How to Answer a 5-Mark Enzyme “Explain” Question (Step by Step)
By Zaheer | Updated 13 October 2026
Disclosure: I wrote the free Enzymes guide linked at the end. Everything in this post is free to use without it. This is independent study material, not affiliated with Cambridge or any exam board.
A 5-mark "explain" question on enzymes looks scary, but it is one of the most predictable questions in IGCSE and O Level Biology. The same few ideas appear every time, and the marks go to students who put them in the right order. This post shows a four-step method, compares a weak answer with a strong one, and gives you three questions to try.
Why "explain" is different from "describe"
- Describe asks what happens: "the rate rises, then falls".
- Explain asks why: you must give the reason behind each change.
Students who only describe the graph get the first one or two marks and then stall. The remaining marks are for the reasons.
The four-step method
- Start with the change. Name what is changing (temperature or pH) and what happens to the rate.
- Give the molecular reason. For a rise in temperature, say the molecules gain kinetic energy, so there are more collisions between enzyme and substrate.
- Mark the turning point. Name the optimum, the temperature where the rate is highest.
- Explain the fall. Above the optimum the enzyme is denatured. The active site changes shape, so it is no longer complementary to the substrate and the substrate cannot fit.
Most 5-mark enzyme answers are these four steps with the key terms filled in.
A weak answer and a strong answer
Question: Explain the effect of increasing temperature on the activity of an enzyme. [5]
| Weak answer | Strong answer |
|---|---|
| "When it gets hotter, the enzyme works faster. Then when it is too hot the enzyme dies and stops working." | "As temperature increases, the molecules gain kinetic energy, so there are more frequent collisions between enzyme and substrate and the rate of reaction increases. This continues up to the optimum temperature. Above the optimum, the enzyme is denatured: the active site changes shape, so it is no longer complementary to the substrate and the rate falls." |
The weak answer is not wrong about the pattern, but it gives no reasons and uses "dies". The strong answer is the same story with the scientific terms added.
Mark the strong answer yourself
Tick one mark for each idea you can find:
- Molecules gain kinetic energy as temperature rises
- More frequent collisions between enzyme and substrate
- Rate of reaction increases up to the optimum
- Above the optimum the enzyme is denatured
- Active site changes shape and is no longer complementary to the substrate
That is five ideas for five marks. Each one is a single idea, so a clear sentence for each is enough.
A data question to practise on
Here is a set of invented results, made up for practice and not from a real experiment. An enzyme was tested at different temperatures.
| Temperature (°C) | Rate of reaction (arbitrary units) |
|---|---|
| 10 | 2 |
| 20 | 5 |
| 30 | 9 |
| 40 | 14 |
| 50 | 3 |
| 60 | 0 |
Two habits that earn marks on questions like this:
- Quote numbers. "The rate increased from 2 to 14 units between 10 and 40 °C" is stronger than "the rate went up".
- Be careful with the optimum. The highest value in the table is at 40 °C, but the true optimum could lie anywhere between 30 and 50 °C. Say "the optimum is around 40 °C".
Try these three questions
Write your answers before reading mine.
- Using the table, describe how the rate of reaction changes from 10 °C to 60 °C. [3]
- Explain the rate at 50 °C compared with 40 °C. [3]
- Pepsin works best at about pH 2. Explain what would happen to its activity if it were placed in an alkaline solution. [3]
Model answers
- The rate increases from 2 units at 10 °C to a maximum of 14 units at 40 °C. It then falls sharply to 3 units at 50 °C and reaches 0 at 60 °C.
- At 50 °C the temperature is above the optimum, so the enzyme is denatured. The active site has changed shape, so it is no longer complementary to the substrate, and fewer enzyme-substrate complexes can form, so the rate is lower than at 40 °C.
- The activity would fall or stop. Alkaline conditions are far from the optimum pH of pepsin, so the enzyme is denatured, and the active site changes shape so the substrate can no longer fit.
Quick checklist before the exam
- I can write the four steps from memory: change, reason, optimum, denaturation.
- I use "kinetic energy" and "collisions" for the rise, and "denatured" for the fall.
- I write "active site" and "complementary" in every denaturation answer.
- I quote numbers from tables and graphs.
- I never write "dies" or "killed".
Want the full topic?
My free 14-page self-study guide covers this from the beginning, with worked answers and a last-minute revision card. You can download it here: IGCSE Biology Enzymes Complete Self-Study Guide (0610/0970), free.
If you read my earlier post on enzyme exam traps, this one builds on it.
Always check the official syllabus and specimen papers for the latest requirements. No exam result is guaranteed.
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