GCSE Biology Formula Sheet – Key Equations, Processes & Must-Know Facts (AQA 8461)

This sheet gathers the equations, processes and facts that come up again and again in GCSE Biology into one place, so you can check a formula quickly instead of hunting through notes. It follows the seven topics of the AQA 8461 specification in order, starting with cells and ending with ecosystems. Use it for short, regular revision sessions, and practise writing each equation from memory before you look at the answer.

How to use this sheet: Cover the teal equation box, try to write the equation yourself, and then check it. After that, read the bullet points for the topic and pick out the one idea you are least sure about, then test yourself on that idea alone. Short, repeated checks like this build recall far better than reading the whole sheet through once. For the full set of biology revision pages, start at the Biology MCQs hub.

1. Cell Biology

• Animal cells contain a nucleus, cytoplasm, a cell membrane, mitochondria and ribosomes. Plant cells usually have those parts too, plus a cell wall made of cellulose, chloroplasts and a permanent vacuole filled with cell sap.
• Bacterial cells are much smaller and have no nucleus. Their genetic material sits as a single loop of DNA in the cytoplasm, often with small extra rings of DNA called plasmids.
• Mitosis produces two cells that are genetically identical to the parent cell. It is how multicellular organisms grow and how damaged tissues are repaired.
• Stem cells are unspecialised cells that can divide and then specialise into other cell types. Embryonic stem cells can form most cell types, adult bone marrow stem cells can form blood cells, and plant meristem cells can form any plant cell type throughout the life of the plant.
• Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. It is faster when the concentration difference is larger, the temperature is higher, or the surface area is larger.
• Osmosis is the movement of water from a dilute solution to a more concentrated solution through a partially permeable membrane.
• Active transport moves substances against a concentration gradient, from a more dilute solution to a more concentrated one, and it needs energy released by respiration.
• A single-celled organism has a large surface area compared with its volume, so diffusion alone can supply it. Larger multicellular organisms need specialised exchange surfaces and a transport system because their surface area to volume ratio is smaller.
• An effective exchange surface has a large surface area, a thin membrane for a short diffusion distance, a good blood supply where relevant, and ventilation where relevant.

2. Organisation

• Organisation follows a clear order: cells group into tissues, tissues group into organs, organs work together in organ systems, and organ systems form an organism.
• Enzymes are biological catalysts. Each enzyme has an active site with a shape that fits one particular substrate, which is why enzymes are specific.
• Enzyme activity rises with temperature up to an optimum, then falls sharply as the enzyme changes shape and stops working. Extreme pH has a similar effect on the active site.
• In the digestive system, carbohydrases break down starch into sugars, proteases break down proteins into amino acids, and lipases break down lipids into fatty acids and glycerol.
• Bile, made in the liver and stored in the gall bladder, neutralises stomach acid in the small intestine and breaks large fat droplets into smaller ones, giving lipases a larger surface area to work on.
• The heart pumps blood around the body in a double circulatory system: one circuit sends blood to the lungs and back, and the other sends blood around the rest of the body and back.
• Arteries carry blood away from the heart at high pressure and have thick, muscular walls. Veins carry blood back to the heart at lower pressure and have valves to stop backflow. Capillaries are very narrow with thin walls, so substances can diffuse quickly between blood and tissues.
• Red blood cells carry oxygen, white blood cells help defend against disease, platelets help blood to clot, and plasma carries dissolved substances including glucose, hormones and urea.
• Non-communicable diseases, such as coronary heart disease, type 2 diabetes and some cancers, cannot be passed from person to person. Risk factors include smoking, poor diet, lack of exercise and obesity, and several risk factors acting together raise the risk further.
• In plants, xylem carries water and dissolved minerals upwards from the roots, while phloem carries sugars made in photosynthesis around the plant in both directions.
• Transpiration is the loss of water vapour from plant leaves. It is faster in bright light, warm air, moving air and dry air.

