Edexcel GCSE Biology Key Definitions You Must Know

Definitions are the foundation of biology. In the Edexcel GCSE Biology (1BI0) exam, precise use of scientific terminology is essential -- AO1 (knowledge and understanding) accounts for 40% of the total marks, and a large proportion of those marks come from recall of key definitions. Examiners use specific wording in their mark schemes, and if your definition is vague or incomplete, you will lose marks even if you broadly understand the concept.

Paper 1 covers Topics 1--5 and Paper 2 covers Topics 6--9 plus synoptic content. Each paper is 1 hour 45 minutes, worth 100 marks, and makes up 50% of your final grade. This guide lists the definitions you must know for every topic in the specification, with exam notes highlighting common mistakes. Learn them precisely and you will pick up marks that many students throw away.

Paper Structure at a Glance

Before diving into the definitions, it is worth understanding exactly how they will be tested.

Paper 1 covers Topics 1--5 (Key Concepts in Biology, Cells and Control, Genetics, Natural Selection and Genetic Modification, Health Disease and Development of Medicines). It is 1 hour 45 minutes, worth 100 marks, and accounts for 50% of your final grade. Paper 1 is heavily weighted towards cellular biology, genetics and health -- all areas where precise definitions are essential.

Paper 2 covers Topics 6--9 (Plant Structures and Functions, Animal Coordination Control and Homeostasis, Exchange and Transport in Animals, Ecosystems and Material Cycles) plus synoptic questions that draw on content from any topic. It is also 1 hour 45 minutes, 100 marks, 50% of grade. Paper 2 tends to feature more extended response questions and data analysis, but definition marks are still available throughout.

Both papers include multiple-choice questions, short-answer questions, calculations, and extended open-response questions worth up to six marks. Definition questions can appear in any of these formats.

Topic 1: Key Concepts in Biology

Topic 1 underpins the entire course. The concepts introduced here -- cells, enzymes, transport mechanisms -- appear in almost every other topic, so these definitions must be second nature.

• Eukaryotic cell -- a cell that contains a nucleus and other membrane-bound organelles, such as animal and plant cells.
• Prokaryotic cell -- a cell that does not contain a true nucleus or membrane-bound organelles, such as bacteria.
• Organelle -- a specialised structure within a cell that has a specific function, such as the nucleus, mitochondria or ribosomes.
• Enzyme -- a biological catalyst that speeds up the rate of a chemical reaction without being used up, made of protein.
• Substrate -- the molecule on which an enzyme acts, which fits into the enzyme's active site.
• Active site -- the specific region of an enzyme where the substrate binds and the reaction takes place.
• Lock and key model -- a model to explain enzyme specificity, where the shape of the active site is complementary to the shape of the substrate, so only one type of substrate can fit.
• Denatured -- when the shape of an enzyme's active site has permanently changed so that the substrate can no longer fit, caused by high temperatures or extreme pH.
• Diffusion -- the net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient.
• Osmosis -- the net movement of water molecules through a partially permeable membrane from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution).
• Active transport -- the movement of particles across a cell membrane from a region of lower concentration to a region of higher concentration, against a concentration gradient, using energy from respiration.
• Cell cycle -- the series of stages through which a cell passes as it grows, replicates its DNA, and divides.
• Mitosis -- a type of cell division that produces two genetically identical daughter cells, used for growth and repair.

Exam note: Students frequently lose marks by defining osmosis without specifying "partially permeable membrane" or by omitting "net movement." For enzymes, always use the phrase "biological catalyst" rather than just "catalyst."

When defining active transport, you must state that it requires energy from respiration -- this is what distinguishes it from diffusion. If asked about the lock and key model, emphasise that the active site and substrate have complementary shapes -- not "the same shape."

Practise Topic 1 with LearningBro's Edexcel GCSE Biology -- Cell Biology course.

Topic 2: Cells and Control

Topic 2 builds on cell division and introduces the nervous system and the brain. You need clear definitions for different types of cell division, the components of the nervous system, and the structure and function of the brain and eye. This topic also covers stem cells and their potential uses in medicine, which is a common context for evaluation questions asking you to weigh up benefits and risks.

