AQA GCSE Biology Paper 2: Complete Revision Guide (Topics 4.5 to 4.7)

This is a complete revision guide for AQA GCSE Biology Paper 2, covering all three topics in the specification: Homeostasis and Response, Inheritance and Evolution, and Ecology. It applies to both Triple Science (8461) and Combined Science (8464). Content labelled Higher Tier only is for Higher students. Foundation students can skip those sections.

Do not read this passively. After each section, cover the page and try to recall the key points from memory. Then practise with past paper questions. Active recall is what moves content into long-term memory.

Topic 4.5: Homeostasis and Response

What is homeostasis?

Homeostasis is the regulation of internal conditions to maintain a stable environment inside the body, despite changes in external conditions. The body regulates body temperature (around 37 degrees Celsius), blood glucose concentration and water balance. This is essential because enzymes only work efficiently within narrow conditions, and any significant deviation can disrupt vital processes.

All control systems in the body share the same three-part structure:

• Receptors: detect a change (stimulus)
• Coordination centre: processes the information (brain, spinal cord or pancreas)
• Effectors: muscles or glands that produce the response

The nervous system

The nervous system enables fast, short-lived responses to stimuli. Nerve impulses travel along neurones.

A reflex arc is automatic, fast and protective. It does not involve the conscious brain. The relay neurone is in the spinal cord, which is why reflex responses happen before the brain registers pain.

A synapse is the gap between two neurones. Neurotransmitters carry the signal across the gap. This slightly slows the signal but ensures it travels in one direction only.

Required Practical 7: reaction time

Aim: investigate the effect of a factor on human reaction time using the ruler drop test.

• A partner holds a ruler vertically. The student positions their fingers at the zero mark without touching it.
• The partner drops the ruler without warning. The student catches it.
• Measure the distance fallen before catching. Repeat multiple times and calculate the mean.

Independent variable: the factor being tested (caffeine, distraction, practice). Dependent variable: reaction time (calculated from the distance fallen). Control variables: same ruler, same person position, same drop method.

The brain (Higher Tier only)

The brain controls complex behaviour. Three key regions: the cerebral cortex controls thinking, memory and intelligence; the cerebellum controls balance and coordination; the medulla controls heart rate and breathing. Neuroscientists study brain function using MRI scanning, electrical stimulation of brain regions, and studying the effects of brain injuries. The brain is complex and delicate, which makes it difficult to treat safely.

The eye

• Cornea: refracts light into the eye
• Iris: controls the size of the pupil
• Lens: focuses light onto the retina
• Ciliary muscles: change the shape of the lens
• Suspensory ligaments: hold the lens in place
• Retina: contains light receptor cells
• Optic nerve: carries electrical signals to the brain

Accommodation is the process of focusing on objects at different distances. For a near object, ciliary muscles contract, suspensory ligaments loosen, and the lens becomes thicker and more curved. For a distant object, ciliary muscles relax, ligaments tighten, and the lens becomes thinner and flatter.

Myopia (short-sightedness): the image focuses in front of the retina. Corrected with a concave lens. Hyperopia (long-sightedness): the image focuses behind the retina. Corrected with a convex lens. Rods are sensitive in low light. Cones detect colour.

Body temperature control

The thermoregulatory centre in the brain monitors and controls body temperature. When too hot: vasodilation (blood vessels near the skin widen, increasing heat loss), sweat is produced and evaporates removing heat. When too cold: vasoconstriction (blood vessels near the skin narrow, reducing heat loss), sweating stops, and shivering generates heat through muscle contractions.

Hormonal coordination

Hormones are chemical messengers secreted by glands and carried in the blood to target organs. Compared to the nervous system, hormones are slower to act, longer-lasting, and travel in the blood rather than along neurones.

Key glands: pituitary gland (controls other glands), pancreas, thyroid, adrenal glands, ovaries and testes.

Blood glucose control

When blood glucose is too high, the pancreas releases insulin. Insulin causes cells to take up glucose and the liver to convert glucose into glycogen for storage. When blood glucose is too low, the pancreas releases glucagon. Glucagon causes the liver to convert glycogen back into glucose, releasing it into the blood.