3. Infection and Response

• Communicable diseases are caused by pathogens and can spread between organisms. The main pathogen groups are viruses, bacteria, fungi and protists.
• Pathogens spread in different ways, including through the air, through contaminated water or food, by direct contact, and by vectors such as insects.
• Measles is a viral disease spread by droplets from coughs and sneezes. HIV is a virus that attacks white blood cells and weakens the immune system over time.
• Salmonella bacteria cause food poisoning, and gonorrhoea is a bacterial disease spread by sexual contact. Rose black spot is a fungal disease of plants, and malaria is caused by a protist carried by mosquitoes.
• The body’s first defences include skin, mucus, stomach acid and cilia that trap and move particles away. If a pathogen gets past these, white blood cells destroy it by engulfing it, by producing antibodies, or by producing antitoxins that neutralise toxins.
• Vaccination introduces a dead or weakened form of a pathogen, or a harmless part of it, so the immune system produces antibodies and memory cells without causing the full disease. If the real pathogen later enters, the response is faster and stronger.
• Antibiotics kill bacteria or stop them growing, but they do not work against viruses, because viruses live inside body cells and reproduce differently. Overuse of antibiotics encourages resistant bacteria to survive and spread.
• New drugs are tested in stages: first in laboratory tests, then in animal tests, then in clinical trials on healthy volunteers and patients, with placebos used to check whether the drug itself is having the effect.
• Plant defences include a waxy cuticle, a tough cell wall, and chemicals such as antibacterial compounds. Some plants also produce mechanical defences such as thorns.

4. Bioenergetics

• Photosynthesis takes place in chloroplasts and is an endothermic reaction, because it takes in light energy and stores it as chemical energy in glucose.
• The rate of photosynthesis is limited by whichever factor is in shortest supply: light intensity, carbon dioxide concentration, or temperature. Raising one factor only helps until another factor becomes limiting.
• Glucose made in photosynthesis can be used in respiration, converted into starch for storage, converted into cellulose for cell walls, or combined with nitrate ions to make amino acids and then proteins. It can also be stored as fats or oils in seeds.
• Respiration is not breathing. It is the chemical process in every living cell that releases energy from glucose, and it happens in mitochondria.
• Aerobic respiration releases much more energy per glucose molecule than anaerobic respiration, which is why anaerobic respiration cannot sustain hard exercise for long.
• During vigorous exercise, muscles may respire anaerobically and build up lactic acid. The extra oxygen needed afterwards to break down lactic acid and repay the energy debt is called oxygen debt.
• Metabolism is the sum of all the chemical reactions in a cell or organism, including building larger molecules from smaller ones and breaking larger molecules down.

5. Homeostasis and Response

• Homeostasis is the control of internal conditions, such as blood glucose concentration, body temperature and water content, so that cells can work properly.
• In the nervous system, a stimulus is detected by a receptor, the signal travels along sensory neurones to the central nervous system, and a response is carried by motor neurones to an effector such as a muscle or gland.
• A reflex action is rapid and automatic, and it does not involve conscious thought. The signal passes through a relay neurone in the spinal cord or brain, which shortens the pathway.
• Synapses are gaps between neurones. A chemical transmitter crosses the gap and triggers the next neurone, which is why signals travel in one direction.
• Insulin is released when blood glucose rises. It causes cells to take up glucose and the liver to convert glucose to glycogen, bringing blood glucose back down. Glucagon has the opposite effect when blood glucose falls.
• In type 1 diabetes the pancreas produces little or no insulin, so blood glucose stays high. In type 2 diabetes the body responds poorly to insulin, and the condition is linked to obesity and lifestyle as well as age and genetics.
• Adrenaline, released in stressful situations, prepares the body for action by increasing heart rate and blood flow to muscles and by releasing glucose from glycogen.
• Thyroxine, released by the thyroid gland, controls metabolic rate. Its release is controlled by negative feedback: high thyroxine levels reduce the release of the hormones that stimulate the thyroid.
• The kidneys remove urea from the blood and control water content. ADH controls how much water is reabsorbed: more ADH means more water reabsorbed and a smaller volume of more concentrated urine.
• Plants respond to light and gravity. Auxin made in shoot tips moves to the shaded side, where it speeds up cell growth, so the shoot bends towards light. Roots bend away from light and towards gravity.

