Explain how competitive inhibitors affect enzyme activity, including the effect of increasing substrate concentration on the degree of inhibition.

IB DP Biology Standard 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)

A competitive inhibitor has a shape complementary to the substrate and binds to the active site of the enzyme.
The inhibitor blocks substrate binding, so fewer enzyme–substrate complexes are formed and the reaction rate falls.
Binding is reversible; the inhibitor can dissociate from the active site.
If the substrate concentration is increased, substrate molecules out‑compete the inhibitor, reducing the degree of inhibition and restoring the rate of reaction.

Examiner tips

Use the term ‘active site’ and ‘reversible’ – examiners look for these key words.
Show the logical sequence: inhibitor binds → blocks substrate → fewer complexes → lower rate.
Explain that higher substrate concentration restores activity – this shows understanding of competitive inhibition.
Avoid unnecessary detail; keep answer concise to fit 4 marks.

Common mistakes

Confusing competitive with non‑competitive inhibition (e.g., saying the inhibitor binds elsewhere).
Forgetting to mention that the inhibition is reversible.
Failing to state that increasing substrate concentration reduces the inhibition effect.

Mark scheme (4 marks)

A competitive inhibitor has a similar shape/complementary shape to the substrate and binds to the active site of the enzyme.
The inhibitor prevents the substrate from binding to the active site, so fewer enzyme–substrate complexes form and the rate of reaction decreases.
Binding of a competitive inhibitor is reversible, so the inhibitor can dissociate from the active site.
Increasing substrate concentration reduces the effect of competitive inhibition because substrate molecules outcompete the inhibitor for the active site, restoring the rate of reaction.

Key terms in this question

competitive inhibitor · substrate concentration