A-Level Biology Topic 3: Enzymes MCQs – 30 Practice Questions with Answers (Cambridge 9700)
Practice for Cambridge International AS and A Level Biology 9700, Topic 3, covering how enzymes work, the factors that affect them, inhibitors and immobilised enzymes, with a short explanation after every question.
Mode of Action
Q1. What type of molecule is an enzyme?
- (A) A fibrous lipid found in cell membranes
- (B) A globular protein
- (C) A polysaccharide
- (D) A nucleic acid
Show Answer
Answer: B – Enzymes are globular proteins that catalyse reactions inside cells or after being secreted outside cells.
Q2. What is an intracellular enzyme?
- (A) An enzyme that catalyses a reaction inside a cell
- (B) An enzyme that only works in the stomach of mammals
- (C) An enzyme that is denatured at all times
- (D) An enzyme found only in plant cells
Show Answer
Answer: A – Intracellular enzymes act inside cells, while extracellular enzymes are secreted to act outside cells.
Q3. What forms when a substrate binds to an enzyme?
- (A) A peptide bond
- (B) A second active site on the same enzyme molecule
- (C) A denatured protein
- (D) An enzyme-substrate complex
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Answer: D – The substrate fits into the active site to form an enzyme-substrate complex, which then breaks down into products.
Q4. How does an enzyme speed up a reaction?
- (A) By raising the temperature of the cell where it acts
- (B) By being used up in the reaction
- (C) By lowering the activation energy of the reaction
- (D) By changing the products into substrates
Show Answer
Answer: C – An enzyme provides an alternative pathway with a lower activation energy, so more molecules can react.
Q5. According to the lock-and-key hypothesis, why is an enzyme specific?
- (A) Its active site shape fits only its substrate
- (B) It is found in only one organ of the body at a time
- (C) It works at only one pH
- (D) It is used up after one reaction
Show Answer
Answer: A – In the lock-and-key model, the active site has a fixed shape complementary to its substrate.
Q6. What does the induced-fit hypothesis add to the lock-and-key model?
- (A) The substrate changes the enzyme into a completely different protein
- (B) The enzyme is destroyed after binding
- (C) The products determine the shape of the substrate
- (D) The active site changes shape slightly to fit the substrate
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Answer: D – In the induced-fit model, binding of the substrate causes a slight change in the active site shape, improving the fit.
Q7. After an enzyme-catalysed reaction, what happens to the enzyme?
- (C) It turns into a substrate
- (B) It becomes part of the product
- (A) It remains unchanged and can be used again
- (D) It is broken down by the reaction
Show Answer
Answer: C – A catalyst is not changed by the reaction it catalyses, so the enzyme is released unchanged.
Q8. How can the progress of a reaction catalysed by catalase be followed?
- (A) By counting the enzyme molecules
- (B) By weighing the substrate before the reaction starts
- (C) By measuring the rate of formation of products
- (D) By measuring the colour of the enzyme
Show Answer
Answer: C – Catalase breaks down hydrogen peroxide to water and oxygen, so product formation, such as oxygen released, can be measured over time.
Q9. How can the progress of a reaction catalysed by amylase be followed?
- (A) By measuring the mass of amylase used in the reaction mixture
- (B) By measuring the rate of disappearance of starch substrate
- (C) By measuring the temperature of the room
- (D) By counting the products in the nucleus
Show Answer
Answer: B – Amylase breaks down starch, so the disappearance of starch, shown by the iodine test, can be timed.
Q10. Why is a colorimeter useful in enzyme investigations?
- (A) It measures progress of reactions involving colour changes
- (B) It counts enzyme molecules directly
- (C) It changes the pH of a solution
- (D) It heats solutions to the optimum temperature for the enzyme
Show Answer
Answer: A – A colorimeter measures absorbance, so it can follow a reaction in which a coloured substrate disappears or a coloured product appears.
Factors Affecting Enzyme Activity
Q11. Why does enzyme activity rise as temperature increases towards the optimum?
- (A) The enzyme gains extra active sites as temperature rises
- (B) The substrate becomes an enzyme
- (C) Molecules gain kinetic energy and collide more often
- (D) Water evaporates from the mixture
Show Answer
Answer: C – Higher kinetic energy increases the frequency of successful collisions between enzyme and substrate.
Q12. What happens to an enzyme well above its optimum temperature?
- (A) It works faster and faster
- (B) It gains a new substrate that fits its active site
- (C) It becomes a different enzyme
- (D) Its active site changes shape and it denatures
Show Answer
Answer: D – Excess heat breaks bonds holding the enzyme shape, so the active site no longer fits the substrate.
Q13. Why are buffer solutions used when investigating the effect of pH on enzymes?
- (A) They keep the pH constant during the reaction
- (B) They increase the enzyme concentration in the mixture
- (C) They denature the enzyme quickly
- (D) They remove all substrate from the mixture
Show Answer
Answer: A – Buffers resist changes in pH, so the effect of one chosen pH can be tested fairly.
Q14. At low substrate concentration, what happens to the rate if more substrate is added?
- (C) The rate stays the same because enzymes are saturated
- (B) The rate falls because the enzyme denatures
- (A) The rate increases because more active sites become occupied
- (D) The rate falls because products block the reaction
Show Answer
Answer: C – With few substrate molecules present, adding more increases collisions with active sites and so raises the rate.
Q15. At very high substrate concentration, why does the rate level off?
- (A) The enzyme has denatured at that substrate concentration
- (B) The pH has changed sharply
- (C) All active sites are occupied at maximum rate
- (D) The substrate has changed shape
Show Answer
Answer: C – When every active site is occupied, adding more substrate cannot increase the rate further.
Q16. If enzyme concentration is increased while substrate is plentiful, what happens to the initial rate?