• Mitosis -- cell division that produces two genetically identical diploid daughter cells from one parent cell, used for growth, repair and asexual reproduction.
• Meiosis -- cell division that produces four genetically different haploid daughter cells (gametes), used in sexual reproduction.
• Diploid -- a cell that contains two complete sets of chromosomes (in humans, 46 chromosomes).
• Haploid -- a cell that contains one complete set of chromosomes (in humans, 23 chromosomes).
• Stem cell -- an undifferentiated cell that is capable of dividing by mitosis to produce more stem cells or differentiating into specialised cell types.
• Differentiation -- the process by which a cell becomes specialised for a particular function.
• Stimulus -- a change in the environment that is detected by a receptor.
• Receptor -- a cell or group of cells that detects a stimulus.
• Effector -- a muscle or gland that carries out a response to a stimulus.
• Synapse -- the junction between two neurones where a nerve impulse is transmitted by the diffusion of neurotransmitter molecules across the gap.
• Reflex arc -- the pathway taken by a nerve impulse from receptor to effector in a reflex action: receptor, sensory neurone, relay neurone, motor neurone, effector.
• Neurotransmitter -- a chemical substance released at a synapse that diffuses across the gap and triggers a nerve impulse in the next neurone.
• Cerebral cortex -- the outer region of the brain responsible for consciousness, memory, language and intelligence.
• Cerebellum -- the region of the brain responsible for coordinating muscular activity and balance.
• Medulla oblongata -- the region of the brain that controls unconscious activities such as heart rate and breathing rate.

Exam note: Many students confuse mitosis and meiosis. Remember: mitosis produces two identical cells and meiosis produces four different cells. The key distinguishing detail is that meiosis produces haploid gametes while mitosis produces diploid cells.

For stem cells, you must include "undifferentiated" -- saying "a cell that can become any cell type" is not precise enough for full marks.

When asked about the brain, you need to link each region to its function: cerebral cortex (consciousness and intelligence), cerebellum (balance and coordination), medulla oblongata (unconscious functions like heart rate). Do not mix these up -- it is a common source of lost marks.

Practise Topics 2 and 3 with LearningBro's Edexcel GCSE Biology -- Cells, Control and Genetics course.

Topic 3: Genetics

Genetics is one of the most definition-heavy topics in the specification. The terms in this section form the vocabulary you need for genetic diagrams, Punnett squares and inheritance questions. Examiners expect you to use these terms precisely -- sloppy use of "gene" when you mean "allele," for example, will cost marks.

• DNA (deoxyribonucleic acid) -- a polymer made of two strands of nucleotides forming a double helix, which carries the genetic code for an organism.
• Gene -- a short section of DNA on a chromosome that codes for a specific protein (or characteristic).
• Allele -- a version of a gene that can give rise to different forms of a characteristic.
• Genotype -- the combination of alleles an organism has for a particular gene.
• Phenotype -- the observable characteristics of an organism, determined by its genotype and environmental factors.
• Dominant allele -- an allele that is always expressed in the phenotype, even when only one copy is present.
• Recessive allele -- an allele that is only expressed in the phenotype when two copies are present (homozygous recessive).
• Homozygous -- having two identical alleles for a particular gene (e.g. BB or bb).
• Heterozygous -- having two different alleles for a particular gene (e.g. Bb).
• Genome -- the entire set of genetic material in an organism.
• Mutation -- a random change in the base sequence of DNA that may alter the protein produced and therefore the phenotype.

Exam note: A very common error is defining a gene as "a section of DNA that codes for a characteristic" without mentioning proteins. Genes code for proteins, and those proteins determine characteristics. The mark scheme requires the link to protein.

Another frequent mistake is defining phenotype without mentioning the environment -- phenotype is the result of both genotype and environmental factors.

When using Punnett squares, always define the letters you are using (e.g. "let B = the dominant allele for brown eyes") before drawing the diagram -- mark schemes award a mark for this.

Topic 4: Natural Selection and Genetic Modification

Topic 4 connects genetics to evolution and applied science. It covers Darwin's theory of evolution by natural selection, modern genetic technologies, and the classification of living organisms. Definitions here often appear in six-mark extended response questions, particularly questions that ask you to describe the process of natural selection or evaluate the advantages and disadvantages of genetic engineering and selective breeding.

