# Explain how the concentration of an enzyme affects the rate of an enzyme-catalysed reaction.

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

> In a cell, the concentration of enzymes is regulated by gene expression. Increasing the amount of enzyme produced can dramatically alter the rate at which metabolic reactions proceed.

## Mark scheme (4 marks)

1. Increasing enzyme concentration increases the rate of reaction (when substrate is not limiting).
2. More enzyme molecules provide more active sites, so more enzyme–substrate complexes can form at any one time.
3. When substrate concentration becomes limiting (substrate is in excess of enzyme), further increases in enzyme concentration continue to raise the rate.
4. If substrate concentration is the limiting factor, a plateau in rate is reached where additional enzyme molecules cannot increase the rate because substrate molecules are insufficient to occupy all active sites.

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