# Explain how temperature affects the rate of an enzyme-catalysed reaction, including the effect of temperatures both below and above the optimum.

> IB DP Biology Standard Level (2023 syllabus) — C1.1 Enzymes and metabolism · Explain · 4 marks

## Mark scheme (4 marks)

1. Below the optimum temperature, lower kinetic energy means fewer enzyme–substrate collisions, reducing the rate of reaction.
2. At the optimum temperature, the rate of reaction is greatest because the frequency of successful enzyme–substrate collisions is maximised.
3. Above the optimum temperature, the enzyme begins to denature — hydrogen bonds (and other bonds) maintaining the tertiary structure break.
4. Denaturation alters the shape of the active site so the substrate can no longer bind, reducing (or stopping) enzyme activity — this change is permanent.

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