• Natural selection -- the process by which organisms with characteristics best suited to their environment are more likely to survive, reproduce and pass on their alleles to the next generation.
• Evolution -- the gradual change in inherited characteristics of a population over many generations, which may result in the formation of new species.
• Selective breeding (artificial selection) -- the process by which humans choose organisms with desirable characteristics and breed them together to produce offspring that inherit those characteristics.
• Genetic engineering -- the process of modifying the genome of an organism by introducing a gene from another organism to produce a desired characteristic.
• Classification -- the organisation of living organisms into groups based on their shared characteristics and evolutionary relationships.
• Species -- a group of organisms that can interbreed to produce fertile offspring.
• Variation -- differences between individuals of the same species, which can be genetic, environmental, or both.
• Adaptation -- a feature of an organism that increases its chance of survival in a particular environment.
• Extinction -- the permanent loss of all members of a species.
• Binomial system -- the internationally agreed system for naming organisms using two parts: the genus name and the species name, written in italics (e.g. Homo sapiens).
• Fossils -- the preserved remains or traces of organisms that lived in the past, providing evidence for evolution.

Exam note: When defining natural selection, you must include the idea of passing on alleles (or genes). Simply saying "survival of the fittest" is not enough and will not score full marks.

For genetic engineering, always state that a gene is taken from one organism and inserted into another -- vague answers about "changing DNA" do not satisfy the mark scheme.

When discussing selective breeding, remember to mention that organisms are chosen over many generations -- it is not a one-step process.

Practise Topic 4 with LearningBro's Edexcel GCSE Biology -- Natural Selection course.

Topic 5: Health, Disease and Development of Medicines

Topic 5 is one of the largest in the specification and is heavily tested on Paper 1. You need to distinguish clearly between communicable and non-communicable diseases, understand how the body defends itself, and know the stages of drug development. This topic also includes content on lifestyle factors (diet, exercise, smoking, alcohol) that contribute to non-communicable diseases -- questions on these frequently appear as data interpretation tasks.

• Health -- the state of physical, mental and social well-being, and not merely the absence of disease or infirmity.
• Communicable disease -- a disease caused by a pathogen that can be spread from one organism to another.
• Non-communicable disease -- a disease that cannot be transmitted from one organism to another, such as cancer, heart disease or diabetes.
• Pathogen -- a microorganism that causes disease, such as a bacterium, virus, fungus or protist.
• Antigen -- a protein on the surface of a cell that triggers an immune response.
• Antibody -- a protein produced by white blood cells (lymphocytes) that binds specifically to an antigen on a pathogen, leading to its destruction.
• Vaccination -- the introduction of a small quantity of dead or inactive pathogen into the body to stimulate the immune system to produce antibodies, providing future immunity.
• Antibiotic -- a substance that kills or inhibits the growth of bacteria. Antibiotics do not work against viruses.
• Drug development -- the process of discovering, testing and trialling a new medical treatment, involving preclinical testing (on cells and animals) followed by clinical trials (on human volunteers), including double-blind trials and peer review.
• Monoclonal antibodies -- identical antibodies produced from a single clone of cells, used in pregnancy tests, diagnosis of disease and targeted drug delivery.
• Placebo -- a substance used in clinical trials that looks like the drug being tested but contains no active ingredient, used to check that the drug is genuinely effective.
• Double-blind trial -- a clinical trial in which neither the patient nor the doctor knows who has received the real drug and who has received the placebo, to prevent bias.
• Antibiotic resistance -- when bacteria evolve and are no longer killed by an antibiotic, due to natural selection of resistant individuals.

Exam note: The definition of health is taken directly from the World Health Organisation and must include all three aspects: physical, mental and social well-being. Students often forget "social" and lose a mark.

For vaccination, always specify that the pathogen is dead or inactive -- writing that a "disease is injected" is a common error that suggests the patient is being given the disease.

Remember that antibiotics only work on bacteria, not viruses -- this is tested almost every year.

For drug development, know the correct order: preclinical testing (cells and animals), then Phase 1 (small group of healthy volunteers to test safety), Phase 2 (small group of patients to test effectiveness), Phase 3 (large-scale double-blind trial with a placebo). Peer review follows before the drug is approved.

Practise Topic 5 with LearningBro's Edexcel GCSE Biology -- Health and Disease course.

