How to Answer "Why" and "How" Questions in Biology

"Why" wants a reason; "how" wants a mechanism

A question that opens with "why" is asking for the reason something happens, the cause that leads to an observed effect. A question that opens with "how" is asking for the way something happens, the mechanism, structure, or sequence of steps involved.

The two overlap in topic but not in what earns the mark: an answer to "why does water move out of a plant cell placed in concentrated sugar solution" needs the reason (a difference in water potential across the membrane draws water out), while an answer to "how does water move out of a plant cell" needs the mechanism (osmosis across the partially permeable membrane, the cell shrinking away from the wall).

Command words back this pattern: "why" questions usually pair with "explain", and "how" questions usually pair with "describe" or "state the process". Recognising which pattern a question is asking for, before writing a single sentence, prevents an answer that is accurate in content but built for the wrong question.

Sentence structures for "why" answers

• Start from the cause and lead to the effect: "Because X, Y happens", for example, "because the sugar solution is more concentrated than the cell sap, water leaves the cell by osmosis."
• Use linking words that show causation directly, because, therefore, as a result, this causes, rather than simply listing two facts next to each other and hoping the connection is read.
• State the mechanism briefly if it strengthens the reason, but keep the sentence anchored on cause and effect rather than turning it into a full step-by-step description.
• Where more than one cause contributes, state each cause and its own effect as a separate sentence rather than combining them into one long clause.

Sentence structures for "how" answers

1. Identify the starting point of the process and state it first, rather than beginning mid-sequence.
2. Present each step in the order it actually occurs, since sequence is often part of what is assessed in a "how" answer.
3. Name the structure or process responsible at each step using the precise biological term, not a general description of movement or change.
4. Finish at the stated end point of the process, so the sequence reads as complete rather than trailing off.
5. Avoid inserting "because" reasoning into every step of a "how" answer, that reasoning belongs in a "why" answer, and repeating it slows the sequence down without adding marks.

A worked example: same topic, two different questions

A common way marks are lost

What about "compare", "define" and "suggest" questions?

• "Compare" wants both things set against each other on the same points, using comparative words such as 'whereas' or 'higher than', describing each one separately, without linking them, usually misses the comparison mark.
• "Define" wants the precise meaning in the terms the syllabus uses, not an example of the term in action.
• "Suggest" wants a reasonable answer applied to a situation you may not have met before, so it rewards sensible biological thinking rather than a memorised line.
• "State the difference" is a compare question in disguise, and still needs both sides named, not just one.
• Reading the question word and the command word together, before deciding on a sentence pattern, keeps the shape of the answer matched to what is actually being asked.

How do you plan a "why" or "how" answer before writing?

A short plan prevents an answer built for the wrong question. Underlining both the question word and the command word first fixes whether a reason or a mechanism is needed.

Glancing at the mark value then shows how many separate points the answer should contain, a two-mark 'why' usually needs two linked causes, and a three-mark 'how' usually needs three ordered steps. Jotting the key terms or the steps in rough before writing means the final answer can be checked against that list rather than trusting memory mid-sentence.

This planning takes only a few seconds but changes what gets written. An answer that names its points in advance is far less likely to blend a reason and a mechanism into one blurred sentence, which is the single most common way these questions lose marks.

A second worked example: a "how" answer built step by step

Question. Describe how oxygen passes from the air in an alveolus into the blood. (3 marks)

Model answer. Oxygen dissolves in the moist lining of the alveolus and moves by diffusion across the thin, one-cell-thick walls of the alveolus and the capillary, from a region of higher oxygen concentration in the alveolus to a region of lower oxygen concentration in the blood, where it is taken up by haemoglobin in the red blood cells.

Notice the shape: this is a 'how' answer, so it is written as an ordered sequence, where the oxygen starts, how it crosses, and where it ends, with each step naming the structure or process responsible, and no 'because' reasoning inserted along the way.

Key facts about where "why" and "how" questions appear

• Most 'why' and 'how' questions appear in Paper 2, the written paper, worth 100 marks over 2 hours 30 minutes.
• Section A alone carries 60 marks across 8 questions (answer all), all compulsory.
• 'Why' usually pairs with 'explain' and expects a reason; 'how' usually pairs with 'describe' and expects a sequence.
• The mark value signals how many linked causes or ordered steps an answer should contain.
• The whole subject is examined through three papers, and clear reasoning is rewarded across all of them.

Frequently asked questions