Energy is a fundamental concept in biology. All living organisms require a source of cellular energy to drive their various activities. ATP is the universal energy currency as its molecules are small, soluble and easily hydrolysed to release energy for cellular activities. All organisms respire to release energy from energy-rich molecules such as glucose and fatty acids and transfer that energy to ATP. Respiration is a series of enzyme-catalysed reactions that release energy in small ‘packets’. In eukaryotes, aerobic respiration occurs in mitochondria. Candidates will be expected to use the knowledge gained in this section to solve problems in familiar and unfamiliar contexts.

12.1 Energy ATP is the universal energy currency as it provides the immediate source of energy for cellular processes.

a) outline the need for energy in living organisms, as illustrated by anabolic reactions, such as DNA replication and protein synthesis, active transport, movement and the maintenance of body temperature

b) describe the features of ATP that make it suitable as the universal energy currency

c) explain that ATP is synthesised in substrate-linked reactions in glycolysis and in the Krebs cycle

d) outline the roles of the coenzymes NAD, FAD and coenzyme A in respiration

e) explain that the synthesis of ATP is associated with the electron transport chain on the membranes of mitochondria and chloroplasts

f) explain the relative energy values of carbohydrate, lipid and protein as respiratory substrates and explain why lipids are particularly energy-rich

g) define the term respiratory quotient (RQ) and determine RQs from equations for respiration

h) carry out investigations, using simple respirometers, to determine the RQ of germinating seeds or small invertebrates (e.g. blowfly larvae)

12.2 Respiration Respiration is the process whereby energy from complex organic molecules is transferred to ATP.

a) list the four stages in aerobic respiration (glycolysis, link reaction, Krebs cycle and oxidative phosphorylation) and state where each occurs in eukaryotic cells

b) outline glycolysis as phosphorylation of glucose and the subsequent splitting of fructose 1,6-bisphosphate (6C) into two triose phosphate molecules, which are then further oxidised to pyruvate with a small yield of ATP and reduced NAD

c) explain that, when oxygen is available, pyruvate is converted into acetyl (2C) coenzyme A in the link reaction

d) outline the Krebs cycle, explaining that oxaloacetate (a 4C compound) acts as an acceptor of the 2C fragment from acetyl coenzyme A to form citrate (a 6C compound), which is reconverted to oxaloacetate in a series of small steps

e) explain that reactions in the Krebs cycle involve decarboxylation and dehydrogenation and the reduction of NAD and FAD

f) outline the process of oxidative phosphorylation including the role of oxygen as the final electron acceptor (no details of the carriers are required)

This process of ATP synthesis using the energy in proton gradients is common to both respiration and photosynthesis.

g) explain that during oxidative phosphorylation:

• energetic electrons release energy as they pass through the electron transport system

• the released energy is used to transfer protons across the inner mitochondrial membrane

• protons return to the mitochondrial matrix by facilitated diffusion through ATP synthase providing energy for ATP synthesis (details of ATP synthase are not required)

h) carry out investigations to determine the effect of factors such as temperature and substrate concentration on the rate of respiration of yeast using a redox indicator (e.g. DCPIP or methylene blue)

i) describe the relationship between structure and function of the mitochondrion using diagrams and electron micrographs

Some organisms and some tissues are able to respire in both aerobic and anaerobic conditions. When yeast and plants respire under anaerobic conditions, they produce ethanol and carbon dioxide as end-products; mammalian muscle tissue produces lactate when oxygen is in short supply.

j) distinguish between respiration in aerobic and anaerobic conditions in mammalian tissue and in yeast cells, contrasting the relative energy released by each (a detailed account of the total yield of ATP from the aerobic respiration of glucose is not required)

k) explain the production of a small yield of ATP from respiration in anaerobic conditions in yeast and in mammalian muscle tissue, including the concept of oxygen debt

l) explain how rice is adapted to grow with its roots submerged in water in terms of tolerance to ethanol from respiration in anaerobic conditions and the presence of aerenchyma

m) carry out investigations, using simple respirometers, to measure the effect of temperature on the respiration rate of germinating seeds or small invertebrates

