IGCSE Biology Topic 5: Enzymes MCQs – 30 Practice Questions with Answers (Cambridge 0610)
Free practice for Cambridge IGCSE Biology 0610 on enzymes, with every question followed by a short worked explanation to help you check your understanding.
How Enzymes Work
Q1. What is an enzyme?
- (A) A fat that stores energy
- (B) A protein that acts as a biological catalyst
- (C) A sugar used in respiration
- (D) A mineral ion needed by plants during the reaction described
Show Answer
Answer: B – Enzymes are proteins that function as biological catalysts in metabolic reactions.
Q2. What does a catalyst do in a chemical reaction?
- (A) It is used up by the reaction
- (B) It increases the rate of the reaction and is not changed by it
- (C) It stops the reaction completely
- (D) It changes the products into reactants in the conditions given in an ordinary cell
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Answer: B – A catalyst increases the rate of a reaction without itself being changed by the reaction.
Q3. Why are enzymes important in all living organisms?
- (A) They make reactions fast enough to sustain life
- (B) They store energy for later use
- (C) They carry oxygen around the body for this process
- (D) They build cell walls
Show Answer
Answer: A – Without enzymes, metabolic reactions would be too slow to sustain life.
Q4. What is the active site of an enzyme?
- (A) The part of the substrate that breaks apart during normal activity
- (B) The region of the enzyme whose shape fits its substrate
- (C) The product formed by the reaction
- (D) The energy released during the reaction
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Answer: B – The active site is the part of the enzyme whose shape is complementary to its substrate.
Q5. What forms when a substrate fits into the active site of an enzyme?
- (A) A new enzyme
- (B) A denatured protein in this type of cell
- (C) An enzyme-substrate complex
- (D) A second substrate
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Answer: C – The substrate fits into the active site to form an enzyme-substrate complex, which then breaks down into products.
Q6. After an enzyme-controlled reaction, what happens to the enzyme?
- (A) It is broken down into products under normal conditions
- (B) It is changed into a substrate
- (C) It becomes part of the product
- (D) It remains unchanged and can be used again
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Answer: D – The enzyme is not changed by the reaction, so it is released unchanged and can act again.
Q7. Why is an enzyme usually specific to one substrate?
- (A) Its active site shape fits only that substrate
- (B) It is found in only one cell
- (C) It works at only one temperature in the system described
- (D) It is used up after one reaction
Show Answer
Answer: A – Specificity comes from the complementary shape and fit between the active site and its substrate.
Q8. Which sequence correctly describes enzyme action?
- (A) Substrate, product, enzyme-substrate complex
- (B) Product, enzyme-substrate complex, substrate
- (C) Enzyme, substrate, product
- (D) Enzyme-substrate complex, enzyme, substrate
Show Answer
Answer: A – The substrate first fits the active site to form a complex, which then releases the products and the unchanged enzyme.
Q9. A substrate with a different shape from the active site is added to an enzyme. What is most likely?
- (A) It will fit and react quickly
- (B) It will not fit, so little or no reaction occurs
- (C) It will change the enzyme into a product when the reaction takes place
- (D) It will denature the enzyme at once
Show Answer
Answer: B – Only a substrate with a complementary shape fits the active site, so a differently shaped molecule reacts little or not at all.
Q10. Which statement about enzymes and metabolic reactions is correct?
- (A) Enzymes are involved in all metabolic reactions
- (B) Enzymes work only outside cells
- (C) Enzymes slow down metabolic reactions
- (D) Enzymes replace the need for substrates for the organism involved
Show Answer
Answer: A – Enzymes are proteins involved in all metabolic reactions, where they act as biological catalysts.
Temperature and Enzyme Activity
Q11. As temperature rises from low values towards the optimum, what happens to enzyme activity?
- (A) It rises
- (B) It falls steadily
- (C) It stays the same
- (D) It stops at once
Show Answer
Answer: A – Raising temperature increases activity up to the optimum temperature.
Q12. What is meant by the optimum temperature of an enzyme?
- (A) The temperature at which it denatures
- (B) The lowest temperature at which it works in most ordinary conditions
- (C) The temperature of boiling water
- (D) The temperature at which its activity is greatest
Show Answer
Answer: D – The optimum temperature is the temperature at which the enzyme works fastest.
Q13. What happens to an enzyme when it is heated well above its optimum temperature?
- (A) It works faster and faster
- (B) Its substrate changes shape instead during the process described
- (C) Its active site changes shape and it denatures
- (D) It becomes a different enzyme
Show Answer
Answer: C – Above the optimum, the enzyme denatures as its active site loses its shape.
Q14. Why does enzyme activity rise as temperature increases towards the optimum?
- (A) The enzyme gains extra active sites in a typical living cell
- (B) The substrate becomes an enzyme
- (C) Particles gain kinetic energy and collide more often
- (D) Water evaporates from the mixture
Show Answer
Answer: C – Higher kinetic energy means more frequent effective collisions between enzyme and substrate.
Q15. After heating well above its optimum, an enzyme is cooled back to its optimum temperature. What is expected?
- (A) It works as fast as before
- (B) It stays denatured and cannot fit its substrate
- (C) It gains a new active site
- (D) It works faster than before under the conditions described
Show Answer
Answer: B – Denaturation is a change in shape, and the denatured enzyme can no longer fit its substrate.
Q16. A graph of enzyme activity against temperature shows a rise, a peak and then a sharp fall. What does the fall show?
