AS Level Biology chapter 3: Enzymes MCQs with answers
By Team Megalecture · Updated 4 Nov 2026
24 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. What is an advantage of using an enzyme immobilised in alginate beads rather than free in solution?
A
the enzyme works faster at all temperatures because it is protected
B
the enzyme can be reused and separated easily from the product afterwards
C
the enzyme can no longer be denatured by high temperatures
D
the substrate no longer has to diffuse to reach the enzyme
Show answer
Answer: B. Beads are easily separated from the product and used again. Immobilised enzymes can still denature, and substrate must diffuse into the bead.
2. With substrate in excess, 4 µg of an enzyme gives an initial rate of 18 µmol min−1. What initial rate, in µmol min−1, is expected with 10 µg of the same enzyme and the same excess of substrate?
A
7.2
B
24
C
45
D
72
Show answer
Answer: C. With substrate in excess, the rate is proportional to enzyme concentration: 18 × 10 ÷ 4 = 45. Adding the extra 6 µg to 18 gives the wrong 24, and 18 × 4 gives 72.
3. Why is the initial rate used when comparing the activity of an enzyme under different conditions?
A
at the start, the substrate concentration is still the value that was set
B
the enzyme concentration rises as more product is formed, speeding the reaction
C
the activation energy is lowest before the enzyme–substrate complex forms
D
the enzyme has not yet been denatured by the product of the reaction
Show answer
Answer: A. As substrate is used up, the rate slows for reasons other than the variable being tested. At the start the substrate concentration is the value that was set.
4. Which is the simplest way to measure the rate of the reaction catalysed by catalase with hydrogen peroxide?
A
the time for a blue-black colour to disappear
B
the decrease in mass of the enzyme
C
the volume of oxygen collected per minute
D
the time for a brick-red precipitate to form
Show answer
Answer: C. Catalase breaks hydrogen peroxide into water and oxygen, so the rate of formation of product can be followed by collecting oxygen gas.
5. Which is an example of an extracellular enzyme?
A
catalase breaking down hydrogen peroxide inside liver cells
B
salivary amylase, secreted into the mouth, breaking down starch in food
C
RNA polymerase making mRNA from a DNA template in the nucleus
D
DNA polymerase joining nucleotides during replication in the nucleus
Show answer
Answer: B. Salivary amylase is secreted by cells and works outside them, in the mouth. The others work inside the cells that make them.
6. In an investigation of the effect of temperature on enzyme activity, why is a buffer solution added to each tube?
A
to keep the temperature of each tube constant in the water bath
B
to provide extra substrate as the reaction continues
C
to stop the reaction at the end of the time period by denaturing the enzyme
D
to keep the pH constant, as pH would also alter activity
Show answer
Answer: D. A buffer resists pH changes, so pH is a controlled variable and the only thing changing is temperature.
7. Which statement describes a non-competitive inhibitor?
A
its effect is reduced by adding a much higher concentration of substrate
B
it has a similar shape to the substrate and binds to the active site
C
it binds away from the active site and distorts the shape of the active site
D
it raises Km but does not change the Vmax of the reaction
Show answer
Answer: C. A non-competitive inhibitor binds elsewhere on the enzyme and distorts the active site, so extra substrate does not help and Vmax falls.
8. Amylase is mixed with starch. Every 30 s a drop of the mixture is added to iodine solution on a spotting tile. Which result shows that all the starch has been broken down?
A
the iodine turns blue-black
B
gas bubbles stop forming
C
a brick-red precipitate forms
D
the iodine stays orange-brown
Show answer
Answer: D. When no starch remains, iodine stays orange-brown. This measures the disappearance of substrate.
9. Before measuring samples, a colorimeter must be set to zero absorbance. What should be placed in it to do this?
A
a cuvette containing the most concentrated coloured sample
B
a cuvette with everything except the coloured substance
C
an empty, clean cuvette with no liquid in the light path
D
a cuvette of the enzyme solution after it has been boiled
Show answer
Answer: B. A blank containing everything except the coloured substance means any absorbance measured afterwards is due only to that substance.
10. Immobilised enzymes are often more stable at high temperatures than the same enzyme free in solution. What is the most likely reason?
A
alginate is itself an enzyme that continues the reaction at high temperature
B
immobilised enzymes have more active sites per molecule than free enzymes
C
the gel supports the enzyme, so its tertiary structure is not easily disrupted by heat
D
alginate raises the activation energy, so the reaction is less affected by heat
Show answer
Answer: C. Being held in the gel restricts the unfolding of the enzyme molecules, so the active site keeps its shape at higher temperatures.
24, each with the answer and a short explanation, across 2 topics: Mode of action of enzymes, Factors that affect enzyme action. Every chapter test draws ten of them, so you can take it many times.
Is the AS Level Biology chapter 3 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.