Water balance: ADH

Water balance is controlled by the pituitary gland, which releases ADH (antidiuretic hormone). ADH acts on the kidneys to control water reabsorption. When the body has too little water, more ADH is released and more water is reabsorbed by the kidneys, producing concentrated urine. When the body has too much water, less ADH is released and less water is reabsorbed, producing dilute urine.

Menstrual cycle and contraception

Four hormones control the menstrual cycle. FSH stimulates egg maturation in the ovaries. LH triggers ovulation. Oestrogen rebuilds the uterus lining. Progesterone maintains the uterus lining.

Hormonal contraception works by disrupting these hormone levels. The pill prevents FSH release, stopping egg maturation. Implants and injections prevent ovulation. Barrier methods such as condoms physically block sperm. IUDs prevent implantation. Sterilisation is a permanent surgical option.

Fertility treatment (Higher Tier only)

Fertility drugs containing FSH and LH can be given to stimulate egg maturation and ovulation, increasing the chance of natural conception. IVF (in vitro fertilisation) involves giving the woman FSH and LH to stimulate the production of multiple eggs, collecting those eggs, fertilising them with sperm in the lab, allowing the embryos to develop for a few days, then transferring one or two embryos into the uterus.

IVF advantages: helps infertile couples have children, allows genetic screening of embryos before transfer. Disadvantages: emotionally stressful, expensive, low success rate, and risk of multiple births from implanting more than one embryo.

Plant hormones (Triple Science only)

Auxin controls the direction of plant growth (tropisms). In phototropism, auxin moves to the shaded side of a shoot. Cells on the shaded side grow faster, causing the shoot to bend towards the light. In gravitropism, shoots grow upward (negative gravitropism) and roots grow downward (positive gravitropism).

Gibberellins (Higher Tier only) stimulate seed germination, increase stem elongation, promote flowering and increase fruit size. Ethene (Higher Tier only) is a gas hormone that controls fruit ripening and is used commercially to ripen fruit during transport.

Agricultural uses of plant hormones: auxins are used in weedkillers to kill broad-leaf weeds and in rooting powder for cuttings. Gibberellins increase fruit size and break seed dormancy. Ethene controls ripening of bananas and tomatoes.

Required Practical 8: plant responses

Aim: investigate the effect of light (phototropism) or gravity (gravitropism) on the growth of seedlings.

For phototropism: germinate seeds on damp cotton wool in a box with a single light source on one side. Leave for several days. Shoots bend towards the light because auxin accumulates on the shaded side.

For gravitropism: germinate seeds in a petri dish and rotate the dish so seedlings are horizontal. Leave in dark conditions. Roots grow downward and shoots grow upward due to auxin distribution.

Independent variable: direction of light or position of seedling. Dependent variable: direction or angle of growth. Control variables: temperature, type of seed, moisture, time. Common evaluation points: light may not be perfectly one-directional, seedlings may not germinate at the same rate, human measurement error in angles.

Topic 4.6: Inheritance, Variation and Evolution

Reproduction

Meiosis

Meiosis is cell division that produces gametes (sex cells). The DNA is first copied, then the cell divides twice, producing four genetically different gametes each with half the normal chromosome number. Fertilisation restores the full chromosome number. Meiosis creates genetic variation and is essential for evolution.

DNA, genes and chromosomes

DNA is a double helix made of two strands linked by bases in complementary pairs: A with T, and C with G. The genome is the entire genetic material of an organism. A gene is a section of DNA that codes for a specific protein. Chromosomes are found in the nucleus, made of DNA, and carry genes. Humans have 23 pairs of chromosomes.

Protein synthesis (Higher Tier only)

Step 1: transcription. The DNA base sequence is copied into a molecule of mRNA in the nucleus. Step 2: translation. The mRNA moves to a ribosome. The ribosome reads the mRNA code and assembles amino acids in the correct order. The chain of amino acids folds into a functional protein.

A mutation is a random change in the DNA base sequence. It may change the shape of the protein produced and could affect the phenotype. Not all mutations are harmful. Some have no effect and some may be beneficial.

Genetic inheritance

• Allele: a version of a gene
• Genotype: the combination of alleles an organism has
• Phenotype: the observable characteristic that results
• Dominant allele: always expressed in the phenotype
• Recessive allele: only expressed when two copies are present
• Homozygous: both alleles are the same (AA or aa)
• Heterozygous: alleles are different (Aa)

Punnett squares show all possible offspring genotypes from a cross. Example: Aa crossed with Aa.

