Cambridge IGCSE Biology 0610. Topic 5: Enzymes

Enzymes

By the IGCSE Biology Team · reviewed for accuracy by our Biology specialists · last reviewed June 2026 · about us

In short

An enzyme is a protein that works as a biological catalyst, speeding up a reaction without being used up. Mark schemes reward the lock-and-key idea: the substrate is complementary to the enzyme's active site. High temperature or unsuitable pH denatures the enzyme. The active site changes shape so the substrate no longer fits.

The exam angle

Enzymes is where careless vocabulary destroys otherwise good answers. The instant fail is writing that enzymes 'die' at high temperature. They denature, because they are not alive. Examiners also test whether you can explain the shape of a graph (not just describe it) and whether you understand that denaturing changes the active site shape so the substrate no longer fits. Get the lock-and-key wording and the denaturation explanation precise and this topic pays out reliably.

What an enzyme is: and the words that matter

Define an enzyme as a biological catalyst that speeds up the rate of a chemical reaction and is not changed/used up by the reaction. Enzymes are proteins. Two facts examiners reward: enzymes are specific (each enzyme catalyses one type of reaction), and they are not used up, so they can be reused.

The substrate is the molecule the enzyme acts on; the products are what it becomes. A key Core idea is that enzymes lower the energy needed for a reaction to start, speeding it up at body temperature without the high heat the reaction would otherwise need. Avoid the loose phrase 'enzymes break down food' as a definition. That is only one role. Use 'catalyst', 'specific' and 'protein' to anchor your answer.

The lock-and-key model: explain, don't just name it

Examiners want the mechanism, not just the label. The enzyme has an active site with a specific shape that is complementary to the shape of its substrate. The substrate fits into the active site like a key into a lock, forming an enzyme–substrate complex. The reaction occurs, products are released, and the enzyme is free to bind another substrate.

This model explains specificity: only a substrate with the matching shape fits the active site, so each enzyme works on only one substrate. When asked why an enzyme cannot break down a different molecule, the answer is that the substrate's shape is not complementary to the active site, so it cannot bind. The phrase 'complementary shape' and the term 'active site' are both mark-scheme staples. Do not say the substrate is 'the same shape' as the active site. It is complementary, fitting into it.

Temperature: explaining the graph (Extended detail)

A rate-against-temperature graph rises to a peak (the optimum, around 37°C for human enzymes) then falls sharply. To score, explain both sides:

Up to the optimum: as temperature increases, particles have more kinetic energy, so enzymes and substrates collide more often and more successfully. Rate increases.
Above the optimum: high temperature causes the enzyme to denature: the active site changes shape (Extended only: the bonds holding the protein's shape break), so the substrate no longer fits and the enzyme–substrate complex cannot form. Rate falls.

The critical word is denature, never 'die' or 'killed'. Denaturing is permanent. Cooling the enzyme does not restore activity, because the shape change cannot be undone. The fall after the optimum is steep precisely because the active site is destroyed, not just slowed.

pH and the rest of the graph story

Each enzyme has an optimum pH at which its rate is highest. Away from the optimum, in conditions that are too acidic or too alkaline, the enzyme denatures: the active site changes shape and the substrate no longer fits. The same denaturation logic applies as for temperature.

Useful exam examples: pepsin works best in the acidic stomach (around pH 2), while amylase and enzymes in the small intestine work best in slightly alkaline conditions. This links directly to human nutrition and digestion, where bile neutralises stomach acid to give the right pH. When a graph shows a symmetrical curve, describe it as a peak at the optimum with reduced activity on either side, then explain the fall using denaturation. Describing without explaining caps your marks on 'explain' questions.

Enzyme practicals: what scores on Paper 5/6

Two classic investigations appear: amylase breaking down starch (tested with iodine solution, which stays orange-brown when starch is gone), and catalase breaking down hydrogen peroxide (measuring oxygen/froth produced). You should be able to:

State the independent variable (temperature or pH) and dependent variable (time for starch to disappear, or volume of gas).
List control variables for a fair test: enzyme concentration, substrate concentration, volume, and the variable you are not testing.
Explain that a shorter time for the iodine to stay orange means a faster reaction.
Use a water bath to control temperature and a buffer solution to control pH.

A common loss is forgetting to mention repeats for reliability and a control. To practise these with a specialist, book a free trial class.

Worked exam question (6 marks)

An enzyme catalyses a reaction at a maximum rate at 40°C. At 60°C, no product is formed even after a long time. (a) Explain why the rate of reaction increases as temperature rises from 10°C to 40°C. (b) Explain why no product is formed at 60°C.

Show the mark-scheme answer

(a) as temperature increases the enzyme and substrate molecules gain (kinetic) energy; they move faster and collide more frequently/successfully; more enzyme-substrate complexes form, so rate increases; (b) at 60°C the high temperature denatures the enzyme; the active site changes shape; so the substrate can no longer fit / no enzyme-substrate complex forms, and no product is made;

Each point separated by a semicolon is worth one mark. This is how real Cambridge mark schemes are written.

Common mistakes that cost marks

Writing that enzymes 'die' or are 'killed' at high temperature. Enzymes are not alive; they denature.
Saying denaturation is reversible. The active site shape change is permanent.
Describing the substrate as 'the same shape' as the active site instead of 'complementary'.
Claiming enzymes are 'used up' in the reaction. They are catalysts and are reused.
On 'explain' graph questions, only describing the curve (it rises then falls) without giving the collision and denaturation reasons.
Forgetting to name the active site when explaining specificity, so the answer reads as vague.

How to phrase the 6-mark answer on this topic

For a 6-mark 'explain how temperature affects enzyme activity' answer, split it into two halves with clear reasoning. For the rise use: 'As temperature increases, molecules gain kinetic energy and collide more frequently/successfully, forming more enzyme-substrate complexes, so rate increases.' For the fall use: 'Above the optimum, the enzyme denatures; the active site changes shape, so the substrate no longer fits and complexes cannot form, so rate decreases.' Always include the word denature and the active-site-shape consequence. Those carry the highest-value marks.

Learn the full 6-mark technique →

For students in Malaysia

Malaysian schools usually cover enzymes alongside digestion, and the amylase-starch-iodine practical is standard in local labs. Both May/June and Oct/Nov 0610 sittings feature enzyme graphs heavily. SPM Biology covers enzymes too, but IGCSE demands the precise 'denature' and 'complementary active site' wording more strictly, so dual-syllabus students should not rely on SPM phrasing in the 0610 exam.

Frequently asked questions

What is the IGCSE definition of an enzyme?

An enzyme is a protein that acts as a biological catalyst, speeding up a chemical reaction without being changed or used up. Both 'protein' and 'biological catalyst' are needed for full marks.

What happens when an enzyme is denatured?

At high temperature or unsuitable pH, the enzyme's active site permanently changes shape, so the substrate no longer fits and the reaction stops. Never write that the enzyme is 'killed'. Enzymes are not alive, and that word loses the mark.

Why must you say 'complementary' not 'same shape' for the active site?

The substrate is complementary to the active site. It fits like a key in a lock. Writing 'the same shape as the active site' is marked wrong, because a substrate the same shape could not fit inside it.

Exam guides for this topic

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Enzymes

What an enzyme is: and the words that matter

The lock-and-key model: explain, don't just name it

Temperature: explaining the graph (Extended detail)

pH and the rest of the graph story

Enzyme practicals: what scores on Paper 5/6

Worked exam question (6 marks)

Common mistakes that cost marks

How to phrase the 6-mark answer on this topic

For students in Malaysia

Frequently asked questions

Frequently asked questions

Related topics

Exam guides for this topic