances limited to acetylcholine and noradrenaline.
v	Understand the formation and effects of excitatory and inhibitory postsynaptic potentials.

9.6 Effects of drugs on the nervous system

	Students should:
i	Understand how the effects of drugs can be caused by their influence on synaptic transmission, including: • nicotine (mimicking effects of acetylcholine) • lidocaine (blocking voltage gated Na⁺ ion channels) • cobra venom (blocking acetylcholine receptors).

9.7 Detection of light by mammals

	Students should:
i	Know the structure of the human retina.
ii	Understand the role of the rhodopsin in initiating action potentials.
iii	Understand how the distribution of human rod and cone cells maintain vision in different light intensities.

9.8 Control of heart rate in mammals

	Students should:
i	Understand how the autonomic nervous system controls heart rate including: • aortic and carotid baroreceptors and chemoreceptors • cardiac centre in the medulla oblongata • sympathetic nerve stimulated to release noradrenaline at the SAN • parasympathetic nerve stimulated to release acetylcholine at the SAN.
ii	Understand the role of the autonomic nervous system in causing the release of adrenaline to increase heart rate.

9.9 Osmoregulation and temperature regulation

	Students should:
i	Know the gross and microscopic structure of the mammalian kidney.
ii	Understand how urea is produced in the liver from excess amino acids (details of the ornithine cycle are not required) and how it is removed from the bloodstream by ultrafiltration.
iii	Understand how solutes are selectively reabsorbed in the proximal tubule and how the Loop of Henle acts as a counter-current multiplier to increase the reabsorption of water.
iv	Understand how the pituitary gland and osmoreceptors in the hypothalamus, combined with the action of antidiuretic hormone (ADH) bring about negative feedback control of mammalian plasma concentration.
v	Understand how the kidney of a kangaroo rat (Dipodomys sp.) is adapted for life in a dry environment.
vi	Understand that an endotherm is able to produce heat through metabolic processes but an ectotherm must rely on the external environment.
vii	Understand how an endotherm is able to regulate its temperature through behaviour, and also physiologically through the autonomic nervous system, including the role of thermoreceptors, hypothalamus and the skin.

Topic 10: Ecosystems

This topic considers the interactions between the organisms and the environment within an ecosystem. It includes details of how biotic and abiotic factors are involved in the development of ecosystems over time. Human influences on ecosystems are also discussed along with the need for conservation.

Opportunities for developing mathematical skills within this topic include: estimating results; constructing and interpreting frequency tables and diagrams, bar charts and histograms; understanding principles of sampling, including use of a formula to calculate an index of diversity; understanding the terms mean, median and mode; selecting and using statistical tests, including the Chi squared test, Student's t-test of difference, Spearman's rank, correlation coefficient; standard deviation and range. (Please see Appendix 6: Mathematical skills and exemplifications for further information.)

10.1 The nature of ecosystems

	Students should:
i	Understand what is meant by the term ecosystem and that they range in size.
ii	Understand what is meant by trophic levels.
iii	Understand the advantages and disadvantages of pyramids of numbers, biomass (dry) and energy as useful representations of ecosystem structure and how biomass and energy are transferred within them.
iv	Know the ecological techniques used to assess abundance and distribution of organisms in a natural habitat, including types of quadrat, transects, ACFOR scales, percentage cover and individual counts.
v	Be able to select appropriate ecological techniques according to the ecosystem and organisms to be studied. CORE PRACTICAL 15: Investigate the effect of different sampling methods on estimates of the size of a population taking into account the safe and ethical use of organisms.
vi	Be able to use statistical tests to analyse data, including t-test and Spearman rank correlation coefficient.

10.2 Energy transfer through ecosystems

	Students should:
i	Understand how energy is transferred between trophic levels using the terms 'net primary productivity' and 'gross primary productivity'.
ii	Be able to calculate the efficiency of energy transfer between different trophic levels and account for the loss of energy at each level.
iii	Understand the significance of microorganisms in the recycling of nutrients within an ecosystem.

10.3 Changes in ecosystems

	Students should:
i	Understand how ecosystems can develop over time, including use of the terms colonisation and succession and types of climax communities.
ii	Understand the effects of biotic and abiotic factors on population size. CORE PRACTICAL 16: Investigate the effect of one abiotic factor on the distribution or morphology of one species taking into account the safe and ethical use of organisms.

10.4 Human effects on ecosystems

	Students should:
i	Understand data relating to human influences on ecosystems, including climate change and depletion of biological resources, including overfishing.
ii	Understand the effect that treaties such as CITES have had on global biodiversity.
iii	Understand the idea that sustainability of resources depends on effective management of the conflict between human needs and conservation, as illustrated by attempts to conserve fish stocks and reduce possible causes of climate change.
iv	Understand the role of the scientific journals, the peer review process and scientific conferences in validating evidence related to the debate about climate change.