Everything in AQA Biology Paper 2 — homeostasis, inheritance and ecology — distilled into spec-accurate notes, mark-scheme model answers and the extra detail that separates a 7 from a 9. Tap any topic to open it.
Topics
5 · 6 · 7
Homeostasis, Inheritance, Ecology
Length
1h 45m
Written exam
Marks
100
50% of GCSE
Questions
Mixed
Multiple choice → 6-mark extended
HTHigher Tier only ·
TRIPLESeparate Biology only (not in Combined Science)
05
Homeostasis & Response
control systems · hormones · the nervous system
Definition
Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes.
What it controls
Blood glucose concentration
Body temperature
Water levels (and ions / nitrogen)
Every control system has three parts
Receptors — detect a stimulus (a change)
Coordination centres — receive & process information (brain, spinal cord, pancreas)
Effectors — muscles or glands that carry out the response to restore optimum levels
Grade 9 detail
Always explain why homeostasis matters: enzymes have an optimum temperature and pH. Outside the optimum, enzymes denature (active site changes shape, substrate no longer fits) and reactions slow, so cells can't function. This is the link examiners want — control conditions → enzymes work → cells survive.
The CNS (central nervous system) is the brain and spinal cord. It lets us react to surroundings and coordinate behaviour.
A synapse is the gap between two neurones. The electrical impulse triggers release of a chemical (neurotransmitter) which diffuses across the gap and binds to receptors on the next neurone, starting a new impulse.
Reflex actions
Reflexes are automatic and rapid — they don't involve the conscious part of the brain. This makes them protective. The pathway is a reflex arc: receptor → sensory neurone → relay neurone (in spinal cord) → motor neurone → effector.
Grade 9 detail
The reflex arc bypasses the brain, so the response happens before you consciously feel the pain — this is why it's faster and reduces harm. Mention that three neurones are involved and that the synapse is where the impulse becomes a chemical signal.
6 marksDescribe how the body responds to touching a hot object, from stimulus to response.
Receptors in the skin detect the stimulus (heat / pain).
An electrical impulse travels along a sensory neurone to the spinal cord (CNS).
At the synapse, a neurotransmitter diffuses across the gap to the relay neurone.
The impulse passes to a motor neurone.
The impulse reaches the effector — a muscle in the arm.
The muscle contracts to move the hand away (the response). This is a reflex, so it is automatic and rapid and protects the body from damage.
Examiner note: name all three neurones in order and use the word "synapse." Sequencing marks are lost if steps are jumbled.
Common trap
"Messages travel along neurones" is too vague. Say electrical impulses along neurones, and chemicals (neurotransmitters) by diffusion at synapses.
The brain HT parts
Cerebral cortex — consciousness, intelligence, memory and language
Cerebellum — coordination of muscular activity & balance
Medulla — unconscious activities, e.g. heart rate and breathing
Neuroscientists map the brain by studying patients with brain damage, electrically stimulating regions, and using MRI scans. Treating brain disorders is hard because the brain is complex and delicate, and easily damaged during surgery.
The eye — accommodation
Near object: ciliary muscles contract, suspensory ligaments loosen, lens becomes thicker and more curved, refracts light strongly.
Myopia (short sight): rays focus in front of the retina → corrected with a concave lens. Hyperopia (long sight): rays focus behind the retina → corrected with a convex lens. New treatments: hard/soft contact lenses, laser surgery to reshape the cornea, replacement lens implants.
The thermoregulatory centre in the brain monitors blood temperature; the skin contains temperature receptors that send impulses to it.
Too hot
Vasodilation — blood vessels near skin surface dilate, more blood flows to surface, more energy radiates away
Sweating — sweat evaporates, transferring energy to the environment (cooling)
Too cold
Vasoconstriction — vessels constrict, less blood to surface, less heat lost
Shivering — muscles contract rapidly, requiring respiration, which releases heat energy
Sweating stops
Common trap
Blood vessels do not "move" up and down. Say they dilate or constrict. Also: sweat evaporating cools you, not the sweat itself.
Glands secrete hormones directly into the bloodstream, which carries them to target organs.
Nervous vs hormonal
Nervous: very fast, very short-lasting, acts on a precise area
Hormonal: slower, longer-lasting, acts in a more general way
Key glands
Pituitary — the "master gland"; releases hormones that act on other glands to release further hormones
Pancreas — insulin & glucagon
Thyroid — thyroxine · Adrenal — adrenaline
Ovaries — oestrogen · Testes — testosterone
The pancreas monitors and controls blood glucose concentration.
