IGCSE Biology: Enzymes Practice Questions
Enzymes are proteins that act as biological catalysts, speeding up chemical reactions without being changed themselves. Each enzyme has an active site with a shape complementary to its substrate, which is why enzymes are specific.
Topic 5 of Cambridge IGCSE Biology 0610 is short but appears on almost every paper, usually as a graph interpretation question about temperature or pH. Marks are lost for saying an enzyme is killed, and for describing the graph rather than explaining it. The questions below train the explanation.
What you need to know for Enzymes
- EnzymeA protein that functions as a biological catalyst. It speeds up the rate of a chemical reaction and is not used up or changed by the reaction.
- Active site and specificityThe active site is the region of the enzyme where the substrate binds. Its shape is complementary to the substrate, so each enzyme catalyses only one type of reaction. This is the lock and key model.
- Effect of temperature below the optimumRaising the temperature gives molecules more kinetic energy, so enzyme and substrate collide more often and the rate increases.
- DenaturingAbove the optimum temperature, or at an extreme pH, the enzyme's shape changes permanently. The active site is no longer complementary to the substrate, so no enzyme substrate complexes form and the reaction stops.
- Optimum pHEach enzyme has a pH at which it works fastest. Pepsin in the stomach works best in acidic conditions around pH 2, while amylase and pancreatic enzymes work best around pH 7 to 8.
- Enzymes are not aliveEnzymes are molecules, so they cannot be killed. Use the word denatured.
IGCSE Biology Enzymes questions and answers
4 exam-style questions written to the 0610 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
A graph shows the rate of an enzyme controlled reaction rising as temperature increases from 10 to 40 degrees Celsius, peaking at 40, then falling sharply to zero by 60. Explain the shape of this graph.
Show the worked answer
- Between 10 and 40 degrees, increasing temperature gives the enzyme and substrate molecules more kinetic energy.
- They therefore move faster and collide more frequently, so more enzyme substrate complexes form per second and the rate increases.
- At 40 degrees the rate is at its maximum. This is the optimum temperature for this enzyme.
- Above the optimum the enzyme molecules begin to denature. The bonds holding the protein in shape break, so the active site changes shape permanently.
- The substrate no longer fits the active site, so fewer complexes form and the rate falls. By 60 degrees all the enzyme has denatured and the rate is zero.
Explain, using the lock and key model, why amylase can break down starch but cannot break down protein.
Show the worked answer
- Each enzyme has an active site with a specific shape.
- The active site of amylase is complementary in shape to a starch molecule, so starch fits into it like a key into a lock and an enzyme substrate complex forms.
- A protein molecule has a different shape and is not complementary to the active site of amylase.
- It therefore cannot bind, no enzyme substrate complex forms, and no reaction is catalysed.
A student investigates the effect of pH on amylase. She mixes amylase with starch at five different pH values and samples the mixture every 30 seconds, testing each sample with iodine solution. Describe how she can use these results to find the rate of reaction, and state one variable she must control.
Show the worked answer
- Iodine solution turns blue black in the presence of starch and stays orange brown when no starch remains.
- She records the time at which the iodine first fails to turn blue black. That is the time taken for all the starch to be broken down.
- A shorter time means a faster reaction, so rate can be compared as 1 divided by time.
- She must keep the temperature constant, for example using a water bath, since temperature also affects enzyme activity. Concentration and volume of amylase and starch must also stay the same.
Explain why a biological washing powder containing protease works poorly if the wash is carried out at 90 degrees Celsius.
Show the worked answer
- Protease is an enzyme, and enzymes are proteins with a precisely shaped active site.
- At 90 degrees the temperature is far above the optimum, so the enzyme denatures and its active site changes shape permanently.
- Protein stain molecules can no longer bind to the active site, so no enzyme substrate complexes form.
- The protein stains are not broken down into soluble amino acids, so they are not washed away and the powder cleans poorly.
Common mistakes in this topic
- Saying enzymes are killed rather than denatured.
- Describing the graph instead of explaining it. The rate goes up is a description. More kinetic energy causing more collisions is an explanation.
- Claiming denaturing is reversible on cooling. At IGCSE, treat it as permanent.
- Saying enzymes are used up in the reaction. Catalysts are unchanged and can be reused.
- Confusing the substrate with the product. Amylase acts on starch, the substrate, and produces maltose, the product.
Exam tips
- Any explanation of an enzyme graph should use four phrases: kinetic energy, collisions, optimum, denatured active site.
- For rate questions, remember that rate is proportional to 1 divided by the time taken.
- Name the enzyme, substrate and product explicitly. Vague answers about food being broken down do not score.
- In practical questions, identify the independent, dependent and controlled variables before you write anything.
- Remember that pepsin has an acidic optimum. Questions often use it to catch students who assume every enzyme prefers pH 7.
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Enzymes FAQs
What is an enzyme in IGCSE Biology?
An enzyme is a protein that acts as a biological catalyst. It increases the rate of a specific chemical reaction and is not used up or permanently changed by that reaction, so a single enzyme molecule can catalyse the same reaction repeatedly.
What does denatured mean?
Denatured means the enzyme's three dimensional shape has been permanently changed, usually by high temperature or an extreme pH. The active site is no longer complementary to the substrate, so the substrate cannot bind and the reaction stops. Enzymes are molecules, so they are never described as killed.
Why does each enzyme only work on one substrate?
The active site of an enzyme has a specific shape that is complementary to one particular substrate. Only that substrate can fit into the active site and form an enzyme substrate complex, which is why enzymes are described as specific. This is the lock and key model.
Why does the rate of an enzyme reaction increase with temperature at first?
A higher temperature gives enzyme and substrate molecules more kinetic energy, so they move faster and collide more frequently with sufficient energy to react. More enzyme substrate complexes form each second, so the rate rises until the optimum temperature is reached.
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Written to the published Cambridge IGCSE Biology (0610) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.