AQA GCSE Biology Revision

The eukaryotic cell is about ten times wider (10:1). No: a bacterium has a DNA loop and may also have plasmids, although neither is enclosed in a nucleus.

A plasmid can carry additional genes, potentially including an antibiotic-resistance gene. A bacterium with that gene may be resistant to that antibiotic.

The projection gives the cell a large surface area in contact with soil, helping it absorb water and mineral ions efficiently.

The cell acquires sub-cellular structures suited to a particular function. Most animal cell types differentiate early, while many plant cells retain the ability to differentiate throughout life.

4 mm = 4000 µm, so magnification = 4000 ÷ 20 = ×200. Both values must use the same unit for the ratio to be valid.

The 25 °C incubation temperature reduces the chance of growing many human pathogens, while inverting the plate prevents condensation from the lid dripping onto and spreading the colonies.

4.8 × 10⁷ bacteria.

They are found in the nucleus, are made of DNA molecules, and each carries many genes.

The cell grows and increases sub-cellular structures; DNA replicates; chromosomes move to opposite ends and the nucleus divides; the cytoplasm and membrane divide.

The embryo has the same genes as the patient, so its stem cells may be recognised as compatible. A possible concern is viral infection transfer or an ethical/religious objection.

The 1 mm cube has SA:V 6 mm⁻¹ and the 2 mm cube 3 mm⁻¹, so doubling side length halves SA:V. Fish gill filaments have many thin lamellae with a rich blood supply and water flow.

(1.8 − 2.0) ÷ 2.0 × 100 = −10%, so it lost 10% of its initial mass.

Active transport, because ions move against their concentration gradient and the movement requires energy from respiration.

It is made of different tissues that work together to pump blood, whereas a tissue is a group of cells with similar structure and function.

Amylase is produced by the salivary glands, pancreas and small intestine. High temperature can change its shape and active site so starch no longer fits; the enzyme is denatured.

Coronary arteries supply oxygenated blood to the heart muscle. A large vein lumen reduces resistance to flow, and valves prevent blood flowing backwards.

Red blood cells transport oxygen using haemoglobin; plasma carries many dissolved substances.

A biological valve may wear out sooner and need replacement. A mechanical valve is durable but usually requires lifelong anticoagulant medicine, which increases bleeding risk.

You can identify a positive correlation. The graph alone cannot establish that one variable causes the disease; sampling and other possible factors need consideration.

Smoking exposes the fetus to carbon monoxide and nicotine, reducing oxygen and nutrient supply and restricting growth. Alcohol crosses the placenta and can disrupt fetal and brain development. Prolonged heavy drinking can damage liver cells and cause cirrhosis.

Malignant cells invade neighbouring tissues and can spread in the blood to form secondary tumours; benign growths stay contained in one area.

They allow gases to move through the leaf, supporting gas exchange for photosynthesis and respiration.

Xylem carries water and mineral ions from roots; phloem carries dissolved sugars from leaves to other parts. A potometer measures water uptake, which is related to but not identical to water lost by transpiration.

Bacteria may produce tissue-damaging toxins; viruses live and reproduce inside cells, causing cell damage.

TMV discolours leaves in a mosaic pattern, reducing photosynthesis. Uncontrolled HIV attacks immune cells and can lead to AIDS, when the immune system cannot deal with other infections or cancers.

Poultry vaccination helps control Salmonella. Gonorrhoea spread can be reduced by antibiotic treatment or a barrier method such as a condom.

It damages leaves, which can yellow and fall early, reducing photosynthesis and therefore the plant’s ability to grow.

It helps prevent mosquito bites, reducing transmission by the vector that carries the malarial protist.

Nasal hairs and mucus trap particles in the nose. Mucus traps particles in the trachea and bronchi, and cilia waft the mucus towards the throat to be swallowed.

It prepares a faster antibody response that can prevent illness in an individual; immunising many people leaves fewer hosts through which the pathogen can spread.

Measles is viral, and antibiotics cannot kill viruses. A painkiller can reduce symptoms but does not kill the pathogen.

