Why exact wording wins marks (and how to use this list)
Most marks in IGCSE Biology are awarded for explaining, applying and interpreting. Definitions are different. They reward recall of a fixed form of words, and that makes them some of the most reliable marks on the whole paper, provided you learn them precisely.
Here is the uncomfortable truth that costs students marks every session: examiners do not award the point for "the right general idea". They award it for specific creditable terms in the mark scheme. "Osmosis is when water moves to where there's more stuff" describes the right idea and scores zero, because none of the load-bearing terms are present. The same student writing "net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane" scores in full. Same understanding, completely different mark. This is exactly the kind of precision our command words guide trains for the rest of the paper.
How to use this list well:
- Learn the bold terms first. In each definition below, the words that typically carry the marks are the technical ones, the terms a casual paraphrase would drop. Those are the ones to protect.
- Test recall, don't reread. Cover the definition, write it from memory, then check it word against word. Rereading feels productive and isn't.
- Say it aloud. If you can recite a clean version unprompted, you own it. If you can only recognise it, you don't.
The definitions here follow current Cambridge 0610/0970 phrasing as closely as a revision aid sensibly can. Where a definition has a standard equation (photosynthesis, respiration), learn that too. Examiners frequently ask for it. For a full method on getting these into long-term memory, see how to memorise IGCSE Biology.
Movement in and out of cells
This trio is examined constantly and confused constantly. The marks live in the precise differences between them, so learn what each one does and does not require.
| Term | Definition (mark-scheme accurate) | Examiner note |
|---|---|---|
| Diffusion | The net movement of particles from a region of their higher concentration to a region of their lower concentration (down a concentration gradient), as a result of their random movement. | The mark-carrying words are net movement, higher to lower concentration and random movement. "Particles spread out" is not enough. No energy from respiration is involved, do not mention it. |
| Osmosis | The net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentrated solution), through a partially permeable membrane. | Three terms must appear: water molecules (not just "water"), water potential (higher to lower), and partially permeable membrane. Never write "semi-permeable" or "selectively permeable" in the current syllabus. |
| Active transport | The movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (against a concentration gradient), using energy from respiration. | The two things that separate it from diffusion: it goes against the concentration gradient (low to high) and it uses energy from respiration. Drop either term and you lose the mark. |
A common trap: students define osmosis using "concentration" rather than "water potential". At Extended level, water potential is the term the mark scheme expects, so train yourself out of the concentration phrasing for water. Our movement in and out of cells notes drill the differences with worked examples.
Enzymes and biological molecules
Enzyme definitions reward two ideas in particular: that enzymes are biological catalysts, and that they are protein. Both phrases tend to be on the mark scheme.
| Term | Definition (mark-scheme accurate) | Examiner note |
|---|---|---|
| Enzyme | A protein that functions as a biological catalyst. | Both words matter: protein and biological catalyst. "A substance that speeds up reactions" omits both and scores poorly. If asked to expand, add that enzymes speed up the rate of a reaction without being changed or used up. |
| Catalyst | A substance that increases the rate of a chemical reaction and is not changed by the reaction. | The two creditable ideas are increases the rate and not changed / used up. An enzyme is a specific kind of catalyst, a biological one. |
Two quick exam pointers. First, enzymes are specific: one enzyme catalyses one reaction because its active site is complementary in shape to its substrate. Be ready to describe this, but do not put "lock and key" into the bare definition of an enzyme. Second, denaturing is not "killing": high temperature or extreme pH changes the shape of the active site so the substrate no longer fits. Enzymes are not alive, so "the enzyme dies" is a guaranteed lost mark. More in our enzymes topic guide.
Photosynthesis and respiration
These are the highest-traffic definitions on the paper, and Cambridge regularly asks for the word equations as well as the definitions. Learn both, exactly.
| Term | Definition (mark-scheme accurate) | Examiner note |
|---|---|---|
| Photosynthesis | The process by which plants make carbohydrates (glucose) from raw materials (carbon dioxide and water) using energy from light, which is absorbed by chlorophyll. | Mark-carriers: carbon dioxide and water as raw materials, light energy, chlorophyll and glucose / carbohydrate as product. Word equation: carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll). |
| Aerobic respiration | The chemical reactions in cells that use oxygen to break down nutrient molecules (glucose) to release energy. | Must include uses oxygen, break down glucose and release energy. Word equation: glucose + oxygen → carbon dioxide + water (+ energy released). |
| Anaerobic respiration | The chemical reactions in cells that break down nutrient molecules to release energy without using oxygen. | The key contrast is without oxygen, and it releases less energy than aerobic respiration. In muscle: glucose → lactic acid. In yeast: glucose → alcohol (ethanol) + carbon dioxide. |
| Respiration (general) | The chemical reactions in cells that break down nutrient molecules and release energy for metabolism. | Define it as a set of chemical reactions in cells, not as "breathing". See the distinction below, it is examined directly. |
The respiration-versus-breathing distinction is one examiners deliberately test, and it catches a startling number of students:
- Respiration is a chemical process inside cells that releases energy from glucose.
