A Level Biology: Enzymes — Revision Notes (Cambridge 9700)

Condensed for the final weeks. For the full explanation, use the Enzymes study guide.

The one fact everything builds from

An enzyme is a biological catalyst — it increases reaction rate and is unchanged at the end. Enzymes are globular proteins, so Topic 2 protein structure is assumed knowledge here.

3.1 Mode of action — two models, both examinable

Model	Core claim
Lock-and-key	Active site has a fixed shape that exactly matches the substrate
Induced-fit (current)	Active site changes shape slightly as substrate binds, improving fit and efficiency

Know both — comparison questions are common. Active site binding forms an enzyme-substrate complex and lowers activation energy, explaining why catalysed reactions proceed faster. Enzyme specificity follows directly from active-site shape.

Intracellular enzymes act inside cells; extracellular enzymes are secreted to act outside cells (e.g. digestive enzymes in the gut). Practical methods: rate of product formation with catalase, rate of substrate disappearance with amylase, both often measured using a colorimeter where colour change is involved.

3.2 Factors affecting rate — five factors

Temperature, pH (buffered to isolate other variables), enzyme concentration, substrate concentration, inhibitor concentration. For each: describe the investigation AND explain the mechanism.

Temperature — rate rises to an optimum (more kinetic energy, more collisions), then falls sharply past it (enzyme denatures, active site shape disrupted).

Michaelis-Menten kinetics

Vmax (maximum rate) is used to derive Km (Michaelis-Menten constant): the substrate concentration at half Vmax. Lower Km = higher affinity — reaches half-maximum rate at a lower substrate concentration. Easy to state, easy to reverse under pressure — fix it with the worked example below.

Worked example: Enzyme A: Km = 2 mmol dm⁻³. Enzyme B: Km = 20 mmol dm⁻³ (same substrate). Enzyme A reaches half-Vmax at a much lower concentration, so A has the higher affinity — more effective at low substrate levels; B needs far more substrate before working efficiently.

Inhibitors — the one reliable test

	Competitive	Non-competitive
Binding site	Active site	Elsewhere on enzyme
Structurally similar to substrate?	Yes	No
Overcome by more substrate?	Yes	No

Test: “does more substrate reduce the inhibitor’s effect?” Yes → competitive. No → non-competitive. Anchor the distinction here, not in memorised mechanism descriptions.

Immobilised enzymes

Enzyme immobilised in alginate vs free in solution — compare activity. Advantages: enzyme recoverable and reusable; product not contaminated with enzyme.

Keeping 3.1 and 3.2 separate

3.1 = why catalysis is faster than uncatalysed (active site, activation energy). 3.2 = what makes that rate go up or down once catalysis is already happening. A “why do enzymes speed up reactions” question wants 3.1 content; a “what affects the rate” question wants 3.2 — don’t cross the two.

Practical assessment note

Paper 5 draws directly on this topic’s practicals: catalase/amylase rate measurement, colorimetry, immobilised enzymes in alginate. Be able to state the method, the controlled variables, and the expected shape of a rate-against-factor graph for each — not just the underlying theory.

Why this topic follows straight on from Biological molecules

Enzymes are globular proteins, so their catalytic behaviour is a direct consequence of the protein structure covered in the previous topic: a specific sequence of amino acids folds into a specific three-dimensional shape, and that shape is what creates the active site enzymes depend on. Revising enzyme structure completely separately from protein structure misses this connection – if a question asks why a change in pH or temperature affects enzyme activity, the underlying reason is always that the enzyme’s protein structure (and therefore its active site shape) has been disrupted, tying this whole topic back to Topic 2’s content on bonding within and between polypeptide chains.

Exam traps

• Getting Km/affinity backwards under pressure — use the worked example’s logic, not raw memorisation.
• Confusing competitive and non-competitive inhibition — always apply the “more substrate” test.
• Answering a 3.1 mechanism question with 3.2 rate-factor content, or vice versa.
• Naming only the induced-fit model when a question asks to compare both hypotheses.
• Forgetting that pH must be buffered when investigating any other factor’s effect on rate.

Self-test

1. What is the core difference between the lock-and-key and induced-fit hypotheses?
2. Why does enzyme activity fall sharply above the optimum temperature?
3. What does a lower Km value indicate about an enzyme’s affinity for its substrate?
4. What single test distinguishes competitive from non-competitive inhibition?
5. Name two advantages of using immobilised rather than free enzymes.

Answers: 1. Lock-and-key proposes a fixed active-site shape matching the substrate exactly; induced-fit proposes the active site changes shape slightly as the substrate binds, improving fit and efficiency. 2. The enzyme denatures, disrupting the active site’s shape so the substrate no longer binds effectively. 3. A lower Km indicates higher affinity — the enzyme reaches half its maximum rate at a lower substrate concentration. 4. Whether increasing substrate concentration reduces the inhibitor’s effect (yes = competitive, no = non-competitive). 5. The enzyme can be recovered and reused, and the product is not contaminated with enzyme.

Official syllabus

Cambridge International, Cambridge International AS & A Level Biology (9700) syllabus for examination in 2025, 2026 and 2027 (Version 1, published September 2022): official syllabus PDF, Subject content, Topic 3 “Enzymes” — the same source already cited by the Enzymes study guide, which first reproduced this topic’s content from it. Verified 2026-09-06.