The low dose supports cautious safety testing; a placebo gives a comparison that helps identify effects attributable to the medicine.

They are made by descendants of one selected hybridoma cell, so they bind the same specific site on the same protein antigen.

It can bind specifically to the target and carry a fluorescent dye, making the target visible. Include that side effects have been greater than expected and weigh risks against benefits.

Magnesium deficiency is associated with chlorosis; magnesium ions are needed to make chlorophyll.

For example, take the plant to a laboratory or use a monoclonal-antibody test kit. Similar symptoms can have different causes, so a sign alone may not establish the pathogen.

A tough waxy cuticle is a physical barrier that helps resist pathogen invasion. Thorns are a mechanical feature that deters animals.

Carbon dioxide and water are reactants; glucose and oxygen are products. It is endothermic because light energy is transferred from the environment into the chloroplasts.

The rate falls because the enzymes denature and their active sites change shape, so substrates bind less effectively.

Carbon dioxide is limiting because the rate rises from 5 to 8 relative units when carbon dioxide increases at high light. The heat option nets £20 - £25 = -£5 per day, so it reduces profit by £5 per day.

Nitrate ions absorbed from the soil.

Muscle cells produce lactic acid; yeast produces ethanol and carbon dioxide. Anaerobic respiration transfers much less energy than aerobic respiration.

They respire anaerobically; incomplete glucose oxidation causes lactic acid build-up and contributes to oxygen debt and fatigue.

It is the extra oxygen needed after exercise to react with accumulated lactic acid and remove it from cells. Blood carries lactic acid to the liver, where it is converted back into glucose.

Three fatty-acid molecules; excess protein can be broken down to form urea for excretion.

Distance triples, so intensity becomes 1 ÷ 3² = 1/9 of the original.

Homeostasis is regulation of internal conditions to maintain optimum conditions for function in response to internal and external changes. The system has receptors, a coordination centre and effectors.

Receptor, sensory neurone, relay neurone in the CNS, motor neurone, effector and response. Reflexes are rapid and automatic, helping protect the body without requiring a conscious decision.

The cerebellum coordinates movement and balance. The medulla controls automatic functions such as breathing.

Structural MRI shows anatomy and possible physical damage; functional MRI maps regions whose activity changes during a task. Mapping can help locate damage, diagnose a problem or plan treatment. Electrical stimulation or surgery can damage delicate tissue, while a scan is non-invasive.

For a near object, ciliary muscles contract, suspensory ligaments loosen and the lens becomes thicker, refracting light strongly. In dim light, radial iris muscles contract and circular muscles relax, dilating the pupil to let in more light.

When too hot, blood vessels dilate and sweat is produced. When too cold, blood vessels constrict, sweating stops and skeletal muscles shiver. Vasodilation and sweating cause energy transfer from the skin to the environment.

Vasodilation increases blood flow close to the skin, allowing more energy to transfer to the surroundings. As sweat evaporates it takes energy from the skin, increasing heat loss and lowering body temperature. Sweat that has not evaporated has not yet produced that cooling effect.

A gland secretes a hormone into the blood, which carries it to a target organ. Hormonal effects are usually slower but longer-lasting than nervous effects. The pituitary secretes hormones that can stimulate other glands to release hormones.

No. The pattern shows an association, but by itself does not establish individual causation; other influences may contribute. A fair recommendation uses evidence, recognises that risk is not destiny, considers access and social circumstances, and offers supportive, non-stigmatising options such as accessible balanced food, enjoyable activity and appropriate healthcare.

The pancreas releases glucagon. It causes glycogen to be converted into glucose and released into the blood, raising blood glucose.

In concentrated external fluid, water leaves the cell and it shrinks. In dilute external fluid, water enters, making it swell and potentially burst if it is an animal cell. Haemodialysis requires repeated sessions, often around four hours three times weekly, whereas a transplant may restore kidney function with fewer food/fluid restrictions but requires a donor, major surgery and immunosuppressants.