6. Inheritance, Variation and Evolution

• A gene is a section of DNA that codes for a particular protein. Genes sit on chromosomes, and most body cells carry two copies of each chromosome, one from each parent.
• An allele is one version of a gene. A dominant allele is expressed even when only one copy is present, while a recessive allele is only expressed when two copies are present.
• Genotype describes the alleles an organism carries, and phenotype describes the observable characteristic that results from the genotype plus the environment.
• A person who carries one dominant and one recessive allele is heterozygous for that gene. A person who carries two identical alleles is homozygous.
• Meiosis produces sex cells with half the usual number of chromosomes. When two sex cells fuse at fertilisation, the full number is restored and the offspring carries a new combination of alleles.
• Variation within a species can be genetic, environmental, or a combination of both. Genetic variation arises from mutations, from meiosis, and from the random combination of alleles at fertilisation.
• Mutations are random changes in DNA. Most have little or no effect, a few are harmful, and a few give an advantage in a particular environment.
• Natural selection works in stages: organisms vary, more offspring are produced than can survive, those best adapted to the environment are more likely to survive and reproduce, and their advantageous alleles become more common over generations.
• Selective breeding, or artificial selection, is when humans choose organisms with wanted characteristics and breed from them over many generations, as in the breeding of wheat with high yield or cattle with high milk production.
• Genetic engineering inserts a gene from one organism into another, for example inserting the human insulin gene into bacteria so the bacteria produce human insulin.
• Fossils, the fossil record, and comparisons of DNA all provide evidence for evolution, and extinct species such as dinosaurs are known mainly from fossils.

7. Ecology

• An ecosystem is a community of organisms plus the non-living parts of their environment, such as soil, water and climate, interacting together.
• Abiotic factors are non-living influences such as light, temperature, water availability and pH. Biotic factors are living influences such as competition, predation and disease.
• Producers, usually green plants and algae, make their own food by photosynthesis. Consumers feed on other organisms, and decomposers break down dead material and waste, returning nutrients to the soil.
• In a food chain, arrows show the direction of energy flow, from the organism being eaten to the organism eating it. A food web shows many linked food chains in one habitat.
• A trophic level is a feeding stage in a food chain. Pyramids of biomass show the mass of living material at each level, and they are usually pyramid-shaped because so much energy is lost between levels.
• Competition happens when organisms need the same limited resources, such as light, water, food or territory. Animals may also compete for mates.
• Materials are cycled through ecosystems. In the water cycle, water evaporates, condenses and falls as precipitation, then returns to rivers, lakes and seas. In the carbon cycle, carbon dioxide is taken in by photosynthesis and released by respiration, decomposition and combustion.
• Decomposers break down dead plants and animals, releasing carbon dioxide by respiration and returning mineral ions to the soil for plants to reuse.
• Human activities affect ecosystems through deforestation, which removes habitats and releases carbon dioxide, and through pollution of land, water and air. Rising carbon dioxide and methane levels are linked to global warming and climate change.
• Biodiversity is the variety of living organisms in an area. Protecting habitats, reducing deforestation, breeding programmes for endangered species, and protecting rare habitats all help maintain biodiversity.
• Sampling with quadrats and transects, and counting techniques such as pitfall traps, let biologists estimate population sizes and distribution without counting every individual.

Keep Revising

Do not try to memorise this sheet in one sitting. Pick one topic at a time, write its equations and key processes from memory, and then check yourself against the sheet. Anything you miss goes straight onto a short list for tomorrow, and anything you get right can wait a few days before you test it again. Mix topics as you get stronger, so you practise switching between cell biology, bioenergetics, genetics and ecology in the same session, just as the papers do.

When you practise calculations, always show your working, keep your units consistent, and check whether the answer makes sense before moving on. A magnification in the thousands, a BMI in the twenties, or a genetic ratio of 3 : 1 should feel familiar by the time you finish revising. If a number looks wildly out of place, re-check the units and the rearrangement before assuming the method was wrong.

Related sheets in this series: IGCSE Biology Formula Sheet, A-Level Biology Formula Sheet and AP Biology Formula Sheet. Return to the Biology MCQs hub for practice questions by topic.

GCSE Biology Section MCQs

Topic-wise practice questions for AQA GCSE Biology 8461 — 30 MCQs per topic with answers and explanations.

• Cell biology
• Organisation
• Infection and response
• Bioenergetics
• Homeostasis and response
• Inheritance, variation and evolution
• Ecology