Explain why enzymes are described as biological catalysts and why the shape of an enzyme is important for its function.

Pearson Edexcel A-Level Biology A (9BI0) — Topic 1 Biological Molecules · Explain · 5 marks · View as Markdown

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

Enzymes are proteins found in all living organisms. They are involved in many reactions, including the breakdown of food molecules during digestion.

Model answer (5 marks)

Enzymes are biological catalysts because they increase the rate of chemical reactions by lowering the activation energy, yet they are not consumed or altered in the process.
The enzyme’s function depends on its three‑dimensional shape, which creates an active site with a specific geometry.
Only a substrate that matches this shape can bind to the active site, forming an enzyme–substrate complex.
If the enzyme’s shape is altered (denaturation), the active site no longer matches the substrate, so the enzyme cannot bind and the reaction stops.

Examiner tips

  • Use the word ‘catalyst’ and explain rate increase and no consumption.
  • Mention active site shape and substrate specificity.
  • Include the effect of denaturation on function.
  • Show clear link between shape and activity.

Common mistakes

  • Saying enzymes are ‘fast’ without mentioning activation energy.
  • Confusing enzyme with substrate or product.
  • Omitting that enzymes are not used up.

Mark scheme (5 marks)

  1. Enzymes speed up chemical reactions
  2. Enzymes are not used up / changed in the reaction
  3. Enzymes have an active site with a specific shape
  4. Only a specific substrate can fit into / bind to the active site
  5. If the shape of the active site changes (denaturation), the enzyme can no longer function / the substrate cannot bind

Key terms in this question

enzyme · catalyst

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