Enzymes — Cambridge IGCSE Biology
Enzymes as biological catalysts, the active site and specificity, and how temperature and pH affect enzyme activity.
Revision notes
4 short notes, in the order of the specification. Each one in short:
Enzymes are proteins that function as biological catalysts in all metabolic reactions. A catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction, so it can be used again and again. Without enzymes, most reactions in cells would happen far too slowly to keep an organism alive.
An enzyme works because its active site has a shape complementary to its substrate, so the substrate fits into it. The substrate binds to the active site to form an enzyme-substrate complex, is changed into products, and the products leave. Enzymes are specific: only a substrate with a complementary shape can fit the active site.
Temperature, pH and enzyme activity
SupplementEnzyme activity is highest at an optimum temperature and an optimum pH. Below the optimum temperature, molecules have less kinetic energy, so there are fewer effective collisions. Above it, the bonds holding the enzyme in shape break, the active site changes shape, and the enzyme is denatured. At extreme pH the enzyme is also denatured.
Investigating temperature and pH
PracticalThe effect of temperature or pH on enzyme activity is investigated by timing how long amylase takes to digest starch. Drops of the mixture are added to iodine every 30 seconds; when it stays orange-brown, no starch is left. Activity rises to an optimum, then falls as the enzyme is denatured. Rate of reaction = 1 ÷ time taken.
Exam questions
7 exam-style questions (20 marks), each with its mark scheme.
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15 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of enzymes on one page, so you can see where this subtopic fits.
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Quick check questions
Define a catalyst.
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A substance that increases the rate of a chemical reaction and is not changed by the reaction.
What are enzymes?
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Proteins that function as biological catalysts in metabolic reactions.
What word describes the shape of an active site compared with its substrate?
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Complementary.
What happens to an enzyme above its optimum temperature?
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The shape of the active site changes and the enzyme is denatured.
Which solution is used to test for starch in the amylase investigation?
Show answer
Iodine solution.
Frequently asked questions
How do enzymes speed up reactions?
Enzymes speed up reactions by acting as biological catalysts. The substrate fits into the enzyme's active site, which has a complementary shape, and is changed into products. The products leave and the enzyme is unchanged, so it can be used again. This makes the reactions in cells fast enough to sustain life.
Why are enzymes specific?
Supplement Enzymes are specific because the shape of each enzyme's active site is complementary to the shape of only one substrate, or a very small group of similar substrates. Only a substrate with a complementary shape can fit into the active site and form an enzyme-substrate complex. A substrate with a different shape does not fit, so no reaction happens.
Why do enzymes denature at high temperatures?
Supplement Enzymes denature at high temperatures because the enzyme molecules vibrate more and the bonds holding the enzyme in shape break. The shape of the active site changes, so the substrate no longer fits and fewer enzyme-substrate complexes form. The rate falls steeply, reaching zero. Say 'denatured', never 'killed', because enzymes are proteins, not living things.
What happens to enzymes at low temperatures?
Supplement At low temperatures enzymes are not denatured; they just work slowly. The enzyme and substrate molecules have less kinetic energy, so they move more slowly and there are fewer effective collisions between substrate and active site. Fewer enzyme-substrate complexes form, so the rate is low. Activity increases again when the enzyme is warmed.
How does pH affect enzyme activity?
Supplement Each enzyme works best at its optimum pH. A pH above or below the optimum changes the bonds that hold the enzyme in shape, so the active site changes shape and the substrate no longer fits. Fewer enzyme-substrate complexes form and activity falls. At extreme pH the enzyme is denatured. Different enzymes have different optimum pH values.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026