The liver deaminates excess amino acids, producing toxic ammonia, and converts the ammonia into urea for kidney excretion. The pituitary releases ADH, which increases kidney-tubule permeability so more water is reabsorbed into blood.

FSH causes an egg to mature in an ovary; LH stimulates egg release. Oestrogen and progesterone are involved in maintaining the uterus lining.

A rise in LH stimulates ovulation. After ovulation, progesterone helps maintain the uterus lining and inhibits release of FSH and LH.

A vasectomy blocks or seals sperm ducts so sperm are not ejaculated; female sterilisation blocks or cuts oviducts so sperm cannot meet an egg. Condoms act as a barrier and reduce STI risk; hormonal contraception changes hormone signals and may cause side effects. A pill needs daily adherence, while some hormonal methods last longer.

FSH and LH stimulate several eggs to mature; eggs are collected and fertilised with sperm in a laboratory; the fertilised eggs develop into embryos, and one or two early embryos are inserted into the uterus. Treatment can be stressful, success rates are not high, and multiple births risk the mother and babies.

Adrenaline is made by the adrenal glands during fear or stress. It increases heart rate and boosts delivery of oxygen and glucose to the brain and muscles. Pituitary TSH stimulates the thyroid to release thyroxine; when thyroxine rises it inhibits further TSH release, reducing thyroxine production. When thyroxine falls, reduced inhibition allows more TSH.

Unequal auxin distributions cause different growth rates in roots and shoots. Record length measurements and careful, labelled biological drawings of the seedling effects.

Gibberellins are important in initiating seed germination; ethene controls cell division and fruit ripening. Their mechanisms are not required.

Auxins are used as weed killers, rooting powders and to promote growth in tissue culture. Ethene controls fruit ripening during storage and transport. Gibberellins can end seed dormancy, promote flowering and increase fruit size.

It is sexual reproduction because pollen and egg cells fuse. The gametes are formed by meiosis.

Meiosis makes gametes with a single, half-sized chromosome set so that fertilisation does not double the chromosome number in every generation. Fusion of two gametes at fertilisation restores the normal number.

The malaria parasite reproduces sexually in a mosquito. Strawberry plants reproduce asexually by runners, or daffodils by bulb division.

A gene is a small section of DNA on a chromosome; the genome is all the genetic material of an organism. One reason to study the human genome is to search for genes linked to disease, understand inherited disorders, or trace past human migration.

A nucleotide has a common sugar, a phosphate group and one of four bases. A sequence of three bases codes for a particular amino acid.

A coding mutation may alter the protein sequence or shape and therefore its activity, changing phenotype. A non-coding variant may change how a gene is switched on or off, altering expression and phenotype. Many mutations have no phenotypic effect.

The child is aa, so each parent passed on a. As they are unaffected, both are carriers, Aa.

One of the four equally likely combinations is aa, so the probability is 1/4 or 25%. Since aa expresses the recessive phenotype, that phenotype also has probability 1/4 or 25%.

Polydactyly is caused by a dominant allele. Cystic fibrosis, a disorder of cell membranes, is caused by a recessive allele.

There are 23 pairs. Females are XX and males are XY.

Variation can arise from inherited genetic differences, environmental differences, or a combination. Most mutations have no effect on phenotype.

They pass the variant to offspring, so the inherited characteristic can become more common and the population's inherited characteristics change over generations.

Breed parents with the desired feature, choose offspring showing it, breed those offspring, and repeat over many generations. Inbreeding can make some breeds more prone to disease or inherited defects.

Crops can be engineered for disease resistance or bigger, better fruit; bacteria can be engineered to make human insulin for diabetes treatment. Concerns include effects on wild flowers and insects or uncertainty about effects on human health.

A vector carries the required gene into target cells. The usual vectors are a bacterial plasmid or a virus.

Cloning multiplies useful or rare plants, but low diversity may leave them vulnerable to one pathogen.

It challenged the idea that God created living things, evidence was insufficient to convince many scientists at the time, and the mechanism of inheritance and variation was not understood until about 50 years later. Any two are sufficient.

