Here ’ s a set of multiple-choice questions (MCQs) on Chapter 5: Enzymes for the IGCSE

curriculum:

MCQs on Enzymes
1. Which of the following best defines an enzyme?
A. A lipid that speeds up chemical reactions
B. A protein that lowers the activation energy of a reaction
C. A carbohydrate that stores energy in the body
D. A nucleic acid that controls cell division
Answer: B
2. What is the specific region on an enzyme where the substrate binds?
A. Active site
B. Allosteric site
C. Catalytic site
D. Binding site
Answer: A
3. What happens to enzymes at very high temperatures?
A. They become more active
B. They are denatured
C. They are replicated
D. They dissolve
Answer: B
4. Enzymes are made up of which type of biological molecule?
A. Lipids
B. Proteins
C. Nucleic acids
D. Carbohydrates
Answer: B
5. Which factor does NOT affect enzyme activity?
A. Temperature
B. pH
C. Substrate concentration
D. Color of the enzyme
Answer: D
6. What is the term for the minimum energy required to start a chemical reaction?
A. Activation energy
B. Kinetic energy
C. Potential energy
D. Chemical energy
Answer: A
7. How does pH affect enzyme activity?
A. Changes the enzyme’s color
B. Affects the shape of the active site
C. Increases the enzyme’s size
D. Reduces substrate concentration
Answer: B
8. What is the optimum temperature for most human enzymes?
A. 25°C
B. 37°C
C. 50°C
D. 100°C
Answer: B
9. What is a coenzyme?
A. A protein that binds with the enzyme to increase its efficiency
B. A molecule that assists enzymes in catalyzing reactions
C. A carbohydrate that inhibits enzyme activity
D. A type of enzyme that acts on lipids
Answer: B
10. What does the term “substrate specificity” mean?
A. One enzyme acts on one specific substrate
B. Enzymes can work at all temperatures
C. Enzymes only work in acidic conditions
D. Enzymes only work in the absence of oxygen
Answer: A
11. What happens when an enzyme is denatured?
A. The enzyme works faster
B. The shape of its active site is altered
C. The enzyme splits into two smaller enzymes
D. The enzyme turns into a substrate
Answer: B
12. What is the lock-and-key model of enzyme action?
A. The enzyme changes its shape to fit the substrate
B. The substrate changes to fit the enzyme
C. The substrate fits perfectly into the enzyme’s active site
D. The enzyme is destroyed after catalysis
Answer: C
13. Which enzyme breaks down starch into maltose?
A. Lipase
B. Amylase
C. Protease
D. Maltase
Answer: B
14. What does the term “enzyme inhibition” mean?
A. Enzymes stop functioning permanently
B. A substance reduces or stops enzyme activity
C. Enzymes are produced in smaller quantities
D. Enzymes are made inactive by high temperatures only
Answer: B
 15. What is the role of enzymes in digestion?
A. To absorb nutrients
B. To catalyze the breakdown of large molecules into smaller ones
C. To transport oxygen to cells
D. To regulate body temperature
Answer: B

16. What is the effect of increasing substrate concentration on enzyme activity, assuming the
enzyme is not saturated?
A. Activity decreases
B. Activity increases
C. Activity remains the same
D. Activity stops
Answer: B
17. Which of the following statements is true about enzyme activity at low temperatures?
A. Enzymes are denatured
B. Enzymes work at their maximum efficiency
C. Enzyme activity is slow due to reduced kinetic energy
D. Enzyme activity increases exponentially
Answer: C
18. Which enzyme is responsible for breaking down proteins into peptides?
A. Lipase
B. Protease
C. Amylase
D. Nuclease
Answer: B
19. Which of the following is a property of enzymes?
A. They are used up in chemical reactions
B. They speed up reactions without being consumed
C. They work at all pH levels equally
D. They are made of carbohydrates
Answer: B
20. What is a competitive inhibitor?
A. A molecule that permanently disables the enzyme
B. A molecule that binds to the active site of the enzyme
C. A molecule that binds to a site other than the active site
D. A molecule that accelerates enzyme activity
Answer: B
21. What is the term for an enzyme that works outside the cell?
A. Intracellular enzyme
B. Extracellular enzyme
C. Isoenzyme
D. Coenzyme
Answer: B
 22. What type of reaction is catalyzed by enzymes?
A. Exothermic reactions only
B. Endothermic reactions only
C. Both anabolic and catabolic reactions
D. Only hydrolysis reactions
Answer: C
23. What happens to the rate of enzyme activity when the pH is far from the optimum level?
A. The rate increases
B. The enzyme is denatured
C. The enzyme forms new active sites
D. The enzyme becomes larger
Answer: B
24. Which enzyme converts hydrogen peroxide into water and oxygen?
A. Amylase
B. Lipase
C. Catalase
D. Pepsin
Answer: C
25. Which type of enzyme inhibitor binds to a site other than the active site?
A. Competitive inhibitor
B. Non-competitive inhibitor
C. Substrate analogue
D. Allosteric activator
Answer: B
26.. What is the optimum pH for the enzyme pepsin, which works in the stomach?
A. pH 2
B. pH 7
C. pH 9
D. pH 14
Answer: A
27. What is the induced fit model of enzyme action?
A. The enzyme and substrate fit perfectly, like a lock and key
B. The active site changes shape slightly to fit the substrate
C. The substrate is permanently attached to the enzyme
D. The enzyme is destroyed after one reaction
Answer: B
28. Which of the following best describes a catalyst?
A. It slows down a chemical reaction
B. It speeds up a chemical reaction without being consumed
C. It changes the direction of a chemical reaction
D. It prevents reactions from occurring
Answer: B
29. What term describes the rate at which an enzyme works at its highest efficiency?
A. Maximum velocity (Vmax)
B. Activation energy
C. Competitive inhibition
D. Optimum energy
Answer: A
30. What type of enzyme catalyzes the breakdown of fats into glycerol and fatty acids?
A. Protease
B. Amylase
C. Lipase
D. Nuclease
Answer: C