Enzymes are biological catalysts made from chains of amino acids. Explain how the structure of an enzyme allows it to carry out its function, and describe what happens to enzyme activity when the temperature rises above the optimum.

Eduqas A-Level Biology — 1.1 Chemical elements and biological molecules · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Enzymes have a specific active site that is shaped to fit the substrate.
The shape of the active site is complementary to the substrate, allowing an enzyme‑substrate complex to form.
When the substrate enters the active site, the complex is formed and the reaction proceeds.
If the temperature rises above the optimum, the active site changes shape and the enzyme denatures.
The altered shape means the substrate can no longer fit, so enzyme‑substrate complexes do not form and the reaction rate falls.

Examiner tips

  • Use the term "active site" and "enzyme‑substrate complex". Show the cause–effect chain: temperature → shape change → loss of fit → reduced rate. Keep the answer concise, matching the five points exactly.

Common mistakes

  • Mixing up the terms "active site" and "substrate". Forgetting to mention that the enzyme denatures. Using vague phrases like "the enzyme gets hot" instead of "the active site changes shape".

Mark scheme (5 marks)

  1. Enzymes have an active site with a specific shape
  2. The active site is complementary to the substrate
  3. An enzyme-substrate complex forms when the substrate enters the active site
  4. Above the optimum temperature, the active site changes shape / the enzyme denatures
  5. The substrate can no longer fit the active site, so no enzyme-substrate complexes form and the rate of reaction decreases

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