Topic 6: Plant Structures and Functions

Topic 6 opens Paper 2 and focuses on how plants make food, transport substances, and respond to their environment. This topic includes the equation for photosynthesis (which you must be able to write as both a word equation and a balanced symbol equation), the mechanism of water transport through xylem, and the role of plant hormones in controlling growth. Core practical work on photosynthesis rate and transpiration rate links directly to these definitions.

• Photosynthesis -- the endothermic reaction in which plants use light energy to convert carbon dioxide and water into glucose and oxygen.
• Chlorophyll -- the green pigment found in chloroplasts that absorbs light energy for photosynthesis.
• Limiting factor -- a factor that, when in short supply, limits the rate of a reaction or process (for photosynthesis: light intensity, carbon dioxide concentration, temperature).
• Transpiration -- the loss of water vapour from the surface of a plant, mainly through the stomata of the leaves.
• Translocation -- the transport of dissolved sugars from the leaves to the rest of the plant through the phloem.
• Xylem -- a plant transport tissue that carries water and dissolved mineral ions from the roots to the leaves in a one-way flow.
• Phloem -- a plant transport tissue that carries dissolved sugars (sucrose) and amino acids both up and down the plant.
• Stomata -- small pores on the underside of leaves that allow gas exchange and water vapour loss, controlled by guard cells.
• Auxin -- a plant hormone that controls cell elongation, involved in phototropism and gravitropism.
• Phototropism -- the growth response of a plant towards or away from a light source.
• Gravitropism (geotropism) -- the growth response of a plant towards or away from gravity.

Exam note: When defining photosynthesis, state that it is endothermic (absorbs energy). Many students lose a mark by omitting this.

For transpiration, note that it is the loss of water vapour specifically -- not liquid water.

The distinction between xylem (water, one direction, upward) and phloem (sugars, both directions) is tested frequently. If asked to compare them, always mention what each transports, the direction of flow, and whether the cells are living (phloem) or dead (xylem).

For tropisms, remember that positive means growth towards the stimulus and negative means growth away from it -- so shoots show positive phototropism (towards light) and roots show positive gravitropism (towards gravity).

Practise Topic 6 with LearningBro's Edexcel GCSE Biology -- Plant Structures course.

Topic 7: Animal Coordination, Control and Homeostasis

Topic 7 is one of the most challenging areas of the specification and carries significant weight on Paper 2. It demands precise knowledge of hormones, feedback mechanisms, and homeostasis. You need to understand how blood glucose is regulated by insulin and glucagon, how body temperature is maintained, how the kidneys control water balance, and how the menstrual cycle is governed by interacting hormones. The concept of negative feedback underpins all of these systems.

• Hormone -- a chemical messenger produced by an endocrine gland, transported in the blood to a target organ where it produces an effect.
• Endocrine system -- the collection of glands in the body that produce hormones to regulate body functions.
• Homeostasis -- the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes.
• Thermoregulation -- the maintenance of a constant core body temperature, controlled by the thermoregulatory centre in the brain.
• Vasoconstriction -- the narrowing of blood vessels near the skin surface to reduce blood flow and heat loss when the body is too cold.
• Vasodilation -- the widening of blood vessels near the skin surface to increase blood flow and heat loss when the body is too hot.
• Type 1 diabetes -- a condition where the pancreas produces little or no insulin, treated with insulin injections.
• Type 2 diabetes -- a condition where the body's cells no longer respond to insulin produced by the pancreas, often linked to obesity and managed through diet and exercise.
• ADH (antidiuretic hormone) -- a hormone released by the pituitary gland that acts on the kidneys to control the reabsorption of water from the collecting duct, regulating the concentration of urine.
• Negative feedback -- a control mechanism where a change in a condition triggers a response that reverses the change, returning the condition to its normal level.
• Insulin -- a hormone produced by the pancreas that causes cells to take up glucose from the blood, lowering blood glucose concentration.
• Glucagon -- a hormone produced by the pancreas that causes glycogen in the liver to be converted back into glucose and released into the blood, raising blood glucose concentration.
• Glycogen -- a storage carbohydrate found in the liver and muscles, formed from excess glucose.
• Menstrual cycle -- the approximately 28-day reproductive cycle in females, controlled by the hormones oestrogen, progesterone, FSH and LH.
• FSH (follicle-stimulating hormone) -- a hormone produced by the pituitary gland that causes an egg to mature in the ovary and stimulates the ovary to produce oestrogen.
• Adrenaline -- a hormone produced by the adrenal glands in response to danger or stress, preparing the body for "fight or flight" by increasing heart rate and blood flow to muscles.