- Breathing (ventilation) is the physical movement of air into and out of the lungs.
Writing "respiration is breathing" is one of the most common and most costly errors in the subject. Use our respiration guide to lock the difference in.
Transport in plants, nutrition and excretion
This group mixes plant transport with two pairs of words that students routinely swap: transpiration/translocation and excretion/egestion. The marks are in keeping them straight.
| Term | Definition (mark-scheme accurate) | Examiner note |
|---|---|---|
| Transpiration | The loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells, followed by diffusion of water vapour out through the stomata. | Key terms: evaporation from mesophyll surfaces and diffusion out through stomata. It is water vapour that is lost, say so. |
| Translocation | The movement of sucrose and amino acids in the phloem, from regions of production (sources) to regions of storage or use (sinks). | Examiners want phloem, the correct substances (sucrose and amino acids), and the source to sink idea. Do not confuse with transpiration, which is water in the xylem. |
| Excretion | The removal from the body of the waste products of metabolism (chemical reactions in cells), toxic materials and substances in excess of requirements. | The defining words are waste products of metabolism. Examples: carbon dioxide, urea. This separates it from egestion below. |
| Egestion | The passing out of undigested food (faeces) through the anus. | The crucial contrast: egestion removes undigested food that was never absorbed, so it is not a waste product of metabolism. Calling faeces "excretion" is a classic lost mark. |
| Deamination | The removal of the nitrogen-containing part of amino acids to form urea (carried out in the liver). | Mention the liver and the product urea. Excess amino acids cannot be stored, hence deamination. |
| Assimilation | The movement of digested food molecules into the cells of the body where they are used, becoming part of the cells. | The idea is that absorbed molecules are built into cells / used by cells. Distinguish from absorption, which is merely crossing into the blood. |
For the plant-transport pair, our transport in plants guide contrasts xylem and phloem side by side, which is the fastest way to stop swapping transpiration and translocation.
Homeostasis and coordination
These definitions reward the precise control vocabulary. Internal environment, set point, corrective mechanisms.
| Term | Definition (mark-scheme accurate) | Examiner note |
|---|---|---|
| Homeostasis | The maintenance of a constant internal environment (within set limits). | The mark words are constant / stable internal environment. Good examples to attach: body temperature, blood glucose, water content. |
| Negative feedback | A control mechanism in which a change in a condition triggers a response that returns the condition back towards the normal (set) level. | Examiners want the change → response → reverses the change logic. The response opposes the original change. Saying it "reduces" or "counteracts" the change earns the point; "keeps it the same" does not explain the mechanism. |
A reliable way to show understanding is to walk a worked example, such as blood glucose: a rise is detected, insulin is secreted, glucose is converted to glycogen in the liver, and the level falls back toward normal. That sequence demonstrates negative feedback far more convincingly than the bare definition. The full machinery is in our coordination and homeostasis guide.
Cell division and inheritance
This is the densest cluster of definitions in the syllabus, and the one where precise wording matters most because so many terms are near-synonyms with crucial differences. Learn them as contrasting pairs.
| Term | Definition (mark-scheme accurate) | Examiner note |
|---|---|---|
| Mitosis | Nuclear division giving rise to genetically identical cells (in which the chromosome number is maintained). | Mark words: genetically identical and chromosome number maintained. Used for growth, repair and asexual reproduction. |
| Meiosis | A reduction division in which the chromosome number is halved, from diploid to haploid, producing genetically different cells (gametes). | The two essentials: chromosome number halved (reduction division) and genetically different. Contrast every word with mitosis above. |
| Gene | A length of DNA that codes for a (specific) protein. | Keep it tight: length of DNA that codes for a protein. The newer phrasing "a unit of inheritance" is acceptable, but the DNA/protein version is safest. |
| Allele | An alternative form (version) of a gene. | The mark is on alternative form / version of a gene. For example, alleles for brown and blue eye colour are alleles of the same gene. |
| Genotype | The genetic make-up of an organism in terms of the alleles present (e.g. Tt). | It is about the alleles present. Often written as letters. Contrast with phenotype. |
| Phenotype | The observable features (physical or biochemical characteristics) of an organism. | The key word is observable features / characteristics, what you can see or measure, e.g. tall, brown-eyed. |
| Dominant | An allele that is expressed if it is present (in the phenotype). | It shows in the phenotype even when only one copy is present. Conventionally written with a capital letter. |
| Recessive | An allele that is only expressed when there is no dominant allele present. | It shows in the phenotype only when two copies are present (homozygous recessive). Written with a lower-case letter. |
| Homozygous | Having two identical alleles of a particular gene (e.g. TT or tt). | Two identical alleles. A homozygous individual is "true-breeding" for that feature. |
| Heterozygous | Having two different alleles of a particular gene (e.g. Tt). | Two different alleles. A heterozygous individual shows the dominant phenotype but can pass on the recessive allele. |
The fastest way to stop muddling these is to practise them on Punnett squares with real crosses, where genotype, phenotype, dominant and recessive all appear at once. Our inheritance guide works through monohybrid crosses step by step.