- (A) It falls steadily
- (B) It stays the same
- (C) It rises then falls at once as more enzyme is added
- (D) It increases, as more active sites are available
Show Answer
Answer: D – More enzyme molecules provide more active sites, so more substrate can be converted at the same time.
Q17. What is Vmax?
- (B) The minimum temperature for enzyme activity
- (A) The maximum rate of reaction when the enzyme is saturated with substrate
- (C) The pH at which an enzyme denatures
- (D) The concentration of inhibitor that stops a reaction
Show Answer
Answer: B – Vmax is the maximum rate reached when all active sites are occupied by substrate.
Q18. Which enzyme has the higher affinity for its substrate: one with a low Km or one with a high Km?
- (A) Neither, because Km does not relate to affinity
- (B) The enzyme with the low Km
- (C) The enzyme with the high Km
- (D) The enzyme with the higher Vmax only
Show Answer
Answer: B – Km is the substrate concentration giving half Vmax, so a low Km means the enzyme reaches half its maximum rate at a low substrate concentration, showing high affinity.
Q19. A graph of rate against substrate concentration rises steeply then levels off. What does the plateau show?
- (A) The enzyme has run out of products to form
- (B) The temperature is too low
- (C) The enzyme is saturated with substrate
- (D) The inhibitor has been removed
Show Answer
Answer: C – The plateau shows saturation: all active sites are occupied and the rate cannot rise further.
Q20. Why does a large change in pH reduce enzyme activity?
- (A) It removes water from the mixture
- (B) It turns the enzyme into a lipid
- (C) It increases substrate concentration in the reaction mixture greatly
- (D) It changes the active site shape, so substrate fits less well
Show Answer
Answer: D – Extreme pH disrupts bonds in the enzyme, altering the active site shape and reducing or stopping activity.
Inhibitors and Immobilised Enzymes
Q21. Where does a competitive inhibitor bind?
- (A) At the active site, competing with the substrate
- (B) At the nucleus of the cell where the DNA is stored
- (C) To the product molecules
- (D) To the cell surface membrane only
Show Answer
Answer: A – A competitive inhibitor has a shape similar to the substrate and competes for the active site.
Q22. How does adding more substrate affect competitive inhibition?
- (A) It has no effect on the rate of reaction at all in the mixture
- (B) It stops the reaction completely
- (C) It reduces inhibition, as substrate outcompetes inhibitor
- (D) It denatures the enzyme
Show Answer
Answer: C – Raising substrate concentration increases the chance that substrate, rather than inhibitor, occupies the active site.
Q23. What effect does a competitive inhibitor have on Vmax and Km?
- (A) Vmax falls and Km falls at the same time
- (B) Vmax stays the same and Km increases
- (C) Vmax rises and Km stays the same
- (D) Vmax stays the same and Km falls
Show Answer
Answer: B – With enough substrate, the original Vmax can still be reached, but more substrate is needed to reach half Vmax, so Km increases.
Q24. Where does a non-competitive inhibitor bind?
- (A) Only to the substrate
- (B) Only to the product
- (C) Only inside the nucleus of the cell where DNA is kept there
- (D) At a site other than the active site, changing its shape
Show Answer
Answer: D – A non-competitive inhibitor binds elsewhere on the enzyme and changes the shape of the active site, so the substrate fits less well.
Q25. What effect does a non-competitive inhibitor have on Vmax and Km?
- (A) Vmax decreases and Km stays the same
- (B) Vmax stays the same and Km increases
- (C) Vmax increases and Km decreases
- (D) Vmax and Km both increase
Show Answer
Answer: A – Fewer active sites can function, so Vmax falls, while the affinity of the remaining working sites is unchanged, so Km stays the same.
Q26. Can adding more substrate overcome non-competitive inhibition?
- (A) Yes, completely, just as with competitive inhibition in the cell
- (B) No, because the inhibitor does not compete for the site
- (C) Yes, but only at low temperatures
- (D) No, because the substrate is destroyed
Show Answer
Answer: B – Adding substrate cannot displace a non-competitive inhibitor, since it binds at a different site, so the inhibition cannot be overcome this way.
Q27. What is meant by an immobilised enzyme?
- (A) An enzyme trapped or attached to a support
- (B) An enzyme that has been denatured by heat or pH
- (C) An enzyme dissolved in water
- (D) An enzyme without an active site
Show Answer
Answer: A – Immobilised enzymes are fixed to a support, for example trapped in alginate beads, while substrate and products can still move around them.
Q28. Which is an advantage of using immobilised enzymes?
- (A) They cannot be reused
- (B) They contaminate the product
- (C) They are easily damaged by heat during the reaction
- (D) They can be reused and separated from the product
Show Answer
Answer: D – Immobilised enzymes can be recovered and used again, and the product is not contaminated with enzyme.
Q29. In an investigation comparing immobilised and free catalase, what should be kept the same?
- (A) Enzyme amount, substrate concentration, temperature and pH
- (B) Only the temperature
- (C) Only the pH
- (D) Nothing needs to be kept the same in this investigation at all
Show Answer
Answer: A – A fair comparison keeps enzyme amount, substrate concentration, temperature and pH the same, varying only whether the enzyme is immobilised.
Q30. Why might an immobilised enzyme show a lower rate than the same enzyme free in solution?
- (A) Its active site has disappeared completely from the enzyme molecule
- (B) Substrate reaches active sites more slowly through the support
- (C) It has become a different enzyme
- (D) Its substrate concentration is always zero
Show Answer
Answer: B – Adding substrate cannot displace a non-competitive inhibitor, since it binds at a different site, so the inhibition cannot be overcome this way.
Work through any questions you missed again, then revise those areas before moving on to the next topic.