Explain how changes in temperature affect the rate of an enzyme-catalysed reaction, including the molecular basis for the optimum temperature.

IB DP Biology Higher Level (2023 syllabus) — C1.1 Enzymes and metabolism · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Increasing temperature raises the kinetic energy of enzyme and substrate molecules, so collisions become more frequent and the rate of reaction rises.
At the optimum temperature the collision frequency is high enough that the rate is maximal; the enzyme’s tertiary structure is still intact, so the active site can bind substrate efficiently.
Beyond the optimum, thermal energy breaks hydrogen bonds and other non‑covalent interactions that stabilise the enzyme’s tertiary structure. The active site changes shape (denaturation), reducing or abolishing enzyme‑substrate complex formation and the reaction rate falls.

Examiner tips

  • Use the word ‘optimum temperature’ and explain why it gives the maximum rate. Show the two opposing effects of temperature: increased collisions vs. loss of structure. Mention the role of hydrogen bonds/non‑covalent interactions in maintaining the active site.
  • common_mistakes
  • :
  • Confusing the optimum temperature with the temperature at which the enzyme is first denatured. Failing to explain that the active site shape changes, not just the enzyme’s overall stability. Using vague terms like ‘too hot’ without linking to molecular disruption.

Mark scheme (4 marks)

  1. Increasing temperature increases the kinetic energy of molecules, leading to more frequent collisions between enzyme and substrate, increasing the rate of reaction.
  2. At the optimum temperature, the rate of reaction is at its maximum because the increase in collision frequency outweighs any thermal disruption of enzyme structure.
  3. Above the optimum temperature, hydrogen bonds and other non-covalent interactions maintaining tertiary structure are disrupted / broken, causing the active site to change shape (denaturation).
  4. The altered active site can no longer form an enzyme-substrate complex effectively, so the reaction rate falls / the enzyme loses catalytic activity.

Key terms in this question

optimum temperature

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