Blood glucose too high
Pancreas releases insulin
Insulin makes glucose move from blood into cells
In the liver & muscles, excess glucose is converted to glycogen for storage
Blood glucose too low HT
Pancreas releases glucagon
Glucagon makes the liver convert glycogen → glucose, released into the blood
Negative feedback
This is a negative feedback loop: a change away from the norm triggers a response that reverses the change, returning levels to normal.
Diabetes
Type 1: pancreas produces too little (or no) insulin → uncontrolled high blood glucose. Treated with insulin injections.
Type 2: body cells stop responding to insulin. Risk factor: obesity. Treated with a carbohydrate-controlled diet and exercise.
Grade 9 detail
Don't confuse the two hormones. Mnemonic: Insulin In (glucose into cells, lowers blood glucose); glucagon brings glucose back when it's gone. Both are made by the pancreas — that's the coordination centre and the gland.
Water leaves the body via the lungs (breathing), skin (sweat) and kidneys (urine). We can't control loss from lungs or skin, so the kidney balances everything.
Urea
Excess amino acids can't be stored, so the liver breaks them down by deamination, producing ammonia (toxic), which is converted to urea and excreted by the kidneys.
How the kidney works
Filters the blood
Selective reabsorption — reabsorbs all the glucose, and the water and ions the body needs
Urea, excess water and ions leave as urine
ADH & water control HT
If blood is too concentrated, the pituitary releases more ADH, which makes kidney tubules more permeable so more water is reabsorbed — controlled by negative feedback.
Kidney failure
Dialysis: blood flows over a partially permeable membrane; dialysis fluid has normal blood concentrations of glucose and ions, so these don't diffuse out, but urea diffuses out down its concentration gradient. Transplant is a long-term cure but risks rejection.
Grade 9 detail
Compare dialysis vs transplant in evaluation questions: dialysis must be done regularly, restricts diet, expensive long-term — but no rejection. Transplant frees the patient but needs a donor, has rejection risk, and requires immunosuppressant drugs.
At puberty, reproductive hormones cause secondary sex characteristics. The main female hormone is oestrogen (ovaries); the main male hormone is testosterone (testes), which stimulates sperm production.
The menstrual cycle — four hormones HT
FSH (pituitary) — matures an egg in the ovary & stimulates oestrogen release
FSH & LH can be given as a "fertility drug." In IVF: FSH/LH mature multiple eggs → eggs collected & fertilised in a lab → embryos develop → one or two inserted into the uterus.
Evaluate IVF
Strengths: gives infertile couples a child. Weaknesses: physically & emotionally stressful, low success rate, expensive, risk of multiple births (risky for mother & babies). Ethical issue: unused embryos.
Adrenaline
Released by the adrenal glands in times of fear or stress. It increases heart rate, boosting oxygen & glucose delivery to the brain and muscles — the "fight or flight" response. (Not controlled by negative feedback.)
Thyroxine
Released by the thyroid; controls metabolic rate; important for growth and development. Controlled by negative feedback involving TSH from the pituitary.
Plant hormones — auxin
Phototropism: shoots grow towards light. Auxin gathers on the shaded side, where it makes cells elongate more, so the shoot bends towards the light.
Auxins: weedkillers, rooting powders, tissue-culture growth media
Gibberellins: end seed dormancy, promote flowering, increase fruit size
Ethene: controls fruit ripening during storage & transport
Grade 9 detail
The classic exam point: auxin makes shoot cells elongate (it does not make more cells). In shoots auxin promotes growth; in roots the same hormone inhibits it — that opposite effect is a favourite question.
06
Inheritance, Variation & Evolution
DNA · genetics · natural selection
Sexual vs asexual
Sexual: two parents · gametes fuse at fertilisation · mixes genetic information · produces variation · involves meiosis
Asexual: one parent · no gametes · only mitosis · offspring are genetically identical clones · no variation
Meiosis
Happens in reproductive organs, producing gametes
Genetic material is copied, then the cell divides twice → four gametes
Each gamete has a single set of chromosomes (haploid) and is genetically different
Fertilisation restores the full (diploid) number; the new cell then divides by mitosis
Grade 9 detail
Link variation to survival: sexual reproduction → variation → if the environment changes, some individuals are more likely to have a useful characteristic and survive (natural selection / selective breeding rely on this). Asexual is faster and needs only one parent, but a single disease could wipe out an identical population.
