Explain how temperature affects the rate of an enzyme-controlled reaction, including what happens at temperatures above the optimum.

AQA A-Level Biology (7402) — 3.1.4 Proteins and Enzymes · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

As temperature rises, the kinetic energy of molecules increases, so enzyme and substrate collide more often and more enzyme‑substrate complexes form, increasing the reaction rate.
The rate continues to rise until an optimum temperature is reached.
Above this optimum, the rate falls because the enzyme becomes denatured – the bonds that maintain its tertiary structure break and the active site changes shape.
The altered active site no longer fits the substrate, so enzyme‑substrate complexes cannot form and the reaction cannot be catalysed.

Examiner tips

  • Use the term ‘optimum temperature’ and explain the kinetic energy effect first. Show the two stages: increase up to optimum, then decrease. Mention denaturation and loss of active‑site shape. Use ‘enzyme‑substrate complex’ and ‘catalyse’ to hit key terms.”]
  • common_mistakes
  • :
  • Confusing temperature with pH, or not mentioning the optimum. Saying the enzyme is ‘destroyed’ instead of ‘denatured’. Forgetting to explain why the active site can’t bind the substrate after denaturation.

Mark scheme (5 marks)

  1. As temperature increases (up to the optimum), the rate of reaction increases
  2. Because molecules have more kinetic energy, so enzyme and substrate collide more frequently / more enzyme-substrate complexes form
  3. Above the optimum temperature, the rate of reaction decreases
  4. The enzyme becomes denatured — bonds holding the tertiary structure break / the shape of the active site changes
  5. The substrate can no longer fit into the active site, so no enzyme-substrate complex can form / the enzyme can no longer catalyse the reaction

Key terms in this question

enzyme

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