Darwin and Wallace published joint writings in 1858; Darwin published On the Origin of Species in 1859. Speciation reaches the endpoint where the populations can no longer interbreed to produce fertile offspring.

Scientists saw that chromosomes and Mendel's inherited units behaved in similar ways during inheritance, leading to the idea that the units, now called genes, were on chromosomes.

The specified examples are fossils and antibiotic resistance in bacteria. Genes show how characteristics can be passed to offspring, supporting inherited change.

A node represents a common ancestor. The pair whose branches meet at the more recent node is more closely related; tip spacing on the page is irrelevant.

Extinction means no individuals of the species remain alive. A pathogen could reduce survival or reproduction so much that the population declines and eventually no individuals remain.

Susceptible bacteria are killed while resistant bacteria survive and reproduce, increasing their proportion. Avoiding inappropriate prescriptions, completing a prescribed course, or restricting agricultural antibiotic use can help reduce the problem.

They share a more recent common ancestor and are more closely related.

They may compete for light, space, water or mineral ions. Interdependence means species rely on other species, for example for food, pollination or shelter, so a change to one can affect others.

Light intensity; temperature; moisture; soil pH and mineral content; wind intensity and direction; carbon dioxide levels for plants; and oxygen levels for aquatic animals.

It can reduce that species' population and change food, competition or other dependencies, causing population changes in species linked to it.

An extremophile lives in an extreme environment such as one with high temperature, pressure or salt concentration. AQA names bacteria living in deep-sea vents.

More predators may increase predation and prey numbers may then fall. Adding values and dividing by their number gives the arithmetic mean.

Carbon dioxide in air is fixed by plant photosynthesis, passed to an animal by feeding, then released as carbon dioxide by respiration or decomposition. Cloud droplets fall as precipitation; water runs off or enters soil and is absorbed by plant roots.

Increasing temperature initially speeds enzyme-controlled reactions and microbial respiration. Above the optimum, enzymes denature and microorganisms may be damaged, reducing the rate.

Temperature, availability of water, and composition of atmospheric gases.

It reduces dependence on a single species for resources and for maintaining the physical environment, so the loss or decline of one species is less likely to destabilise the whole system.

Water: sewage, fertiliser or toxic chemicals; air: smoke or acidic gases; land: landfill or toxic chemicals. Pollution can kill plants and animals, reducing biodiversity.

It reduces peatland habitat and its species variety, and decay or burning of peat releases carbon dioxide.

To provide land for cattle and rice fields, and to grow crops for biofuels.

Examples include a species shifting its distribution and a change in migration, breeding or flowering time. For example, warmer temperatures can make a former habitat unsuitable, so a population may move towards a cooler region or higher elevation.

Any three: endangered-species breeding programmes; protection/regeneration of rare habitats; restoring field margins and hedgerows; reducing deforestation and carbon dioxide emissions; recycling rather than landfill.

Level 1 producers (plants/algae), level 2 primary consumers (herbivores), level 3 secondary consumers, level 4 tertiary consumers. Decomposers secrete enzymes outside and small soluble products diffuse into them.

Trophic level 1 is at the bottom. Bar width should be proportional to the biomass at that level.

45 ÷ 500 × 100 = 9%.

Food security is having enough food to feed a population. Threats include rising birth rate, changing diets and transport of scarce resources, pests/pathogens, environmental change or failed rains, input costs, and conflict affecting water or food.

They restrict energy transfer from the animals to the environment, leaving more energy available for growth; the methods also require evaluation for ethical and welfare concerns.

They help conserve stocks: nets can allow some fish to escape and breed, while quotas limit catches so stocks can remain at a level where reproduction continues.

Fusarium is grown on glucose syrup in aerobic conditions, then its biomass is harvested and purified. Mycoprotein is protein-rich and suitable for vegetarians.

Broth stayed free of growth while dust and microorganisms were kept from it, and growth followed exposure. This supported the view that microorganisms came from other microorganisms, but by itself did not prove that every disease was caused by a particular pathogen.

