A Level Biology: Enzymes — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Enzymes study guide

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Section A

1. State two ways enzymes can be classified according to where they act, giving one example of each. [2]

2. Explain what is meant by enzyme specificity in terms of the active site. [2]

Section B

3. Compare the lock-and-key hypothesis with the induced-fit hypothesis of enzyme action, and state what the induced-fit hypothesis explains that the lock-and-key hypothesis does not. [4]

4. Describe how the progress of an enzyme-catalysed reaction could be followed using catalase, and separately using amylase, naming what is actually measured in each case. [4]

5. Explain how an enzyme lowers the activation energy of a reaction, referring to the enzyme-substrate complex. [3]

6. Describe the shape of a graph of rate of reaction against temperature for an enzyme-catalysed reaction, and explain the shape on both sides of the optimum. [5]

7. A student investigates the effect of pH on an enzyme-catalysed reaction. Explain why a buffer solution must be used. [2]

8. Distinguish between competitive and non-competitive inhibition, and describe a test that could be used to tell them apart experimentally. [5]

9. Explain what is meant by Km, and state what a low Km value indicates about an enzyme’s affinity for its substrate. [3]

10. State two advantages of using an enzyme immobilised in alginate rather than the same enzyme free in solution. [2]

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Answers

1. Intracellular enzymes act inside the cell that makes them, e.g. catalase, which breaks down hydrogen peroxide within liver cells [1]. Extracellular enzymes are secreted out of the cell to act outside it, e.g. amylase, secreted into the gut to digest starch [1].

2. An enzyme’s active site has a shape (and chemical groups) complementary to only one substrate, or a small group of very similar substrates [1], so the enzyme will only catalyse a reaction involving that particular substrate and no others [1].

3. Lock-and-key: the active site has a fixed, rigid shape that exactly matches the substrate’s shape, like a key fitting a lock [1]. Induced-fit: the active site is flexible and changes shape slightly as the substrate binds, moulding around it to improve the fit [1]. What induced-fit explains that lock-and-key does not: how binding strains specific bonds in the substrate as the site closes around it, increasing catalytic efficiency beyond what a perfectly rigid, pre-formed site could achieve [1] [1].

4. Catalase: measure the volume of oxygen gas produced over time (formation of product), for example by collecting the gas in an inverted measuring cylinder or gas syringe [1] [1]. Amylase: measure the disappearance of starch over time (disappearance of substrate), for example by sampling the reaction mixture at set time intervals and testing each sample with iodine solution until it no longer turns blue-black [1] [1].

5. The active site binds the substrate(s), forming an enzyme-substrate complex [1]. This holds the substrate molecule(s) in an orientation that strains particular bonds [1], lowering the activation energy needed for the reaction to proceed, so more substrate molecules have sufficient energy to react at a given temperature, increasing the rate of reaction [1].

6. Rate increases with temperature up to an optimum, because higher kinetic energy increases the frequency of successful enzyme-substrate collisions [1] [1]. Beyond the optimum, rate falls sharply, because the enzyme’s tertiary structure is disrupted – hydrogen and ionic bonds holding it in shape break – denaturing the active site so it no longer complements the substrate’s shape [1] [1], and the enzyme becomes permanently unable to catalyse the reaction [1].

7. A buffer solution maintains a constant pH throughout the investigation [1], so that any change in rate observed can be attributed only to the factor being deliberately varied, rather than to an uncontrolled drift in pH that would itself alter or denature the enzyme [1].

8. Competitive inhibitors are structurally similar to the substrate and compete for the active site [1]; their effect can be reduced by increasing substrate concentration, since more substrate molecules outcompete the inhibitor for access to the active site [1]. Non-competitive inhibitors bind at a site other than the active site, changing the enzyme’s overall shape, including the active site [1]; their effect cannot be overcome by increasing substrate concentration [1]. Test: run the reaction at a range of substrate concentrations with a fixed inhibitor concentration present – if the rate eventually recovers towards the uninhibited maximum, the inhibitor is competitive; if the rate plateaus below the uninhibited maximum regardless of substrate concentration, it is non-competitive [1].

9. Km is the substrate concentration at which the reaction proceeds at half its maximum rate (half of Vmax) [1] [1]. A low Km indicates high affinity – the enzyme reaches half its maximum rate at a low substrate concentration, meaning it binds its substrate effectively even when the substrate is scarce [1].

10. Any two: the enzyme can be recovered and reused, which is economically efficient [1]; the product is not contaminated with enzyme, avoiding a separate purification step [1]; an immobilised enzyme is often more stable to changes in temperature and pH than the free enzyme in solution [1].

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Where marks are usually lost

• Describing only one hypothesis of enzyme action when a question asks for a comparison, or failing to state what induced-fit adds beyond lock-and-key.
• Naming catalase and amylase without correctly pairing each with what it is used to measure – product formed for catalase, substrate disappearing for amylase.
• Explaining the fall in rate above the optimum temperature as simply “the enzyme is destroyed” rather than the specific mechanism: bonds breaking, tertiary structure changing shape, active site no longer complementary.
• Confusing competitive and non-competitive inhibition, particularly getting backwards which one is overcome by raising substrate concentration.
• Getting the Km-affinity relationship the wrong way round under exam pressure – a lower Km means higher, not lower, affinity.

Approaching enzyme questions

Most marks on this topic are lost not from missing a fact but from stating it without the mechanism behind it – “the enzyme denatures” alone rarely earns full credit, since the expected answer traces the chain from bond-breaking to shape change to loss of complementary fit at the active site. The same discipline applies to the competitive/non-competitive distinction: state the mechanism (does the inhibitor bind the active site or somewhere else) before stating the consequence (can raising substrate concentration overcome it), since examiners can ask for either half separately. For practical-based questions, always be ready to name exactly what is measured and how – volume of gas, disappearance of colour with iodine, or absorbance on a colorimeter – since “measure the rate” alone, without saying what is actually observed changing, will not gain the method marks.