O Level / IGCSE Biology chapter 5: Enzymes MCQs with answers
By Team Megalecture · Updated 4 Nov 2026
19 exam-style questions on Enzymes, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.
Try each one, then open the answer. These ten stay the same; the test draws new ones every time.
1. A student adds protease to a solution of amylase. The amylase then stops digesting starch. What is the best explanation?
A
the protease changes the pH so the amylase is denatured
B
the protease uses up the starch that amylase needs
C
the protease digests the amylase, because amylase is a protein
D
the protease raises the temperature of the mixture
Show answer
Answer: C. Enzymes are proteins, so a protease digests amylase into amino acids and it can no longer work. The protease does not act on starch.
2. A student investigates the effect of pH on amylase activity. Which variables should be kept constant?
A
the colour of the iodine after the reaction has finished in each tube
B
the time taken for the starch to disappear from each mixture
C
the pH of the buffer solution added to each tube
D
the temperature, and the volume and concentration of enzyme and starch
Show answer
Answer: D. To make a fair test of pH, only pH (the independent variable) is changed. The temperature and the volume and concentration of enzyme and starch are controlled. The time taken is the dependent variable.
3. The enzyme sucrase splits sucrose into glucose and fructose. What are the products of this reaction?
A
sucrase only
B
sucrose and glucose
C
glucose and fructose
D
sucrase and fructose
Show answer
Answer: C. Sucrase is the enzyme and sucrose is the substrate. The products are the two smaller sugars, glucose and fructose.
4. What is the part of an enzyme molecule where the substrate binds called?
A
the product
B
the active site
C
the enzyme-substrate complex
D
the antigen
Show answer
Answer: B. The substrate fits into the active site. Enzyme and substrate joined together form the enzyme-substrate complex.
5. Catalase is added to hydrogen peroxide and the total volume of oxygen collected is measured. It is 0 cm3 at 0 minutes, 6 cm3 at 1 minute, 11 cm3 at 2 minutes, 15 cm3 at 3 minutes and 18 cm3 at 4 minutes. What is the mean rate of oxygen production between 2 and 4 minutes?
A
4.5 cm3 per minute
B
3.5 cm3 per minute
C
7 cm3 per minute
D
9 cm3 per minute
Show answer
Answer: B. Between 2 and 4 minutes the volume increases by 18 − 11 = 7 cm³ in 2 minutes, so the mean rate is 7 ÷ 2 = 3.5 cm³ per minute. The rate falls as the substrate is used up.
6. Which sequence describes enzyme action?
A
substrate binds to the active site → enzyme-substrate complex forms → products are released and the enzyme is unchanged
B
substrate binds to the active site → the enzyme changes into the product → a new enzyme is made
C
product binds to the active site → enzyme-substrate complex forms → the substrate is made
D
enzyme and product form a complex → the substrate is released → the enzyme is used up
Show answer
Answer: A. The substrate binds to the active site to form an enzyme-substrate complex, the reaction occurs and the products leave. The enzyme is unchanged and can be used again.
7. In the 'lock and key' hypothesis, what does the key represent?
A
the enzyme
B
the active site
C
the substrate
D
the product
Show answer
Answer: C. The active site of the enzyme is the lock; the substrate, with its complementary shape, is the key.
8. What is a catalyst?
A
a substance that is changed into the product during a reaction
B
a substance that speeds up a reaction but is used up in doing so
C
a substance that provides the energy needed for a reaction to start
D
a substance that speeds up a reaction and is not changed by it
Show answer
Answer: D. A catalyst increases the rate of a chemical reaction and is unchanged at the end, so it can be used again.
9. Pepsin has an optimum pH of about 2. What will its activity be at pH 7?
A
very low, because pH 7 is far from its optimum
B
at its highest, because pH 7 is neutral
C
the same as at pH 2
D
higher than at pH 2, because there is less acid
Show answer
Answer: A. Each enzyme works best at its own optimum pH. Pepsin works in the acidic stomach; at pH 7 its active site shape is changed and activity is very low.
10. Why can amylase break down starch but not protein?
A
amylase is denatured when it meets protein, so it cannot bind to it
B
protein molecules are too big to reach the enzyme in the mouth and gut
C
amylase is used up completely when it digests starch, so none is left
D
the active site of amylase has a shape that fits only starch molecules
Show answer
Answer: D. Enzymes are specific: the shape of the active site is complementary only to its own substrate. Amylase fits starch, not protein.
19, each with the answer and a short explanation, across 2 topics: Enzyme action, Effects of temperature and pH. Every chapter test draws ten of them, so you can take it many times.
Is the O Level / IGCSE Biology chapter 5 test free?
Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.
How is the chapter test marked?
The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.