Cells function most efficiently if they are kept in near constant conditions. Cells in multicellular animals are surrounded by tissue fluid. The composition, pH and temperature of tissue fluid are kept constant by exchanges with the blood as discussed in the section on Transport in mammals. In mammals, core temperature, blood glucose concentration and blood water potential are maintained within narrow limits to ensure the efficient operation of cells. Prior knowledge for this section includes an understanding that waste products are excreted from the body – a role that is fulfilled by the kidneys – and an outline of the structure and function of the nervous and endocrine systems. In plants, guard cells respond to fluctuations in environmental conditions and open and close stomata as appropriate for photosynthesis and conserving water.

Candidates will be expected to use the knowledge gained in this section to solve problems in familiar and unfamiliar contexts.

14.1 Homeostasis in mammals Homeostasis in mammals requires complex systems to maintain internal conditions near constant. The kidneys remove wastes from the blood and are the effectors for controlling the water potential of the blood.

a) discuss the importance of homeostasis in mammals and explain the principles of homeostasis in terms of internal and external stimuli, receptors, central control, co-ordination systems and effectors (muscles and glands)

b) define the term negative feedback and explain how it is involved in homeostatic mechanisms

c) outline the roles of the nervous system and endocrine system in co-ordinating homeostatic mechanisms, including thermoregulation, osmoregulation and the control of blood glucose concentration

d) describe the deamination of amino acids and outline the formation of urea in the urea cycle (biochemical details of the urea cycle are not required)

e) describe the gross structure of the kidney and the detailed structure of the nephron with its associated blood vessels using photomicrographs and electron micrographs

f) describe how the processes of ultrafiltration and selective reabsorption are involved with the formation of urine in the nephron

g) describe the roles of the hypothalamus, posterior pituitary gland, ADH and collecting ducts in osmoregulation

h) explain how the blood glucose concentration is regulated by negative feedback control mechanisms, with reference to insulin and glucagon

i) outline the role of cyclic AMP as a second messenger with reference to the stimulation of liver cells by adrenaline and glucagon j) describe the three main stages of cell signalling in the control of blood glucose by adrenaline as follows:

• hormone-receptor interaction at the cell surface (see 4.1c)

• formation of cyclic AMP which binds to kinase proteins

• an enzyme cascade involving activation of enzymes by phosphorylation to amplify the signal

k) explain the principles of operation of dip sticks containing glucose oxidase and peroxidase enzymes, and biosensors that can be used for quantitative measurements of glucose in blood and urine

l) explain how urine analysis is used in diagnosis with reference to glucose, protein and ketones.

14.2 Homeostasis in plants Stomatal aperture is regulated in response to the requirements for uptake of carbon dioxide for photosynthesis and conserving water.

a) explain that stomata have daily rhythms of opening and closing and also respond to changes in environmental conditions to allow diffusion of carbon dioxide and regulate water loss by transpiration

b) describe the structure and function of guard cells and explain the mechanism by which they open and close stomata

c) describe the role of abscisic acid in the closure of stomata during times of water stress (the role of calcium ions as a second messenger should be emphasised)

Genetic information is transmitted from generation to generation to maintain the continuity of life. In sexual reproduction, meiosis introduces genetic variation so that offspring resemble their parents but are not identical to them. Genetic crosses reveal how some features are inherited. The phenotype of organisms is determined partly by the genes they have inherited and partly by the effect of the environment. Genes determine how organisms develop and gene control in bacteria gives us a glimpse of this process in action.

Candidates will be expected to use the knowledge gained in this section to solve problems in familiar and unfamiliar contexts.

