In short
The 2025 IB Biology guide defines 28 command terms across three groups: AO1 recall (define, state, list), AO2 application (describe, outline, calculate) and AO3 reasoning (explain, evaluate, discuss). Learn each IB definition, match your answer length to the marks, and avoid confusing describe with explain.
A command term is the verb at the start of an IB Biology question. It tells you how much to write and what kind of answer earns the marks. The 2025 IB Biology guide defines 28 of them. The table gives the IB's own wording for every one, with the typical marks you can expect.
| Group | Command term | IB definition | Typical marks |
|---|---|---|---|
| AO1 | Define | Give the precise meaning of a word, phrase, concept or physical quantity. | 1–2 |
| AO1 | Draw | Represent by means of a labelled, accurate diagram or graph, using a pencil. A ruler (straight edge) should be used for straight lines. Diagrams should be drawn to scale. Graphs should have points correctly plotted (if appropriate) and joined in a straight line or smooth curve. | 2–4 |
| AO1 | Label | Add labels to a diagram. | 1 per label |
| AO1 | List | Give a sequence of brief answers with no explanation. | 1 per item |
| AO1 | Measure | Obtain a value for a quantity. | 1 |
| AO1 | State | Give a specific name, value or other brief answer without explanation or calculation. | 1 |
| AO2 | Annotate | Add brief notes to a diagram or graph. | 1–3 |
| AO2 | Calculate | Obtain a numerical answer showing the relevant stages in the working. | 1–3 |
| AO2 | Describe | Give a detailed account. | 2–4 |
| AO2 | Distinguish | Make clear the differences between two or more concepts or items. | 2 |
| AO2 | Estimate | Obtain an approximate value. | 1 |
| AO2 | Identify | Provide an answer from a number of possibilities. | 1 |
| AO2 | Outline | Give a brief account or summary. | 2–3 |
| AO3 | Analyse | Break down in order to bring out the essential elements or structure. | 2–4 |
| AO3 | Comment | Give a judgement based on a given statement or result of a calculation. | 1–2 |
| AO3 | Compare | Give an account of the similarities between two (or more) items or situations, referring to both (all) of them throughout. | 2–3 |
| AO3 | Compare and contrast | Give an account of similarities and differences between two (or more) items or situations, referring to both (all) of them throughout. | 3–4 |
| AO3 | Construct | Display information in a diagrammatic or logical form. | 1–3 |
| AO3 | Deduce | Reach a conclusion from the information given. | 1–2 |
| AO3 | Design | Produce a plan, simulation or model. | 3–6 |
| AO3 | Determine | Obtain the only possible answer. | 1–2 |
| AO3 | Discuss | Offer a considered and balanced review that includes a range of arguments, factors or hypotheses. Opinions or conclusions should be presented clearly and supported by appropriate evidence. | 3–6, or a Section B part |
| AO3 | Evaluate | Make an appraisal by weighing up the strengths and limitations. | 2–4 |
| AO3 | Explain | Give a detailed account including reasons or causes. | 2–4 |
| AO3 | Justify | Give valid reasons or evidence to support an answer or conclusion. | 1–3 |
| AO3 | Predict | Give an expected result. | 1–2 |
| AO3 | Sketch | Represent by means of a diagram or graph (labelled as appropriate). The sketch should give a general idea of the required shape or relationship, and should include relevant features. | 1–3 |
| AO3 | Suggest | Propose a solution, hypothesis or other possible answer. | 1–3 |
The typical marks are patterns, not IB rules. The real guide is the mark in square brackets after each question, for example [2]. Aim for about one clear point per mark, and match the command term to the mark total.
Every question on both papers tests AO1, AO2 and AO3, and calculators are allowed. See the IB exam papers guide for how Paper 1 and Paper 2 are built, and the IB Biology glossary for key terms.
The IB treats "compare" as similarities only, and "distinguish" as differences only. Many UK exam boards use "compare" for both. If you have used GCSE habits, unlearn this one.
AO1 command terms: recall
These ask for a fact with no explanation. Writing more than the question asks for earns nothing extra.
Define (1–2 marks)
Examiners want: The precise meaning, using the key idea in the right order. Example: *Define osmosis.* Model answer: The passive movement of water molecules across a partially permeable membrane from a region of lower solute concentration (higher water potential) to a region of higher solute concentration (lower water potential).
State (1 mark)
Examiners want: One specific name, value or short answer. No reasons. Example: *State the enzyme that joins Okazaki fragments together on the lagging strand.* Model answer: DNA ligase.