Exam note: The definition of homeostasis must include the idea of maintaining "optimum conditions for function" -- simply saying "keeping things constant" is not enough.

For diabetes, make sure you distinguish clearly between Type 1 (no insulin produced, treated with insulin injections) and Type 2 (cells do not respond to insulin, managed through diet and exercise). Confusing the two is one of the most common errors in Paper 2.

When discussing ADH, always link the hormone to the kidney and water reabsorption.

For the menstrual cycle, you do not need to memorise exact day numbers, but you must know which hormone does what: FSH stimulates egg maturation, oestrogen repairs the uterus lining and inhibits FSH, LH triggers ovulation, and progesterone maintains the uterus lining.

Practise Topic 7 with LearningBro's Edexcel GCSE Biology -- Animal Coordination course.

Topic 8: Exchange and Transport in Animals

Topic 8 covers the circulatory system, gas exchange and respiration. These are among the most frequently examined areas on Paper 2. You need to know the structure and function of the heart, blood vessels and blood components, as well as the equations for both aerobic and anaerobic respiration. Definitions here underpin many calculation and extended response questions.

• Heart -- a muscular organ that pumps blood around the body through a double circulatory system.
• Double circulatory system -- a system in which blood passes through the heart twice during one complete circuit of the body: once to the lungs (pulmonary circuit) and once to the rest of the body (systemic circuit).
• Artery -- a blood vessel that carries blood away from the heart, with thick muscular walls and a narrow lumen to withstand high pressure.
• Vein -- a blood vessel that carries blood towards the heart, with thinner walls, a larger lumen and valves to prevent backflow.
• Capillary -- a tiny blood vessel with walls one cell thick, allowing exchange of substances between the blood and surrounding tissues.
• Plasma -- the liquid component of blood that transports dissolved substances, including glucose, amino acids, carbon dioxide, urea, hormones and antibodies.
• Aerobic respiration -- the chemical reaction that breaks down glucose using oxygen to release energy, producing carbon dioxide and water: glucose + oxygen -> carbon dioxide + water.
• Anaerobic respiration -- respiration without oxygen. In animals: glucose -> lactic acid. In plants and yeast (fermentation): glucose -> ethanol + carbon dioxide.
• Oxygen debt -- the amount of extra oxygen the body needs after anaerobic respiration to break down the accumulated lactic acid.
• Gas exchange -- the movement of oxygen and carbon dioxide between the air in the alveoli and the blood in the surrounding capillaries, occurring by diffusion across a thin, moist surface with a large surface area and a rich blood supply.
• Alveoli -- tiny air sacs in the lungs where gas exchange takes place, adapted with a large surface area, thin walls (one cell thick), a moist lining and a rich blood supply.
• Red blood cell -- a biconcave disc-shaped cell with no nucleus, containing haemoglobin that binds to oxygen to form oxyhaemoglobin for transport around the body.
• White blood cell -- a cell of the immune system that defends the body against pathogens by producing antibodies (lymphocytes) or engulfing and digesting microorganisms (phagocytes).
• Platelet -- a cell fragment that helps blood to clot at the site of a wound, preventing blood loss and the entry of pathogens.

Exam note: For blood vessels, you must link structure to function. Saying an artery has "thick walls" is not enough -- say "thick muscular walls to withstand high pressure."

For respiration, note that anaerobic respiration transfers less energy than aerobic respiration because glucose is only partially broken down. A common error is stating that anaerobic respiration produces no energy -- it does, just less than aerobic respiration.

When describing blood components, remember that red blood cells have no nucleus (to make room for more haemoglobin) and are biconcave (to increase surface area for oxygen absorption). These structural details are what examiners look for when they ask about adaptations.

Practise Topics 8 and 9 with LearningBro's Edexcel GCSE Biology -- Exchange and Ecosystems course.