	A	a
A	AA	Aa
a	Aa	aa

This gives a 3:1 ratio of dominant phenotype to recessive phenotype. Remember: genetics gives probabilities, not certainties. Always express results as ratios or fractions.

Inherited disorders

Cystic fibrosis is caused by a recessive allele (ff). Both alleles must be recessive for the condition to be expressed. It affects cell membranes and causes a build-up of thick mucus in the lungs and digestive system. Polydactyly is caused by a dominant allele, so only one copy is needed for it to be expressed. It causes extra fingers or toes.

Embryo screening (Higher Tier only) allows embryos created by IVF to be tested for genetic conditions before implantation. Ethical issues include concerns about selecting embryos, the cost of screening, and questions about the value placed on lives affected by genetic conditions.

Sex determination

Sex is determined by chromosome pair 23. Females have XX chromosomes and males have XY. Eggs always carry an X chromosome. Sperm carry either X or Y. There is a 50% probability of producing a male and a 50% probability of producing a female offspring.

Variation and evolution

Genetic variation is inherited from parents and caused by genes. Environmental variation is caused by external factors such as diet and climate. Most characteristics result from a combination of both. Mutations are random changes in DNA that are a source of new alleles and rare but important for evolution.

Evolution is the change in species over time through natural selection. The steps are: variation exists in a population; organisms compete for limited resources; those best adapted to their environment survive; survivors reproduce and pass on their beneficial alleles to offspring; over many generations the population changes.

Selective breeding

Humans select parent organisms with desired traits and breed them over many generations. Examples include cows bred for high milk yield, crops bred for disease resistance, and animals bred for fast growth. Disadvantages include reduced genetic diversity in the population and increased risk of inherited diseases due to inbreeding.

Genetic engineering

The process: the desired gene is isolated from the donor organism; it is inserted into a vector such as a bacterial plasmid; the plasmid is inserted into a host cell; the host organism expresses the gene and produces the desired protein. Uses include producing insulin in bacteria, creating pest-resistant GM crops and developing disease-resistant animals. Concerns include unknown long-term effects, ethical issues and potential impact on wild species.

Cloning

Plant cloning is done by taking cuttings or through tissue culture. Animal cloning by adult cell cloning: the nucleus is removed from an egg cell; a nucleus from the adult cell to be cloned is inserted into the empty egg; an electric shock stimulates cell division; the embryo is implanted into a surrogate mother. Advantages include preserving endangered species and replicating desirable traits. Disadvantages include ethical concerns, low success rate and reduced genetic diversity.

Evidence for evolution

Fossils show that life has changed over time. They form when organisms are preserved in rock, replaced by minerals, or leave impressions. The fossil record is incomplete because soft-bodied organisms rarely fossilise. DNA comparisons between species show how closely related they are. Antibiotic resistance in bacteria is a direct example of evolution by natural selection: random mutations produce resistant individuals; antibiotics kill non-resistant bacteria; resistant bacteria survive and reproduce, passing on the resistance gene.

To reduce antibiotic resistance: only use antibiotics when necessary, complete the full course, and avoid overuse in farming.

Classification

The Linnaean system organises life into groups from broadest to most specific: Kingdom, Phylum, Class, Order, Family, Genus, Species. The binomial naming system uses Genus and species together (for example, Homo sapiens). Modern classification is based on DNA evidence rather than observable features alone. The three-domain system divides all life into Bacteria, Archaea and Eukaryota.

Topic 4.7: Ecology

Ecosystems and communities

Levels of biological organisation: an individual is a single organism; a population is all individuals of the same species in an area; a community is all populations of different species in an area; an ecosystem is the community combined with all the abiotic (non-living) factors.

Abiotic factors include light intensity, temperature, moisture, soil pH, oxygen levels and carbon dioxide levels. Biotic factors include predators, disease, competition and food availability.

Organisms compete for food, water, space and mates. Organisms are interdependent: removing one species affects others through food chains, pollination and seed dispersal.

Adaptations

Adaptations help organisms survive in their environment. Structural adaptations are physical features. Behavioural adaptations are actions. Functional adaptations are internal processes. A cactus stores water in its stem (structural), has spines instead of leaves to reduce water loss (structural), and carries out CAM photosynthesis at night (functional).

