A Level Biology: Enzymes (Cambridge 9700)

Topic 3 Enzymes is AS Level content in Cambridge International AS & A Level Biology (9700), following directly from Topic 2 Biological molecules, where proteins are first introduced. Enzymes are globular proteins, so understanding protein structure from Topic 2 is assumed knowledge here. The syllabus frames this topic around a single prior fact – that an enzyme is a biological catalyst that increases the rate of a reaction and remains unchanged when the reaction is complete – and builds outward from it into mechanism and kinetics.

3.1 Mode of action of enzymes

This sub-topic covers how enzymes actually work, not just what they do. You need to be able to:

• State that enzymes are globular proteins that catalyse reactions inside cells (intracellular enzymes) or are secreted to catalyse reactions outside cells (extracellular enzymes) – for example, digestive enzymes secreted into the gut.
• Explain the mode of action of enzymes in terms of the active site, the enzyme-substrate complex, the lowering of activation energy, and enzyme specificity, covering both historical models: the lock-and-key hypothesis (the active site has a fixed shape that exactly matches the substrate) and the more current induced-fit hypothesis (the active site changes shape slightly as the substrate binds, improving the fit and catalytic efficiency). Both models must be known, not just the more modern one, since exam questions can ask you to compare them.
• Investigate the progress of enzyme-catalysed reactions practically, using two named methods: measuring the rate of formation of products with catalase, and measuring the rate of disappearance of substrate with amylase.
• Outline the use of a colorimeter for measuring the progress of enzyme-catalysed reactions that involve a colour change – a practical skill likely to be tested alongside the theory.

3.2 Factors that affect enzyme action

This sub-topic moves from mechanism to kinetics – what changes the rate at which enzymes work.

Five factors must be investigated and explained: temperature, pH (using buffer solutions to hold it constant while varying other factors), enzyme concentration, substrate concentration, and inhibitor concentration. For each, you need to both describe the practical investigation and explain the underlying reason for the effect on rate – for example, temperature increases rate up to an optimum by increasing kinetic energy and collision frequency, then decreases it sharply beyond the optimum as the enzyme denatures.

Michaelis-Menten kinetics: you need to explain that the maximum rate of reaction (Vmax) is used to derive the Michaelis-Menten constant (Km), and that Km is used to compare the affinity of different enzymes for their substrates. A lower Km indicates a higher affinity (the enzyme reaches half its maximum rate at a lower substrate concentration), a distinction that is easy to state correctly with practice but easy to get backwards under exam pressure.

Worked comparison. Enzyme A has a Km of 2 mmol dm⁻³ and enzyme B has a Km of 20 mmol dm⁻³ for the same substrate. Enzyme A reaches half of its maximum rate at a much lower substrate concentration than enzyme B, so enzyme A has the higher affinity for the substrate – it is more effective at low substrate concentrations, whereas enzyme B needs a much higher substrate concentration before it works anywhere near as efficiently.

Inhibitors: you need to explain the effects of reversible inhibitors on enzyme activity, distinguishing:

• Competitive inhibitors – structurally similar to the substrate, compete for the active site, and their effect can be overcome by increasing substrate concentration
• Non-competitive inhibitors – bind elsewhere on the enzyme (not the active site), change the enzyme’s shape, and their effect cannot be overcome by increasing substrate concentration

Immobilised enzymes: you need to investigate the difference in activity between an enzyme immobilised in alginate and the same enzyme free in solution, and state the advantages of using immobilised enzymes (for example, the enzyme can be recovered and reused, and the product is not contaminated with enzyme).

Related resources

• Enzymes practice questions
• Biological molecules — the previous topic

How the two sub-topics connect

3.1 explains why an enzyme-catalysed reaction proceeds faster than an uncatalysed one (lowered activation energy via the active site); 3.2 explains what makes that rate go up or down once catalysis is already happening. A well-structured exam answer keeps these separate: a question about why enzymes speed up reactions should reference active site binding and activation energy from 3.1, not restate a factor from 3.2, and vice versa.

How to approach it

The lock-and-key versus induced-fit distinction is frequently tested by asking you to critique or compare the two models rather than simply define them, so practise explaining specifically what induced-fit adds that lock-and-key does not (a degree of flexibility in the active site, allowing a better fit and improved catalytic efficiency as the substrate binds). For 3.2, competitive versus non-competitive inhibition is the single most commonly confused pair in this topic – anchor the distinction in one clear test: “does increasing substrate concentration reduce the inhibitor’s effect?” (yes for competitive, no for non-competitive) rather than trying to memorise the mechanisms in isolation. Since this topic carries several practical investigations assessed in the Paper 5 Practical Assessment (catalase/amylase rate measurement, colorimetry, immobilised enzymes in alginate), make sure you can describe the method, the variables controlled, and the expected shape of a rate-against-factor graph for each – practical-based questions on this topic are common on both the AS and A Level papers.

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”. Verified 2026-09-02.