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 revision notes

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Section A

1. Define the term enzyme. [2]

2. Explain what is meant by enzyme specificity, using the lock-and-key model. [3]

Section B

3. A student measures the rate at which amylase breaks down starch at temperatures from 10 °C to 70 °C.

(a) Sketch or describe the expected shape of the graph. [2] (b) Explain the increase in rate up to the optimum. [2] (c) Explain the sharp decrease above the optimum. [3] (d) State two variables that must be controlled. [2]

4. Pepsin works best at pH 2 and amylase at pH 7.

(a) Suggest where in the body each is found. [2] (b) Explain what happens to amylase at pH 2. [3]

5. Explain why denaturation is irreversible but the slowing of an enzyme at 5 °C is not. [3]

6. Give two uses of enzymes in industry and explain the benefit of each. [4]

7. (Beyond this syllabus — substrate concentration is not one of the enzymes outcomes in O Level 5090, which cover only enzyme action and the effects of temperature and pH.) A student investigates the effect of substrate concentration on the rate of an enzyme-catalysed reaction, keeping enzyme concentration, temperature and pH constant.

(a) Describe the shape of the graph of rate against substrate concentration. [2]

(b) Explain why the rate plateaus at high substrate concentration. [2]

(c) Suggest one change to the reaction mixture that would raise the plateau to a higher rate. [1]

8. Trypsin has an optimum pH of about 8, unlike pepsin (pH 2) or amylase (pH 7).

(a) State where in the body trypsin is most likely to act, and explain your answer. [2]

(b) Describe a suitable practical method for measuring the rate of the reaction between catalase and hydrogen peroxide. [3]

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Answers

1. A biological catalyst [1] that speeds up a reaction without being used up or permanently changed [1]. (Enzymes are proteins.)

2. Each enzyme has an active site with a specific shape [1] that is complementary to only one substrate [1]; only that substrate can fit to form an enzyme–substrate complex, so the enzyme catalyses only one reaction [1].

3. (a) The rate rises to a peak at the optimum, around 37 °C [1], then falls steeply to zero [1]. (b) Molecules gain kinetic energy and move faster [1], so there are more frequent successful collisions between enzyme and substrate [1]. (c) The increased vibration breaks the bonds holding the enzyme’s shape [1]; the active site changes shape [1] so the substrate no longer fits and no enzyme–substrate complexes form — the enzyme is denatured [1]. (d) Any two: pH, enzyme concentration, substrate (starch) concentration, volume of solution [1] [1].

4. (a) Pepsin — the stomach, which contains hydrochloric acid [1]. Amylase — the mouth or small intestine [1]. (b) The high H⁺ concentration disrupts the bonds holding the tertiary structure [1]; the active site changes shape [1] and the enzyme is denatured, so it stops working [1].

5. Denaturation involves the permanent breaking of the bonds that hold the enzyme’s 3-D shape [1], so the active site cannot re-form [1]. At 5 °C the enzyme is only working slowly because the molecules have little kinetic energy — the shape is unchanged, so warming restores activity [1].

6. Any two, each for 2 marks: biological washing powders contain proteases and lipases [1] that digest protein and fat stains at lower temperatures, saving energy [1]. Lactase is used to make lactose-free milk [1] for people who are lactose intolerant [1]. Pectinase is used in fruit juice production [1] to increase juice yield and clarity [1].

7. (a) Rate rises steeply at first, then the increase slows and levels off into a plateau at high substrate concentration [1] [1]. (b) At high substrate concentration all the active sites are occupied at any given moment [1], so the enzyme concentration becomes the limiting factor and extra substrate cannot be processed any faster [1]. (c) Increase the enzyme concentration [1], which provides more active sites for the excess substrate to bind to.

8. (a) The small intestine [1], because it is alkaline there (unlike the acidic stomach), matching trypsin’s alkaline optimum pH [1]. (b) Mix catalase with hydrogen peroxide and collect the oxygen gas produced in a gas syringe [1], timing how much gas is collected in fixed time intervals [1]; a steeper volume-against-time graph indicates a faster reaction [1].

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Where marks are usually lost

• Saying enzymes are “killed” by heat — they are denatured, not alive.
• Explaining the fall above the optimum without mentioning the active site’s shape.
• Listing control variables that are not actually relevant to the investigation.
• Saying cold denatures enzymes.
• Explaining the substrate-concentration plateau by reaching for the temperature/pH denaturation explanation instead of the correct one — the enzyme’s active sites simply become saturated, nothing is denatured.
• Assuming every digestive enzyme has an optimum pH of 7 — trypsin’s alkaline optimum (~8) is easily confused with amylase’s neutral one (~7) if the specific enzyme in the question isn’t checked carefully.

Questions 7 and 8 draw on the substrate-concentration and reaction-progress sections of the Enzymes revision notes — material the earlier questions on this page don’t reach, since they focus on temperature and pH rather than substrate concentration or practical method.