Key definitions
DNA — a polymer of two strands forming a double helix, found in chromosomes in the nucleus. Gene — a small section of DNA coding for a sequence of amino acids → a specific protein. Genome — the entire genetic material of an organism.
Why the human genome matters
Find genes linked to diseases
Understand & treat inherited disorders
Trace human migration patterns
DNA structure Triple
Made of nucleotides = sugar + phosphate + base. The sugar–phosphate forms the backbone. Four bases pair up: A–T and C–G (complementary base pairing). A sequence of three bases codes for one amino acid.
Protein synthesis Triple
DNA is the template for making mRNA (transcription); mRNA leaves the nucleus
mRNA attaches to a ribosome; carrier molecules bring specific amino acids in the correct order (translation)
The amino-acid chain folds into a unique 3D shape → enzymes, hormones, structural proteins
Grade 9 detail
Mutations are random changes to DNA. Most have no effect; some change the protein. A mutation in the active site of an enzyme can change its shape so the substrate no longer fits, stopping its function. Non-coding DNA mutations can switch genes on/off.
Vocabulary you must spell correctly
Allele — a version of a gene · Dominant — needs only one copy to show (capital letter) · Recessive — needs two copies (lowercase) · Homozygous — two identical alleles · Heterozygous — two different alleles · Genotype — the alleles present · Phenotype — the physical characteristic.
Inherited disorders
Polydactyly (extra fingers/toes) — caused by a dominant allele
Cystic fibrosis (disorder of cell membranes) — caused by a recessive allele
Sex determination
The 23rd pair of chromosomes: XX = female, XY = male. A Punnett square gives a 50:50 ratio.
4 marksTwo parents are both heterozygous for cystic fibrosis (Ff). Use a Punnett square to find the probability a child has cystic fibrosis.
Parents' alleles: F f × F f.
Offspring genotypes: FF, Ff, Ff, ff.
Only ff has cystic fibrosis (recessive needs two copies).
Probability = 1 in 4 = 25%. (Carriers Ff = 50%.)
Examiner note: draw the grid, label parent gametes outside it, and write a clear conclusion as a ratio, fraction and percentage to secure all marks.
Common trap
"Embryo screening" questions want a balanced answer: it can prevent suffering and reduce treatment costs, BUT raises ethical issues (designer babies, who decides?) and screening itself carries risk. Always give both sides.
Variation arises from genes, the environment, or both. Mutations are the source of all new alleles.
Evolution
The change in the inherited characteristics of a population over time, through natural selection, which may result in the formation of new species.
Natural selection — the four steps
There is variation within a species (caused by mutation)
Individuals with advantageous characteristics are more likely to survive and reproduce
They pass on the advantageous alleles to their offspring
Over many generations, these alleles become more common — the population evolves
Speciation Triple
If two populations are isolated and face different selection pressures, their phenotypes diverge until they can no longer interbreed to produce fertile offspring — they are now separate species. Wallace co-proposed natural selection and worked on this.
Grade 9 detail
Use exact wording: organisms are "more likely to survive" — not "they want to" or "they try to." Evolution has no intention. The mutation happens first, by chance; the environment then selects for it.
Selective breeding (artificial selection)
Humans choose organisms with desired traits and breed them together over generations — e.g. disease-resistant crops, high-yield cattle, docile dogs. Risk: reduced gene pool → inbreeding → more disease/genetic defects.
Genetic engineering
Transferring genes from one organism to another. E.g. bacteria modified to make human insulin; GM crops for pest resistance or higher yield (e.g. golden rice with added vitamin A).
Process HT: enzymes cut out the required gene → it's inserted into a vector (a bacterial plasmid or a virus) → the vector inserts the gene into the target cells → done at an early stage of development so all cells carry the gene.
Cloning Triple
Tissue culture — small groups of plant cells grown into many identical plants
Cuttings — simple, older method for plants
Embryo transplants — split an embryo, implant clones into host mothers
Adult cell cloning — nucleus from an adult body cell put into an empty egg cell → electric shock → embryo → implanted (e.g. Dolly the sheep)
Evaluate GM
Benefits: higher yields, added nutrients, less pesticide use. Concerns: effects on wild populations & insects, uncertain long-term health effects, ethical & economic issues for farmers. Give both sides for the marks.
Scientists
Darwin — On the Origin of Species (1859); natural selection. Accepted slowly: it challenged religious belief, there was limited evidence, and the mechanism of inheritance (genes) was unknown.
