IB Biology Command Terms — What Examiners Actually Want

Students consistently lose 3–5 marks per Paper 2 paper by describing when they were asked to explain, or giving a one-sided argument when the command term was discuss. These are the cheapest marks to reclaim in your IB Biology revision.

This guide decodes every command term that appears on the 2025 Paper 2, explains the mark-tariff pattern, and walks through worked examples for the four highest-yield terms.

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The Full Command Term Decoder

Memorise the tariff column. Seeing "Discuss" on a 6-mark question should instantly trigger: I need multiple perspectives, balanced evidence, and a concluding judgement.

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The Four Highest-Yield Terms

These four command terms appear most often in Paper 2 and carry the biggest mark swings.

1. Explain — "Give reasons or causes"

What goes wrong: Students describe the phenomenon instead of explaining why it happens.

Test: Every explain answer should be able to be written in the form "X happens because Y." If you cannot insert "because" in your paragraph, you are describing, not explaining.

Example question:

Explain how stomatal closure helps a plant survive drought stress. (4 marks)

Describe-style answer (scores 2/4):

"Stomata close during drought. The guard cells become flaccid and the stomatal aperture narrows. Water loss is reduced."

Explain-style answer (scores 4/4):

"Under drought stress, abscisic acid (ABA) accumulates in guard cells, because the leaf senses low water potential. ABA triggers potassium ion efflux from the guard cells, because it opens anion channels in the plasma membrane. This reduces guard-cell turgor pressure, because water follows the osmotic gradient out of the cells. The stomatal aperture therefore narrows, because the guard cells can no longer push the pore open against surrounding epidermal cells."

Four "because" links = four marks. The structure matches the tariff exactly.

2. Discuss — "Balanced argument with multiple perspectives"

What goes wrong: Students give a one-sided argument.

Test: A discuss answer must have at least two genuine perspectives. A discuss answer that only presents one side caps at half the available marks.

Example question:

Discuss the claim that natural selection is the sole driver of evolution. (6 marks)

One-sided answer (scores 3/6):

Natural selection causes differential survival. This drives adaptation over generations. Example: antibiotic resistance in bacteria. Example: peppered moths. Therefore natural selection drives evolution.

Discussion-style answer (scores 5-6/6):

"On the one hand, natural selection clearly drives adaptation, as shown by antibiotic resistance (where resistant bacterial alleles increase in frequency under antibiotic pressure). On the other hand, genetic drift and gene flow also change allele frequencies — genetic drift is particularly important in small populations, and gene flow can counteract local adaptation. Mutation itself provides the raw material without which selection cannot operate. A more accurate claim is that natural selection is one of several evolutionary forces — the dominant driver of adaptation but not the sole driver of evolution per se."

Balanced evidence on both sides, with a conclusion that moves past the binary. That is 5-6 territory.

3. Evaluate — "Judgement at the end"

What goes wrong: Students describe strengths and weaknesses but never commit to a judgement.

Test: The last sentence of an evaluate answer should contain a verdict. If you cannot identify the verdict sentence, examiners cannot either.

Example question:

Evaluate the use of CRISPR-Cas9 for treating genetic diseases. (6 marks)

Describe-style answer (scores 3/6):

"CRISPR is a gene-editing tool. It uses a guide RNA and Cas9 enzyme. Benefits include precision. Risks include off-target edits. It has been used for sickle-cell disease."

Evaluate-style answer (scores 5-6/6):

"Strengths of CRISPR-Cas9 include its base-pair-level precision, relatively low cost per edit, and proven clinical success in treating sickle-cell disease (Casgevy, 2023). Weaknesses include off-target editing, ethical concerns around germline editing, and long-term unknowns. Weighing these, CRISPR is currently a safe and effective somatic-cell therapy for monogenic diseases with a single-nucleotide cause, but germline editing remains outside the ethical threshold for clinical use in 2025. The tool's strengths outweigh its weaknesses for targeted somatic applications but not for heritable edits."

Explicit judgement (bold sentence) = full marks.

4. Suggest — "Plausible explanation in a novel context"

What goes wrong: Students revert to describing what they already know, rather than proposing an explanation for something new.

Test: Suggest questions appear at the end of Paper 2 in novel contexts you have not seen before. Resist the urge to dump memorised content. Start your answer with "A possible explanation is…" or "One hypothesis is…".

Example question:

In a novel experiment, mice raised in complete darkness for 30 days showed decreased bone density. Suggest a biological mechanism. (3 marks)

Describe-style answer (scores 1/3):

"Vitamin D is required for bone density. Bone density depends on calcium uptake."

Suggest-style answer (scores 3/3):

"One hypothesis is that darkness suppresses endogenous vitamin D₃ synthesis, because cutaneous synthesis of cholecalciferol requires UV-B exposure. Reduced vitamin D₃ would impair intestinal calcium absorption, because vitamin D upregulates calcium transporter expression in the small intestine. Lower circulating calcium would trigger parathyroid hormone release, increasing osteoclast activity and resulting in net bone resorption."

A proposed mechanism grounded in IB-level biology, with "because" links.

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The 3-Step Paper 2 Command-Term Drill

Use this drill on every Paper 2 past-paper question you practise:

1. Circle the command term with a pen before you read the rest of the question.
2. Write the mark tariff next to it — this is the scaffold for your answer structure (a 6-mark discuss = at least 3 points, 2 marks each).
3. Write the first word of your answer to match the term:
   • Explain → "Because…"
   • Discuss → "On the one hand…"
   • Evaluate → start with strengths OR weaknesses; end with "Overall, …"
   • Suggest → "A possible explanation is…"
   • Describe → neutral narrative

Do this consistently for 6 past papers and you will add 3–5 marks on Paper 2 without learning any new content.

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Where This Fits in the 2025 IB Biology Plan

• Paper 2 Guide — full extended-response strategy
• Past Papers Hub — what to practise vs what to skip in 2025
• How to Score 7 — the 6-step score-7 playbook

Command term mastery is the single cheapest mark unlock in IB Biology. The content you already know is fine; the phrasing you use to present it is what examiners reward.