Topic 9: Ecosystems and Material Cycles

Topic 9 covers ecology, interdependence and nutrient cycling. This is typically the final topic taught and is tested on Paper 2. Questions frequently involve interpreting data from fieldwork, analysing food webs, and explaining the impact of human activity on ecosystems. The definitions below appear regularly in data-analysis questions and six-mark responses.

• Ecosystem -- the interaction of a community of living organisms (biotic factors) with the non-living (abiotic) parts of their environment.
• Community -- all the populations of different species living and interacting in the same area at the same time.
• Population -- the total number of organisms of one species living in an area.
• Habitat -- the place where an organism lives, which provides the conditions and resources it needs.
• Biodiversity -- the variety of different species of organisms in an ecosystem.
• Food chain -- a diagram showing the transfer of energy from one organism to the next, beginning with a producer.
• Food web -- a network of interconnected food chains showing the feeding relationships within a community.
• Producer -- an organism that makes its own food by photosynthesis (a plant or alga), forming the first trophic level.
• Consumer -- an organism that obtains energy by eating other organisms.
• Decomposer -- an organism (such as bacteria or fungi) that breaks down dead organic matter, returning nutrients to the soil.
• Carbon cycle -- the continuous movement of carbon between the atmosphere, living organisms, the oceans and the Earth's crust through processes including photosynthesis, respiration, combustion and decomposition.
• Trophic level -- the position of an organism in a food chain. Energy is lost at each trophic level, mainly through respiration.
• Abiotic factor -- a non-living factor that affects an ecosystem, such as temperature, light intensity, water availability, soil pH or wind speed.
• Biotic factor -- a living factor that affects an ecosystem, such as competition, predation, disease or availability of food.
• Interdependence -- the way in which all organisms in an ecosystem depend on each other for survival, so that a change in one population affects other populations.
• Water cycle -- the continuous movement of water between the atmosphere, land and oceans through evaporation, condensation, precipitation and transpiration.

Exam note: When defining ecosystem, you must mention both the living organisms and the non-living environment -- stating just one part will lose you a mark.

For biodiversity, the key word is "variety" -- do not confuse biodiversity (number of different species) with population size (number of organisms of one species).

In questions about food webs and energy transfer, always state that energy is lost at each trophic level through respiration, not that it is "used up" or "destroyed."

When asked about the impact of humans on ecosystems, structure your answer around specific activities (deforestation, pollution, overfishing) and link each to a specific consequence (loss of habitat, reduced biodiversity, disruption of food chains).

Practise ecology and ecosystems with LearningBro's Edexcel GCSE Biology -- Exchange and Ecosystems course.

How to Use Definitions in Exam Answers

Knowing the definitions is only half the battle -- you also need to deploy them correctly under exam conditions. Many students can recite a definition at home but fail to score the mark in the exam because they paraphrase incorrectly, include the wrong detail, or bury the key phrase in a rambling answer. Here are practical strategies to avoid these pitfalls.

Use the exact terminology. If the mark scheme says "partially permeable membrane," writing "semi-permeable membrane" is accepted, but writing "a membrane with tiny holes" is not. Match the scientific vocabulary wherever possible.

Front-load your definitions. When a question says "Define..." or "What is meant by...," write the definition in your first sentence. Do not bury it in the middle of a paragraph.

Link definitions to context. In longer questions, use the definition and then apply it to the scenario in the question. For example, if asked to explain why a plant wilts, start by defining osmosis, then explain how water moves out of the cells by osmosis when placed in a concentrated solution.

Watch for command words. "State" means give a brief factual answer. "Explain" means give a reason using scientific knowledge. "Describe" means say what happens without necessarily explaining why. See our guide to Edexcel GCSE Exam Command Words Explained for the full breakdown.

Do not over-explain on short questions. A one-mark "State the definition of..." question needs one clear sentence. Writing a paragraph wastes time and increases the chance of contradicting yourself. Save your detail for six-mark questions where the mark scheme rewards depth.

Practise with mark schemes. After attempting a past paper question, read the mark scheme carefully. Note the exact phrases that earn marks. You will quickly see that the same key phrases appear again and again -- "net movement," "concentration gradient," "partially permeable membrane," "optimum conditions." These are the phrases you must memorise.