Lamarck — proposed (incorrectly) that characteristics acquired in life are inherited.
Fossils — formed from hard body parts, casts/impressions, or preservation where decay is prevented. The fossil record is incomplete because many soft-bodied organisms decayed and many fossils were destroyed.
Extinction causes: new predators, new diseases, new competitors, environmental change, or catastrophic events.
Antibiotic-resistant bacteria (e.g. MRSA)
A random mutation makes some bacteria resistant
The antibiotic kills non-resistant bacteria; resistant ones survive
They reproduce, passing on the resistance allele → resistant population spreads
Reduce it: don't over-prescribe antibiotics, always finish the course, restrict agricultural use. New antibiotics are slow & costly to develop.
Classification
Linnaean system: Kingdom → Phylum → Class → Order → Family → Genus → Species, with binomial naming. Improved microscopes & biochemistry led Carl Woese to the three-domain system: Archaea, Bacteria, Eukaryota. Evolutionary trees show relationships.
07
Ecology
ecosystems · cycles · human impact
Levels of organisation: individual → population → community → ecosystem. A stable community is one where all species and environmental factors are in balance, so population sizes stay roughly constant.
Interdependence
Species depend on each other for food, shelter, pollination and seed dispersal. Remove one species and others can be affected.
Food availability, new predators, new pathogens, and competition (one species out-competing another)
Competition
Plants compete for light, space, water and mineral ions. Animals compete for food, mates and territory.
Adaptations
Structural (body features), behavioural (actions) and functional (internal processes). Extremophiles are adapted to extreme conditions — high temperature, pressure or salt concentration.
Food chains begin with a producer (usually a green plant or alga that photosynthesises). Then come primary, secondary and tertiary consumers. Predators eat prey.
Predator–prey cycles
Numbers rise and fall in cycles that are roughly out of phase: more prey → predators increase → prey eaten → prey fall → predators fall → prey recover.
Grade 9 detail — sampling maths
Estimate population = mean number per quadrat × (total area ÷ quadrat area). Use a quadrat with random sampling for abundance (avoids bias), and a transect (a line) to measure how distribution changes along an environmental gradient, e.g. from shade into light.
The carbon cycle
Photosynthesis removes CO₂ from the air (carbon → biomass)
Respiration, combustion (burning) and decomposition return CO₂ to the air
Carbon is passed along food chains as biomass
The water cycle
Energy from the Sun causes evaporation (and transpiration from plants) → condensation into clouds → precipitation (rain/snow) → provides fresh water before draining back to the sea.
Decomposition Triple
Microorganisms break down dead material. Rate of decay depends on temperature, water/moisture and oxygen. Gardeners use these to make compost. Anaerobic decay produces methane / biogas, used as a fuel in biogas generators.
Grade 9 detail — decay maths
Rate of decay is often calculated as rate = 1000 ÷ time (per day). On a graph, optimum temperature gives the fastest rate; too hot denatures enzymes in the microorganisms, so the rate falls.
Definition
Biodiversity is the variety of all the different species of organisms on Earth, or within an ecosystem. High biodiversity makes ecosystems more stable.
A rising human population and standard of living means more resources used and more waste produced.
Pollution reduces biodiversity
Water: sewage, fertiliser, toxic chemicals
Air: smoke and acidic gases
Land: landfill and toxic chemicals
Land use & deforestation
Humans use land for building, quarrying, farming and dumping waste. Destroying peat bogs releases stored CO₂ and reduces habitat. Deforestation (for timber, cattle and crops) reduces biodiversity, removes CO₂-absorbing trees and releases CO₂ when wood is burnt.
Global warming
Rising CO₂ and methane (greenhouse gases) trap heat → global warming → climate change. Consequences: melting ice, rising sea levels, flooding, changes in species distribution & migration, and falling biodiversity.
Maintaining biodiversity
Breeding programmes for endangered species
Protecting & regenerating habitats
Replanting hedgerows & field margins on farms
Reducing deforestation and carbon emissions
Recycling rather than dumping in landfill
These programmes can conflict with human needs and have economic costs — a common evaluation question.
Trophic levels
Level 1 — producers (plants/algae)
Level 2 — primary consumers (herbivores)
Level 3 — secondary consumers (carnivores)
Level 4 — apex predators
Decomposers break down dead organisms, recycling materials
Biomass transfer
Only about 10% of biomass is transferred to the next trophic level. Losses occur because: not all of an organism is eaten, some passes out as faeces (egestion), energy is lost in respiration (as heat), and material is lost in excretion (urea).
