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IGCSE Biology · 0610

IGCSE Biology: Enzymes Practice Questions and Answers

Enzyme questions turn on precise language: complementary shape, active site, denatured, optimum. These worked answers show exactly how to describe the effect of temperature and pH without losing the easy marks.

What you need to know

  • Enzymes are biological catalysts, made of protein, that speed up reactions and are not used up.
  • Each enzyme has an active site with a shape complementary to one substrate, described by the lock and key model.
  • Activity rises with temperature to an optimum, then falls as the enzyme denatures.
  • Denaturation permanently changes the shape of the active site, so the substrate no longer fits.
enzyme substrate active site complementary shape: only the correct substrate fits
Lock and key model: the substrate fits the complementary active site of the enzyme.

Practice questions with answers

Question 14 marks

Describe and explain the effect of increasing temperature on the activity of an enzyme.

As temperature increases up to the optimum, the rate of reaction increases because the enzyme and substrate molecules have more kinetic energy and collide more often. Above the optimum temperature the enzyme denatures. The shape of the active site changes, so the substrate no longer fits, and the rate of reaction falls.

Mark schemeRate rises with temperature up to optimum (1). More kinetic energy, more collisions (1). Above optimum enzyme denatures (1). Active site shape changes so substrate no longer fits, rate falls (1).

Common mistakeSaying the enzyme is killed. Enzymes are not alive; they are denatured.

Exam tipDescribe both sides of the optimum. Many candidates only explain the rise and lose two marks.

Question 22 marks

Explain why an enzyme is specific to one substrate.

The active site of the enzyme has a shape that is complementary to only one substrate. Only that substrate can bind to the active site, following the lock and key model, so the enzyme catalyses only one reaction.

Mark schemeActive site is a complementary shape to the substrate (1). Only that substrate fits or binds, lock and key (1).

Common mistakeSaying the active site is the same shape as the substrate. It is complementary, not identical.

Exam tipUse the word complementary. It is the key term examiners look for.

Question 32 marks

State what is meant by the term denatured.

Denatured means that the shape of the enzyme, including its active site, has been permanently changed. The substrate can no longer bind, so the enzyme no longer works.

Mark schemeShape of active site changed, permanently (1). Substrate no longer fits or binds, so enzyme stops working (1).

Common mistakeDescribing denaturation as temporary. It is a permanent change.

Exam tipLink the shape change to the substrate no longer fitting for the second mark.

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Frequently asked questions

What is an enzyme?

An enzyme is a biological catalyst made of protein. It speeds up a chemical reaction in living things and is not used up in the reaction.

What happens to enzymes at high temperatures?

Above the optimum temperature the enzyme denatures. The active site changes shape, the substrate cannot bind, and activity falls sharply.

What is the lock and key model?

It describes how only a substrate with a shape complementary to the enzyme's active site can fit and react, like a key fitting one lock.

Does denaturation reverse when it cools down?

No. Denaturation is permanent. Once the active site has changed shape, cooling does not restore enzyme activity.

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