Variation, selection and classification
These definitions underpin the evolution and classification questions, where vague wording is heavily penalised.
| Term | Definition (mark-scheme accurate) | Examiner note |
|---|---|---|
| Continuous variation | Variation that results in a range of phenotypes between two extremes, with no distinct categories (e.g. height, mass). | The mark is on a range / continuum between extremes. Usually shown on a line graph or histogram. Controlled by many genes and the environment. |
| Discontinuous variation | Variation that results in a limited number of distinct phenotypes with no intermediates (e.g. blood group, ability to roll the tongue). | The mark is on distinct categories with no intermediates. Usually controlled by a single gene; shown on a bar chart. |
| Natural selection | The process by which organisms better adapted to their environment tend to survive, reproduce and pass on their alleles to the next generation. | Examiners want the full chain: variation → better-adapted survive → reproduce → pass on alleles. "Survival of the fittest" alone is not a definition. |
| Adaptive feature | An inherited feature that helps an organism to survive and reproduce in its environment. | Two ideas: it is inherited and it aids survival and reproduction. Be ready to give a worked example, e.g. a feature reducing water loss. |
| Species | A group of organisms that can reproduce to produce fertile offspring. | The single most important word is fertile. "Can breed together" is not enough, because some different species can breed but produce infertile offspring (e.g. a mule). |
| Binomial system | An internationally agreed system of naming species in which the scientific name is made of two parts: the genus and the species. | Two parts: genus then species. The genus takes a capital letter, the species a lower-case letter (e.g. Homo sapiens). |
For the wider framework of kingdoms and classification keys, see characteristics and classification; for the selection side, variation and selection.
Reproduction and disease
The final high-value cluster covers reproduction and the immune-system terms, where the immunity definitions in particular are tested almost every session.
| Term | Definition (mark-scheme accurate) | Examiner note |
|---|---|---|
| Sexual reproduction | A process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote and the production of offspring that are genetically different from each other. | Mark words: fusion of gamete nuclei and genetically different offspring. Involves two parents and meiosis. |
| Asexual reproduction | A process resulting in the production of genetically identical offspring from one parent. | The two essentials: one parent and genetically identical offspring (clones). Involves mitosis, no gametes. |
| Pathogen | A disease-causing organism (microorganism). | Keep it simple: an organism that causes disease. Examples include certain bacteria, viruses and fungi. |
| Antibody | A protein, made by lymphocytes, that binds to (the antigens on) a specific pathogen and helps destroy it. | Mark words: protein, made by lymphocytes, and specific to a particular antigen. Each antibody has a complementary shape to its antigen. |
| Active immunity | Defence against a pathogen by the production of antibodies in the body, gained after infection or by vaccination. | The defining idea is that the body makes its own antibodies, so it is long-lasting and involves memory cells. |
| Passive immunity | Short-term defence against a pathogen by antibodies acquired from another individual (e.g. across the placenta or in breast milk). | The contrast: antibodies are received, not made by the body, so protection is short-lived with no memory cells. |
| Vaccination | Putting a weakened, dead or modified pathogen (or its antigens) into the body to stimulate the production of antibodies and memory cells, giving active immunity. | Examiners want the chain: antigen introduced → lymphocytes make antibodies → memory cells formed → faster response next time. Just "an injection" scores nothing. |
For the full immune response and the difference between antigens and antibodies, work through our diseases and immunity guide alongside this table.
How to actually memorise these
A list of perfect definitions is worthless if it stays on the page. Converting it into reliable exam recall takes a method, not just repeated reading. Here is the routine we recommend.
- Active recall, not rereading. Cover the definition column, write each one from memory, then check word-for-word against the version here. Mark every missing creditable term. Those gaps are exactly where you would lose the mark.
- Spaced repetition. Review the ones you got wrong after one day, then three days, then a week. Flashcards (paper or an app) are ideal for this, with the term on one side and the mark-scheme wording on the other.
- Teach them aloud. Recite each definition to a parent, a sibling or an empty room. If you can produce a clean version unprompted, it is genuinely learned. If you can only nod along when you read it, it is not.
- Test in context. Pull "define" and "state what is meant by" questions from past papers and answer them cold, then mark against the real mark scheme. This trains you to deploy the wording under pressure, which is the only condition that counts.
Two final cautions. First, beware the near-synonyms. Osmosis vs diffusion, mitosis vs meiosis, excretion vs egestion, active vs passive immunity. Learn each pair together and define each by how it differs from its partner. Second, do not paraphrase to sound natural. In a definition, the textbook phrasing is the answer, and "putting it in your own words" is precisely how creditable terms get lost.
For the full memorisation system, including how to schedule reviews across an eight-week run-in, see how to memorise IGCSE Biology, and use our glossary as a quick-reference companion to this list. Master these definitions and you have secured a block of marks that many candidates leave on the table, before you have answered a single "explain" question. For the technique that wins the rest, start with how to get an A*.