Explain why enzymes stop working when the temperature is increased above their optimum temperature.

Edexcel GCSE Biology (1BI0) — 1.4 Enzymes · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Enzymes are biological catalysts that speed up chemical reactions in living organisms. Each enzyme has an optimum temperature at which it works best.

Model answer (4 marks)

Enzymes have a specific three‑dimensional shape that creates an active site for the substrate. Above the optimum temperature the kinetic energy of the molecules increases, causing the weak bonds that maintain the enzyme’s shape to break or be disrupted. This changes the shape of the active site – the enzyme becomes denatured – so the substrate can no longer fit into or bind to it, and the reaction cannot be catalysed.

Examiner tips

Use the terms ‘active site’, ‘denatured’ and ‘substrate cannot bind’ – these are the key phrases the mark scheme looks for.
Show the cause (high temperature) and effect (denaturation) in a clear, logical order.
Mention that the shape change prevents substrate binding – this demonstrates understanding of the mechanism.

Common mistakes

Confusing enzyme inactivation with substrate depletion; the enzyme itself is denatured, not the substrate.
Using vague terms like ‘too hot’ without explaining bond disruption or shape change.
Omitting the word ‘active site’ or ‘denatured’, which are explicitly required by the mark scheme.

Mark scheme (4 marks)

The enzyme has an active site with a specific shape
Above the optimum temperature, the bonds holding the enzyme's shape break / are disrupted
The shape of the active site changes / the enzyme is denatured
The substrate can no longer fit into / bind to the active site, so the reaction cannot be catalysed

Key terms in this question

optimum temperature