List (1 mark per item)
Examiners want: Short items only. Give exactly the number asked for, because extra wrong items can cancel correct ones. Example: *List two products of the light-dependent reactions of photosynthesis.* Model answer: ATP and NADPH (oxygen is also released).
Draw, label and measure (usually 1 mark per point)
Draw: pencil, a ruler for straight lines, drawn to scale, with a clear outline and no shading. Label: a label line that ends exactly on the structure. Measure: read a value with units from a ruler, scale bar or instrument. Our exam questions include drawing and labelling practice.
AO2 command terms: apply what you know
Identify (1 mark)
Examiners want: Pick the answer from the options given. Do not explain. Example: *Identify, from mitochondrion, ribosome, lysosome and Golgi apparatus, the organelle where the Krebs cycle takes place.* Model answer: Mitochondrion (the matrix).
Outline (2–3 marks)
Examiners want: Brief, with the main points only and no detail or reasons. Example: *Outline the role of the ribosome in translation.* Model answer: It binds to mRNA; tRNA anticodons pair with mRNA codons; it forms peptide bonds between amino acids, building the polypeptide.
Describe (2–4 marks)
Examiners want: A detailed account of what happens, a pattern or a structure. For graphs, quote data with units. Do not give reasons. Example: *A graph shows enzyme activity rising from 10 °C to a peak at 40 °C, then falling to almost zero at 60 °C. Describe the trend.* Model answer: Activity increases as temperature rises from 10 °C to a maximum at 40 °C; above 40 °C it decreases rapidly, reaching almost zero by 60 °C.
Distinguish (2 marks)
Examiners want: Differences only, stated as a clear contrast about the same feature. Example: *Distinguish between transcription and translation.* Model answer: Transcription makes mRNA using DNA as a template (in the nucleus in eukaryotes); translation makes a polypeptide using the mRNA code at a ribosome.
Calculate (1–3 marks)
Examiners want: Show the working, the answer, and the units. Correct working can still earn marks if you slip on the arithmetic. Example: *A drawing of a cell is 45 mm long at a magnification of ×300. Calculate the actual length.* Model answer: Actual size = image size ÷ magnification = 45 ÷ 300 = 0.15 mm (150 µm).
Annotate means short notes on a diagram or graph (for example, "steepest gradient: rate is highest here"). Estimate means an approximate value, such as reading a value between gridlines. Give a sensible figure with units and do not over-work it.
Put the units in your final answer line, and keep the working visible. Our IB Biology notes have the maths skills built into each topic.
AO3 command terms: reason, judge and create
Explain, justify, suggest, predict, deduce
Explain (2–4 marks)
Examiners want: Reasons or causes: each mark is a cause-and-effect link, usually using "because" or "so". Example: *Explain why the activity in the graph above falls above 40 °C.* Model answer: High temperature breaks hydrogen and ionic bonds in the enzyme; the tertiary structure changes so the active site shape changes; substrate can no longer bind, so the enzyme is denatured. See enzymes and metabolism.
Justify (1–3 marks)
Examiners want: Evidence or valid reasons for a stated conclusion, from the data or from biology. Example: *Justify classifying a whale as a mammal rather than a fish.* Model answer: It has mammary glands and feeds milk to its young; it has lungs and breathes air; it is endothermic and gives birth to live young.
Suggest (1–3 marks)
Examiners want: A sensible, biology-based idea. Often the situation is unfamiliar, and a range of answers is accepted. Example: *Fertiliser run-off into a lake is followed by the death of fish. Suggest why.* Model answer: Nutrients cause an algal bloom that blocks light, so submerged plants die; bacteria decomposing the dead matter use up oxygen, so fish suffocate.
Predict (1–2 marks)
Examiners want: The expected result, usually with a reason implied by the question wording. Example: *Predict the effect of increased wind speed on the rate of transpiration.* Model answer: The rate increases, because wind removes water vapour near the stomata, keeping the diffusion gradient steep.
Deduce (1–2 marks)
Examiners want: Reach a conclusion from the information given, using logic. Show the reasoning if marks allow. Example: *A child has blood group O. Her father is group A and her mother is group B. Deduce the parents' genotypes.* Model answer: The child is ii, so each parent carries i: father IAi, mother IBi. Practise this in inheritance.
Compare, analyse, comment, evaluate, discuss
Compare (2–3 marks)
Examiners want: Similarities only, mentioning both items in each point. Example: *Compare aerobic and anaerobic cell respiration.* Model answer: Both start with glycolysis in the cytoplasm; both produce ATP; both break down glucose. See cell respiration.
