Explain how enzymes act as biological catalysts by reference to their structure and interaction with substrates.

IB DP Biology Standard Level (2023 syllabus) — B1.2 Proteins · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Enzymes are proteins whose three‑dimensional shape is fixed by their amino‑acid sequence.
The active site is a pocket or cleft whose shape and chemical properties match the specific substrate.
When the substrate fits into the active site an enzyme–substrate complex is formed, which stabilises the transition state and lowers the activation energy of the reaction.
After the reaction the product is released and the enzyme is unchanged, ready to bind another substrate.

Examiner tips

  • Use the word ‘active site’ and describe its complementary shape. Show that the enzyme lowers activation energy. Mention that the enzyme is unchanged and can act again.

Common mistakes

  • Confusing the enzyme with the substrate. Forgetting that the enzyme is unchanged after the reaction. Using vague terms like ‘helps the reaction’ without mentioning activation energy.

Mark scheme (4 marks)

  1. Enzymes are proteins with a specific three-dimensional shape determined by their amino acid sequence.
  2. The active site is a region of the enzyme with a shape complementary to the specific substrate(s).
  3. The enzyme–substrate complex forms when the substrate binds to the active site, lowering the activation energy of the reaction.
  4. The enzyme is released unchanged after the reaction, allowing it to catalyse further reactions.

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