16.1 Passage of information from parent to offspring

Diploid organisms contain pairs of homologous chromosomes. The behaviour of maternal and paternal chromosomes during meiosis generates much variation amongst individuals of the next generation.

a) explain what is meant by homologous pairs of chromosomes

b) explain the meanings of the terms haploid and diploid and the need for a reduction division (meiosis) prior to fertilisation in sexual reproduction

c) outline the role of meiosis in gametogenesis in humans and in the formation of pollen grains and embryo sacs in flowering plants

d) describe, with the aid of photomicrographs and diagrams, the behaviour of chromosomes in plant and animal cells during meiosis, and the associated behaviour of the nuclear envelope, cell surface membrane and the spindle (names of the main stages are expected, but not the sub-divisions of prophase)

e) explain how crossing over and random assortment of homologous chromosomes during meiosis and random fusion of gametes at fertilisation lead to genetic variation including the expression of rare, recessive alleles.

16.2 The roles of genes in determining the phenotype

Patterns of inheritance are explained by using genetic diagrams. The results of genetic crosses are analysed statistically using the chi-squared test.

(Studies of human genetic conditions have revealed the links between genes, enzymes and the phenotype.)

a) explain the terms gene, locus, allele, dominant, recessive, codominant, linkage, test cross, F1 and F2, phenotype, genotype, homozygous and heterozygous

b) use genetic diagrams to solve problems involving monohybrid and dihybrid crosses, including those involving autosomal linkage, sex linkage, codominance, multiple alleles and gene interactions (the term epistasis does not need to be used; knowledge of the expected ratio for various types of epistasis is not required. The focus is on problem solving)

c) use genetic diagrams to solve problems involving test crosses

d) use the chi-squared test to test the significance of differences between observed and expected results (the formula for the chi-squared test will be provided) (see Mathematical requirements)

e) explain that gene mutation occurs by substitution, deletion and insertion of base pairs in DNA and outline how such mutations may affect the phenotype

f) outline the effects of mutant alleles on the phenotype in the following human conditions: albinism, sickle cell anaemia, haemophilia and Huntington’s disease

g) explain the relationship between genes, enzymes and phenotype with respect to the gene for tyrosinase that is involved with the production of melanin

16.3 Gene control

Some genes are transcribed all the time to produce constitutive proteins; others are only ‘switched on’ when their protein products are required.

a) distinguish between structural and regulatory genes and between repressible and inducible enzymes

b) explain genetic control of protein production in a prokaryote using the lac operon

c) explain the function of transcription factors in gene expression in eukaryotes

d) explain how gibberellin activates genes by causing the breakdown of DELLA protein repressors, which normally inhibit factors that promote transcription

Charles Darwin and Alfred Russel Wallace proposed a theory of natural selection to account for the evolution of species in 1858. A year later, Darwin published On the Origin of Species providing evidence for the way in which aspects of the environment act as agents of selection and determine which variants survive and which do not. The individuals best adapted to the prevailing conditions succeed in the ‘struggle for existence’.

Candidates will be expected to use the knowledge gained in this section to solve problems in familiar and unfamiliar contexts

17.1 Variation

The variation that exists within a species is categorised as continuous and discontinuous. The environment has considerable influence on the expression of features that show continuous (or quantitative) variation.

a) describe the differences between continuous and discontinuous variation and explain the genetic basis of continuous (many, additive genes control a characteristic) and discontinuous variation (one or few genes control a characteristic) (examples from 16.2f may be used to illustrate discontinuous variation; height and mass may be used as examples of continuous variation)

b) explain, with examples, how the environment may affect the phenotype of plants and animals

c) use the t-test to compare the variation of two different populations (the formula for the t-test will be provided) (see Mathematical requirements)

d) explain why genetic variation is important in selection

17.2 Natural and artificial selection

Populations of organisms have the potential to produce large numbers of offspring, yet their numbers remain fairly constant year after year.