Required Practical 9: sampling populations

Aim: estimate population size and investigate the effect of environmental factors on distribution.

Using quadrats: place a quadrat randomly in the study area, count the number of organisms inside, repeat many times across the area, calculate the mean count per quadrat, then use the formula below to estimate the total population.

Transects are used when investigating changes across a gradient, such as from a path edge into a meadow. Improvements to the method: use more quadrats, place them randomly, sample a larger area.

Food chains and trophic levels

A food chain always starts with a producer (a plant or algae that makes food by photosynthesis). Trophic level 1 is producers, level 2 is primary consumers, level 3 is secondary consumers, and level 4 is tertiary consumers.

Only approximately 10% of biomass is transferred from one trophic level to the next. Energy and biomass are lost at each level through respiration (heat loss and movement), waste such as faeces and urine, and parts of organisms that are not eaten.

Pyramids of biomass show the total mass of living material at each trophic level. They are always widest at the base (producers) and narrowest at the top. Unlike pyramids of numbers, they are never inverted.

Increasing efficiency in food production

Farmers can increase the efficiency of energy transfer by keeping animals warm so less energy is lost to maintaining body temperature, and by limiting movement so less energy is used for respiration. These methods increase the proportion of food energy converted to biomass.

Material cycles

The carbon cycle: photosynthesis removes carbon dioxide from the atmosphere; respiration, decomposition and combustion release it back. The water cycle: water evaporates and transpires from organisms, condenses to form clouds, and returns to Earth as precipitation.

Required Practical 10: effect of temperature on decay

Aim: investigate how temperature affects the rate of decay using milk.

Method: place milk samples in containers held at different temperatures; measure pH change over time using an indicator or pH probe; record how quickly acidity increases as bacteria break down the milk.

Expected results: higher temperature leads to faster decay because microorganisms respire more quickly. Independent variable: temperature. Dependent variable: pH change or rate of decay. Control variables: same volume of milk, same container, same time period.

Improvements: use a pH probe instead of an indicator for greater precision, repeat readings at each temperature and calculate a mean, use a water bath to maintain temperature precisely.

Biodiversity

Biodiversity is the variety of all living organisms in an area. High biodiversity leads to stable ecosystems, balanced food chains and resilience to change. Reduced genetic diversity increases extinction risk.

Human impacts that reduce biodiversity: land use for farming and urban development reduces habitats; deforestation clears habitat and increases atmospheric carbon dioxide; pollution through fertilisers causes eutrophication in water; global warming disrupts ecosystems and may cause species migration or extinction.

Methods to maintain biodiversity: captive breeding programmes protect endangered species; nature reserves and national parks protect habitats; replanting hedgerows increases habitat variety; recycling reduces demand for raw materials and reduces pollution; reducing deforestation and carbon emissions slows climate change.

Food security

Food security means having access to enough food for the whole population. Threats include climate change, pests and disease, poor farming practices and an increasing global population. Solutions include sustainable farming methods, GM crops with higher yields or pest resistance, and more efficient food production.

Sustainable fishing methods include fishing quotas to limit how much can be caught and net size controls to allow young fish to escape, maintaining breeding populations and preventing long-term decline in fish stocks.

Biotechnology can improve food security: bacteria have been genetically engineered to produce human insulin; mycoprotein (Quorn) is produced from Fusarium fungus and provides protein with lower environmental cost than meat; GM crops can be designed for improved yield, drought resistance or pest resistance.

Quick reference: key equations for Biology Paper 2

Both equations below may be needed in calculation questions:

Most common exam mistakes across all three topics

• Mixing up insulin and glucagon: insulin lowers blood glucose, glucagon raises it
• Forgetting negative feedback in homeostasis: the response reduces the original change
• Confusing sensory and motor neurones
• Saying "blood temperature receptor in skin" only: the brain thermoregulatory centre must also be mentioned
• Saying plants "move" towards light: it is unequal growth caused by auxin, not movement
• Confusing meiosis and mitosis
• Mixing up genotype and phenotype
• Forgetting to express genetic results as probabilities or ratios
• Saying evolution happens to individual organisms: it happens to populations over generations
• Saying energy is recycled in ecosystems: energy flows and is lost as heat. Matter is what is recycled.
• Drawing inverted pyramids of biomass: these are always wider at the base
• Not linking human activity to biodiversity loss in ecology questions