Grade 9 detail — efficiency calc
Efficiency = (biomass at higher level ÷ biomass at lower level) × 100. This 10% rule explains why food chains rarely have more than 4–5 levels — there isn't enough energy left to support a higher one.
Food security & production
Threats to food security: growing population, changing diets, new pests/pathogens, environmental change, rising costs of farming, and conflicts.
Efficient production: restricting animal movement & controlling temperature reduces energy loss — but raises ethical concerns (factory farming)
Sustainable fisheries: fishing quotas and controlled net mesh sizes protect breeding populations
Biotechnology:mycoprotein (from Fusarium fungus grown in fermenters), GM bacteria producing insulin, and GM crops such as golden rice
Required Practicals
Paper 2 can examine these directly. Learn the variables, the method and one improvement for each — that's where marks hide.
RP — Reaction time
Method: a partner drops a ruler between your open fingers; you catch it and record the distance (or convert to time). Repeat and take a mean.
Independent variable: e.g. caffeine / no caffeine, or before/after practice
Control variables: same person catching, same hand, ruler held at the same point
Improve: use a computer reaction-time test for more precise, repeatable timing
RP — Effect of light or gravity on seedling growth Triple
Method: grow cress/seedlings in different light conditions (e.g. all-round light, light from one side, darkness) and measure growth direction & length.
Control variables: water, temperature, type & number of seeds
Result: shoots bend towards the light (positive phototropism) due to auxin
RP — Measuring population size (quadrats & transects)
Method: place quadrats randomly (use random coordinates) to find mean abundance, then scale up to estimate the population. Use a transect line to study distribution along a gradient.
Why random? Avoids bias and makes results representative
Estimate: mean per quadrat × (total area ÷ quadrat area)
RP — Rate of decay Triple
Method: investigate how temperature affects the rate of decay — e.g. milk + lipase + an indicator, timing the colour change at different temperatures.
Calculate rate: rate = 1000 ÷ time
Too hot → enzymes denature → rate falls
Grade 9 Exam Technique
Knowing the biology isn't enough at grade 9 — you have to answer the exact command word with the right depth.
DescribeSay what happens / what you see. No reasons needed. Often for trends in data.
ExplainGive reasons — use "because," "so," "this causes." The most common verb in Paper 2.
CompareUse "whereas" / "but." Give linked points for both things in the same sentence.
EvaluateGive advantages AND disadvantages, then a justified conclusion / judgement.
SuggestApply your knowledge to an unfamiliar context. There may be more than one right answer.
CalculateShow every step, include units, and don't round too early. Carry through to a final answer.
The 6-mark question strategy
These are marked by levels, not points. To reach the top level (5–6 marks): (1) write a logical sequence in the right order, (2) use specific scientific vocabulary (denature, vasodilation, advantageous alleles…), and (3) link cause to effect throughout. Plan a quick spider of key terms before writing. A short, well-sequenced answer beats a long, jumbled one.
Top mark-droppers
Confusing insulin/glucagon · saying organisms "want to" or "try to" adapt · forgetting that auxin makes cells elongate · vague "messages" instead of impulses/neurotransmitters · giving only one side in evaluate questions · no units in calculations.
Active Recall Flashcards
Tap the card to flip. Test yourself out loud before flipping — recall beats re-reading. Filter by topic below.
Topic 5Define homeostasis.
click card to flip
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Definition Bank
The exact definitions examiners reward. Learn these word-for-word.
HomeostasisRegulation of internal conditions to maintain optimum conditions for function.
Reflex actionAn automatic, rapid response that does not involve the conscious brain.
HormoneA chemical messenger secreted by a gland and carried in the blood to a target organ.
Negative feedbackA change is detected and reversed to return a condition to its normal level.
GeneA small section of DNA that codes for a particular protein.
GenomeThe entire genetic material of an organism.
AlleleA different version of the same gene.
MeiosisCell division producing four genetically different haploid gametes.
EvolutionChange in inherited characteristics of a population over time via natural selection.
SpeciationFormation of a new species when populations can no longer interbreed to give fertile offspring.
BiodiversityThe variety of all species in an ecosystem or on Earth.
CommunityAll the populations of different species living in a habitat.
EcosystemThe interaction of a community with the abiotic parts of its environment.
Selective breedingChoosing organisms with desired traits to breed over generations.