Test it against observations or an experiment; a model may simplify a complex system or omit relevant features.

Data may be uncertain, incomplete or unavailable, so conclusions are limited and additional evidence may be needed.

Personal, social, economic and environmental implications.

Voluntary versus imposed risk and familiar versus unfamiliar (or visible versus invisible) hazards.

It helps detect false claims and establish consensus; popular-media reports are not peer-reviewed and may be oversimplified, inaccurate or biased.

It is based on a scientific explanation and makes a specific, testable prediction that evidence could support or challenge.

Independent: changed or selected; dependent: measured for each independent-variable value; control: kept the same to make the comparison fair.

The suitable technique or instrument and a reason showing how its measurement capability meets the specified purpose.

Identify the actual hazard and state a suitable control that reduces the chance or severity of harm while allowing the procedure to be carried out safely.

It makes conclusions about the larger population more trustworthy; biased or insufficient samples may not reflect the population.

An appropriate unit in the heading, and a reading recorded at a precision justified by the instrument scale.

It addresses a specific identified weakness, explains how that weakness affects results, and states how the proposed change improves validity, precision or sufficiency.

A histogram represents continuous grouped data with adjoining bars; a bar chart represents categories and normally has separated bars.

The data values, variable meaning, units and relationship or trend.

(12 - 4) ÷ (6 - 2) = 8 ÷ 4 = 2 response units per x-axis unit.

The range of the measurements about the mean is used as a measure of uncertainty.

An observation describes the recorded evidence; an inference is an explanation interpreted from that evidence.

No. It shows the data fit the hypothesis to some extent, but other explanations may also fit and further evidence may be needed.

Random error is reduced by more measurements and a mean; systematic error is not removed because it shifts results consistently.

The scientific rationale, method, findings/data analysis and reasoned conclusion.

Terms communicate the exact process, variable, observation or conclusion, so they are important in both subject explanations and practical accounts.

What it measures, how it was determined, and the appropriate unit.

Examples include kg, g, mg; km, m, mm; and kJ, J.

Micro is 10^-6 and nano is 10^-9, so 1 micrometre = 1000 nanometres. Therefore 3 micrometres = 3000 nanometres.

2.5 g = 2500 mg because 1 g = 1000 mg.

The input measurements have limited precision, so excessive digits imply unjustified accuracy; round to an appropriate number of significant figures.

Photosynthesis captures light energy to make organic compounds; those compounds supply biomass through food chains, while chemicals in ecosystems are continually recycled.

0.00042 = 4.2 × 10⁻⁴; 18% of 250 = 45.

The three even outcomes are 2, 4 and 6. There are 3 favourable outcomes out of 6 equally likely outcomes, so P(even) = 3/6 = 1/2.

∝ means is proportional to. Divide both sides by 2 to obtain x = y ÷ 2.

(13 - 5) ÷ (6 - 2) = 8 ÷ 4 = 2.

Surface area = 6 × 3² = 54 cm²; volume = 3³ = 27 cm³.

Coarse focus can make the objective hit and break the slide or damage the lens. Start at low power, focus with coarse then fine adjustment, centre the specimen, switch to higher power and use fine focus only.

A = π × 4² = 16π ≈ 50.3 mm².

(1.8 − 2.0) ÷ 2.0 × 100 = −10%; the sample lost 10% of its starting mass.

Ethanol is flammable. Keep it away from naked flames and do not heat it; heat the separate aqueous Benedict's test in a hot-water bath.

Test every 30 seconds. Starch has been completely digested when the sample no longer turns iodine blue-black.

It reduces heating of the pondweed by the lamp so temperature is controlled and the light-intensity comparison is fair.

Record 0.14 s, which is 140 ms. Use the supplied table; no physics equation is required for this lookup.

Length measurements and accurate, labelled biological drawings showing the effects of light or gravity.

Density = 8 ÷ 0.5 = 16 plants per m². Estimated population = 16 × 200 = 3200 plants.

A decrease of 1.2 ÷ 6 = 0.20 pH units per hour.