- (A) The substrate has run out
- (B) The pH has become optimum
- (C) The enzyme has denatured
- (D) The reaction has reversed
Show Answer
Answer: C – The sharp fall above the optimum is caused by denaturation of the enzyme.
Q17. At a low temperature, why is enzyme activity low?
- (A) The enzyme has denatured
- (B) The active site has changed shape permanently during the reaction described
- (C) Particles have low kinetic energy, so effective collisions are rare
- (D) The products block the reaction
Show Answer
Answer: C – At low temperatures particles move slowly, so enzyme and substrate collide effectively less often.
Q18. Two identical enzyme mixtures are kept at the optimum temperature and at a temperature far above the optimum. Which works faster?
- (A) The mixture far above the optimum
- (B) Both work at the same rate
- (C) Neither works at all
- (D) The mixture at the optimum
Show Answer
Answer: D – Activity is greatest at the optimum, while far above it the enzyme denatures and activity falls.
Q19. An enzyme that has been boiled is added to its substrate and no products form. What best explains this?
- (A) The substrate was used up by boiling
- (B) The enzyme denatured, so its active site no longer fits the substrate
- (C) Boiling added a new substrate
- (D) Boiling lowered the pH to the optimum in the conditions given in an ordinary cell
Show Answer
Answer: B – Boiling denatures the enzyme, changing the shape of its active site so the substrate cannot fit.
Q20. Which change would increase the rate of an enzyme-controlled reaction that is currently below its optimum temperature?
- (A) Cooling the mixture further
- (B) Warming the mixture towards the optimum
- (C) Adding a differently shaped substrate for this process
- (D) Removing all the enzyme
Show Answer
Answer: B – Warming towards the optimum increases kinetic energy and the frequency of effective collisions.
pH, Specificity and Investigations
Q21. What is meant by the optimum pH of an enzyme?
- (A) The pH at which it denatures fastest during normal activity
- (B) The pH of pure water
- (C) The lowest pH at which it works
- (D) The pH at which its activity is greatest
Show Answer
Answer: D – Each enzyme has an optimum pH at which it works fastest.
Q22. What happens when the pH moves far from an enzyme’s optimum?
- (A) The enzyme gains a new substrate in this type of cell
- (B) The substrate becomes an enzyme
- (C) The reaction speeds up steadily
- (D) The active site shape changes and activity falls
Show Answer
Answer: D – Moving far from the optimum pH changes the shape and fit of the active site, reducing activity.
Q23. Why does a large change in pH reduce enzyme activity?
- (A) It removes water from the mixture
- (B) It turns the enzyme into a fat
- (C) It changes the shape of the active site, so the substrate fits less well
- (D) It increases the substrate concentration in the system described under normal conditions
Show Answer
Answer: C – A large pH change alters the active site shape and fit, and can denature the enzyme.
Q24. A graph of activity against pH for one enzyme shows a peak in the middle and lower activity on both sides. What does the peak show?
- (A) The point of denaturation
- (B) The point where the enzyme is used up
- (C) The pH of the substrate
- (D) The optimum pH
Show Answer
Answer: D – The peak of the graph marks the optimum pH, where activity is greatest.
Q25. To investigate the effect of pH on enzyme activity, which variables should be kept the same?
- (A) Only the pH
- (B) The pH and the temperature, while substrate is varied when the reaction takes place
- (C) Temperature and substrate concentration, while pH is varied
- (D) Nothing needs to be kept the same
Show Answer
Answer: C – A fair test varies only pH and keeps other factors, such as temperature, constant.
Q26. How can the rate of an enzyme-controlled reaction be followed in an investigation?
- (A) By counting the number of enzymes present for the organism involved
- (B) By measuring the mass of the container
- (C) By waiting until the enzyme disappears
- (D) By measuring the amount changed in a set time
Show Answer
Answer: D – Rate can be followed as the amount of substrate used, or product formed, in a given time.
Q27. Why should an enzyme investigation be repeated?
- (A) To use up all the substrate in most ordinary conditions
- (B) To check that the results are reliable
- (C) To change the optimum pH
- (D) To denature the enzyme
Show Answer
Answer: B – Repeating measurements helps show whether the results are consistent and reliable.
Q28. An enzyme works well at pH 7 but very slowly at pH 2. What is the best conclusion?
- (A) Its optimum pH is near 7
- (B) Its optimum pH is 2
- (C) It has denatured at pH 7
- (D) It has no active site
Show Answer
Answer: A – Activity is greatest near the optimum, so an optimum near pH 7 fits the results better than pH 2.
Q29. Which statement best explains enzyme specificity?
- (A) Every enzyme has many active sites for many substrates during the process described
- (B) Enzymes work at every pH equally well
- (C) Substrates change shape to fit any enzyme
- (D) The complementary shape of the active site fits only its substrate
Show Answer
Answer: D – Specificity is explained by the complementary shape and fit of the active site with its substrate.
Q30. Which is the correct order of events in enzyme action?
- (A) The enzyme-substrate complex forms, then products are released and the enzyme is unchanged
- (B) Products form, then the substrate fits the active site under the conditions described in a typical living cell
- (C) The enzyme is used up, then products form
- (D) The substrate denatures, then the enzyme fits it
Show Answer
Answer: A – The substrate fits the active site to form a complex, products are then released, and the enzyme remains unchanged.
Work through any questions you missed again, then revise those areas before moving on to the next topic.