Compare and contrast (3–4 marks)
Examiners want: At least one similarity and one difference, always naming both items. Example: *Compare and contrast active transport and facilitated diffusion.* Model answer: Both use specific membrane proteins. Active transport needs ATP and moves substances against the concentration gradient; facilitated diffusion is passive and moves them down the gradient.
Analyse (2–4 marks)
Examiners want: Pick out the key features, trends and relationships in the data. Quote numbers. Example: *Analyse a graph of atmospheric CO2 at Mauna Loa from 1958 to 2023.* Model answer: CO2 rises from about 315 ppm to over 420 ppm, with the rate of increase speeding up. A repeating annual wave sits on top: the dips follow summer photosynthesis in the northern hemisphere, the peaks come in the northern spring. See climate change.
Comment (1–2 marks)
Examiners want: A short judgement on a statement or result, not a description. Example: *Potato cylinders in sucrose solution show a -8% change in mass. Comment on this result.* Model answer: A negative change means water left the cells by osmosis, so the solution had a lower water potential than the potato tissue.
Evaluate (2–4 marks)
Examiners want: Strengths and limitations, then a weighed overall view. Example: *Evaluate captive breeding as a conservation method.* Model answer: Strengths: protects against extinction and preserves genetic diversity, and animals can be reintroduced. Limitations: small populations risk inbreeding, it is expensive, and captive animals may lose natural behaviours. It helps most when habitat protection runs alongside it. See conservation of biodiversity.
Discuss (3–6 marks)
Examiners want: A balanced range of arguments or factors with evidence, and a conclusion if the data allow one. Example: *Discuss the use of antibiotics in livestock feed.* Model answer: For: prevents disease and raises growth in crowded farms. Against: selects for antibiotic-resistant bacteria, which may pass to humans. Weigh the evidence and give a justified view.
Construct, design, sketch and determine
Construct (1–3 marks)
Examiners want: The information in the requested form (a table, chart or logical diagram), complete and labelled. Example: *Construct a Punnett square for a cross between two heterozygous (Tt) plants.* Model answer: Gametes T and t on each axis give TT, Tt, Tt and tt, a 3:1 ratio of dominant to recessive phenotype.
Design (3–6 marks)
Examiners want: A plan that someone else could follow: variables, controls, method, and how the data will be processed. Example: *Design an investigation into the effect of light intensity on photosynthesis in pondweed.* Model answer: Independent variable: distance of lamp. Dependent variable: bubbles of oxygen per minute. Control temperature (a water bath), pondweed length and CO2 concentration; repeat three times and calculate the mean.
Determine (1–2 marks)
Examiners want: The one possible answer, found from data or by a calculation. Example: *Determine the rate of reaction from the gradient of the graph.* Model answer: Gradient = change in product ÷ time, with units, for example 0.6 cm³ s⁻¹.
Sketch (1–3 marks)
Examiners want: Correct overall shape and key features, with labelled axes. Exact values are not needed. Example: *Sketch a graph of population growth in a habitat with limited resources.* Model answer: An S-shaped curve: slow start, rapid growth, levelling off at the carrying capacity, with a labelled line at K.
The glossary lists only these 28 terms, but the guide says other terms may be used to direct you to present an argument in a specific way. Always read the whole question. Our topic tests let you practise a mix of command terms.
Common mistakes: terms students mix up
Describe vs explain
Describe says what happens. Explain says why. "Enzyme activity falls above 40 °C" is a description. "The active site changes shape because bonds break" is an explanation. If the mark total is high, the answer usually needs both.
Outline vs explain
Outline is a brief summary of main points. Explain needs causes and reasons. An outline of active transport lists the steps; an explanation says why each step moves the ions. Check the membrane transport notes to practise both.
Evaluate vs discuss
Discuss gives a balanced range of arguments. Evaluate also weighs strengths against limitations to reach an appraisal. If you only list points on both sides, you have discussed. To evaluate, state which side is stronger and why.
Define vs describe
Define is one precise meaning, with no examples needed. Describe is a detailed account. "Define homeostasis" needs one sentence; "describe the control of blood glucose" needs a sequence.
Using "compare" as if it means similarities and differences. In IB Biology it means similarities only. If the question wants both, it says "compare and contrast".
How to use this in revision
Take a question from our IB Biology exam questions and underline the command term first. Then decide the style of answer: a fact, a pattern, a cause, or a judgement. After that, check your answer against the mark scheme and count how many separate points you made. The same skills are tested in the internal assessment, where "evaluate" and "explain" drive the conclusion and evaluation. Standard Level students can use the SL course in the same way.
If you want help turning knowledge into full-mark answers, we offer 1-to-1 tutoring online and in Dubai.