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A-Level Biology May/June 2024 Q4(b): Describe and explain the mode of action of laccase when catalysing the formation of lig…
A-Level Biology · Paper 9700/23 · May/June 2024 · Question 4(b) · [5 marks]
Describe and explain the mode of action of laccase when catalysing the formation of lignin.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
The mode of action of laccase is described by the induced-fit model. The enzyme's active site is not initially perfectly complementary in shape to its substrate, the monolignols.
When a monolignol substrate enters the active site, it binds and induces a conformational change in the enzyme's tertiary structure. This causes the active site to change shape to fit the substrate more closely, becoming fully complementary. This forms a temporary enzyme-substrate complex.
By holding the substrate(s) in the correct orientation, this binding puts strain on the bonds within the substrate molecules, which lowers the activation energy for the reaction. This allows monolignols to be polymerised into lignin. After the product (lignin) is released, the laccase enzyme returns to its original shape and can be reused.
How the marks are awarded
- B1 — Stating that laccase uses the induced-fit model, as seen in 'The mode of action of laccase is described by the induced-fit model'.
- B1 — Explaining that the active site is not initially a perfect match for the substrate, as in 'The enzyme's active site is not initially perfectly complementary in shape to its substrate, the monolignols'.
- B1 — Describing how the active site changes shape to fit the substrate upon binding, as in '...it induces a conformational change...causing the active site to change shape to fit the substrate more closely'.
- B1 — Correctly identifying the formation of an enzyme-substrate complex, stated as 'This forms a temporary enzyme-substrate complex'.
- B1 — Explaining that the enzyme's action lowers the activation energy, as written in '...which lowers the activation energy for the reaction'.
Common mistakes
- Describing the lock-and-key model instead of the induced-fit model, incorrectly stating the active site is a rigid and perfect fit from the start.
- Stating that the enzyme is 'used up' or permanently changed by the reaction, instead of explaining that it returns to its original shape and is reused.
- Failing to mention the lowering of activation energy as the mechanism for catalysis, instead just stating that the enzyme 'speeds up the reaction' without explanation.
- Confusing the roles of substrate and product, for example by stating that laccase breaks down lignin rather than catalysing its formation from monolignols.
Examiner tip: Always apply your knowledge of core biological principles, like the induced-fit model of enzyme action, to the specific context provided in the question, using the correct terminology for the named enzyme and substrate.
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
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