Avoid common language traps. Do not say enzymes are "killed" by high temperatures -- they are denatured (enzymes are not alive). Do not say energy is "created" or "made" during respiration -- energy is transferred or released. Do not say organisms "choose" to evolve or "need" to adapt -- evolution happens through random variation and natural selection, not through choice. Examiners specifically penalise these errors because they reveal a misunderstanding of the underlying science.

A Revision Strategy for Definitions

Learning 100+ definitions can feel overwhelming. Here is a structured approach that works.

Week 1--2: Make definition flashcards. Write the term on one side and the full definition on the other. Organise them by topic. Use the definitions from this guide as your starting point.

Week 3--4: Active recall testing. Test yourself daily on one topic at a time. Cover the definition and try to write it from memory. Check your answer against the flashcard. Any definitions you get wrong go into a separate "retry" pile.

Week 5 onwards: Past paper practice. Work through past papers and examiner reports. When you encounter a definition question, write your answer before looking at the mark scheme. Compare your wording to the mark scheme wording and adjust your flashcards if needed.

Ongoing: Spaced repetition. Return to earlier topics regularly. Definitions you learned in Week 1 will fade if you do not revisit them. Aim to cycle through all nine topics at least once a week in the final month before the exam. Research consistently shows that spaced repetition is the most effective way to commit factual knowledge to long-term memory.

Prepare for exam day with LearningBro's Edexcel GCSE Biology -- Exam Prep course.

Definitions That Cross Multiple Topics

Some terms appear across several topics in the specification. Examiners can test these in any context, so you must be comfortable applying them wherever they appear.

• Surface area to volume ratio -- the relationship between the surface area of an object and its volume. As organisms get larger, the ratio decreases, making diffusion alone insufficient for exchange of substances, which is why large organisms need specialised exchange surfaces and transport systems.
• Concentration gradient -- the difference in concentration of a substance between two regions. Particles move down a concentration gradient in diffusion and osmosis, and against it in active transport.
• Specialised cell -- a cell that has a specific structure adapted to carry out a particular function, such as a red blood cell, sperm cell, root hair cell or nerve cell.
• Respiration -- an exothermic reaction that occurs continuously in all living cells to release energy from glucose, which is needed for all life processes.
• Metabolism -- the sum of all the chemical reactions that take place in the cells of an organism, including respiration, photosynthesis, protein synthesis and breakdown of molecules.

Exam note: The concept of surface area to volume ratio connects Topic 1 (cells), Topic 6 (leaves), Topic 8 (alveoli and villi) and Topic 9 (exchange surfaces). Whenever you are asked why a biological surface is adapted for efficient exchange, your answer should reference: large surface area, thin walls, good blood supply (for animals), and a concentration gradient maintained by blood flow or ventilation. This pattern earns marks across multiple topics. Respiration is another cross-cutting concept -- it provides the energy for active transport (Topic 1), muscle contraction (Topic 8), nerve impulse transmission (Topic 2) and maintaining body temperature (Topic 7). Whenever a question asks where the energy comes from for a biological process, the answer is almost always respiration.

Pulling It All Together

The definitions in this guide cover the core terminology you need for both Paper 1 and Paper 2 of Edexcel GCSE Biology. There are over 100 defined terms across the nine topics, and while that sounds like a lot, many of them follow patterns -- transport mechanisms all involve concentration gradients, hormones are all chemical messengers carried in the blood, and exchange surfaces all share the same adaptations. Once you recognise these patterns, the definitions become easier to remember.

Work through them topic by topic as you revise, test yourself regularly, and practise writing them out under timed conditions. When you encounter these terms in past papers, you will find that precise definitions lead directly to marks. Pay particular attention to the exam notes in each section -- they highlight the exact errors that cause students to lose marks every year.

Biology rewards students who learn definitions thoroughly. Unlike calculation-heavy subjects, many biology marks come from simply stating the right words in the right order. AO1 recall accounts for 40% of the total marks available, and a significant portion of that comes from definition questions worth one or two marks each. These are marks you can secure with straightforward memorisation -- treat every definition as a guaranteed mark waiting to be collected.

For a broader overview of the specification, see our Edexcel GCSE Biology Revision Guide. To prepare for core practicals, read our Edexcel GCSE Biology Core Practicals Guide. For advice on how examiners award marks, see How Edexcel Mark Schemes Work.

Explore all Edexcel GCSE Biology courses on LearningBro and start practising with exam-style questions today.