Enzymes
Practise Enzymes for GCSE Biology with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Use the 'lock and key' model to explain enzyme specificity, and why extremes of temperature or pH denature the active site.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Enzymes are biological catalysts: proteins that speed up chemical reactions without being used up themselves.
Each enzyme has an active site with a specific shape, and only a substrate with a complementary shape will fit. This is the lock and key theory, and it explains why enzymes are specific to one reaction.
If the temperature or pH moves too far from the optimum, the active site changes shape and the substrate no longer fits. The enzyme is then described as denatured. It is important to say the active site changes shape — saying the enzyme is killed is wrong, because enzymes are not alive.
Revision notes
Lock and key theory
The substrate fits into the enzyme's active site like a key into a lock.
The shapes are complementary, so each enzyme catalyses only one type of reaction. After the reaction the products leave and the enzyme is free to work again.
Effect of temperature
As temperature rises, the rate increases because particles move faster and collide more often.
Above the optimum the rate falls sharply because the active site changes shape and the substrate no longer fits — the enzyme is denatured. This change is permanent.
Effect of pH
Each enzyme has an optimum pH. Moving away from it reduces the rate, and a large change denatures the enzyme.
Stomach protease works best in acidic conditions around pH 2, whereas pancreatic enzymes work best in the slightly alkaline conditions of the small intestine.
Key points
- Enzymes are biological catalysts made of protein.
- They speed up reactions without being used up.
- The substrate fits the active site — the lock and key theory.
- Each enzyme is specific to one substrate.
- High temperature or extreme pH denatures the enzyme.
- Denaturing changes the shape of the active site.
Worked examples
Example 1
Explain what is meant by an enzyme being denatured. [2 marks]
Model answer
The shape of the enzyme's active site changes (1 mark), so the substrate no longer fits and the enzyme can no longer catalyse the reaction (1 mark).
Example 2
Explain why an enzyme can catalyse only one type of reaction. [2 marks]
Model answer
The active site has a specific shape (1 mark) which is complementary only to one substrate, so no other substrate can fit into it (1 mark).
Example 3
Explain why the rate of an enzyme-controlled reaction increases and then falls as temperature rises. [3 marks]
Model answer
At first the rate increases because the particles have more kinetic energy, so there are more frequent successful collisions between enzyme and substrate (1 mark). Above the optimum temperature the active site changes shape (1 mark), so the substrate no longer fits and the rate falls sharply (1 mark).
Common mistakes
Saying the enzyme is killed.
Enzymes are not alive. The correct term is denatured, and the active site changes shape.
Saying the enzyme is used up.
Catalysts are not used up — the enzyme is free to work again after the reaction.
Forgetting to mention the active site.
It is the active site whose shape changes, and that detail usually carries the mark.
Saying all enzymes work best at pH 7.
Stomach protease works best at about pH 2.
Exam tips
- Always refer to the active site changing shape when explaining denaturing.
- Use the words specific and complementary when explaining enzyme action.
- Explain the rate increase using collisions and kinetic energy.
- Never say an enzyme is killed.
Key terms
- Enzyme
- A biological catalyst made of protein.
- Active site
- The region of an enzyme where the substrate binds.
- Substrate
- The molecule an enzyme acts upon.
- Denatured
- Having an active site whose shape has changed so the substrate no longer fits.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.