(Humans use selective breeding (artificial selection) to improve features in ornamental plants, crop plants, domesticated animals and livestock.)

a) explain that natural selection occurs as populations have the capacity to produce many offspring that compete for resources; in the ‘struggle for existence’ individuals that are best adapted are most likely to survive to breed and pass on their alleles to the next generation

b) explain, with examples, how environmental factors can act as stabilising, disruptive and directional forces of natural selection

c) explain how selection, the founder effect and genetic drift may affect allele frequencies in populations

d) use the Hardy–Weinberg principle to calculate allele, genotype and phenotype frequencies in populations and explain situations when this principle does not apply

e) describe how selective breeding (artificial selection) has been used to improve the milk yield of dairy cattle

f) outline the following examples of crop improvement by selective breeding:

• the introduction of disease resistance to varieties of wheat and rice

• the incorporation of mutant alleles for gibberellin synthesis into dwarf varieties so increasing yield by having a greater proportion of energy put into grain

• inbreeding and hybridisation to produce vigorous, uniform varieties of maize

17.3 Evolution

Isolating mechanisms can lead to the accumulation of different genetic information in populations, potentially leading to new species. Over prolonged periods of time, some species have remained virtually unchanged, others have changed significantly and many have become extinct

a) state the general theory of evolution that organisms have changed over time

b) discuss the molecular evidence that reveals similarities between closely related organisms with reference to mitochondrial DNA and protein sequence data

c) explain how speciation may occur as a result of geographical separation (allopatric speciation), and ecological and behavioural separation (sympatric speciation)

d) explain the role of pre-zygotic and post-zygotic isolating mechanisms in the evolution of new species

e) explain why organisms become extinct, with reference to climate change, competition, habitat loss and killing by humans

The biodiversity of the Earth is threatened by human activities and climate change. Classification systems attempt to put order on the chaos of all the organisms that exist on Earth. Fieldwork is an important part of a biological education to appreciate this diversity and find out how to analyse it. There are opportunities in this section for candidates to observe different species in their locality and assess species distribution and abundance. Conserving biodiversity is a difficult task but is achieved by individuals, local groups, national and international organisations. Candidates should appreciate the threats to biodiversity and consider the steps taken in conservation, both locally and globally.

Candidates will be expected to use the knowledge gained in this section to solve problems in familiar and unfamiliar contexts.

18.1 Biodiversity

Biodiversity is much more than a list of all the species in a particular area.

a) define the terms species, ecosystem and niche

b) explain that biodiversity is considered at three different levels:

• variation in ecosystems or habitats

• the number of species and their relative abundance

• genetic variation within each species

c) explain the importance of random sampling in determining the biodiversity of an area

d) use suitable methods, such as frame quadrats, line transects, belt transects and mark-release-recapture, to assess the distribution and abundance of organisms in a local area

e) use Spearman’s rank correlation and Pearson’s linear correlation to analyse the relationships between the distribution and abundance of species and abiotic or biotic factors (the formula for these correlations will be provided) (see Mathematical requirements)

f) use Simpson’s Index of Diversity (D) to calculate the biodiversity of a habitat, using the formula and state the significance of different values of D.

18.2 Classification

Organisms studied locally may be used to show how hierarchical classification systems are organised.

a) describe the classification of species into the taxonomic hierarchy of domain, kingdom, phylum, class, order, family, genus and species

b) outline the characteristic features of the three domains Archaea, Bacteria and Eukarya

c) outline the characteristic features of the kingdoms Protoctista, Fungi, Plantae and Animalia

d) explain why viruses are not included in the three domain classification and outline how they are classified, limited to types of nucleic acid (RNA or DNA) and whether these are single stranded or double stranded

18.3 Conservation

Maintaining biodiversity is important for many reasons. Actions to maintain biodiversity must be taken at local, national and global levels. It is important to conserve ecosystems as well as individual species.

a) discuss the threats to the biodiversity of aquatic and terrestrial ecosystems (see 18.1b)

b) discuss the reasons for the need to maintain biodiversity

c) discuss methods of protecting endangered species, including the roles of zoos, botanic gardens, conserved areas (national parks and marine parks), ‘frozen zoos’ and seed banks

d) discuss methods of assisted reproduction, including IVF, embryo transfer and surrogacy, used in the conservation of endangered mammals

e) discuss the use of culling and contraceptive methods to prevent overpopulation of protected and non-protected species

f) use examples to explain the reasons for controlling alien species

g) discuss the roles of non-governmental organisations, such as the World Wide Fund for Nature (WWF) and the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), in local and global conservation

h) outline how degraded habitats may be restored with reference to local or regional examples