Enzymes are one of the most examined topics in IGCSE Biology (Cambridge 0610). They appear in multiple-choice, structured, and practical papers, and examiners expect you to define key terms, interpret graphs, and explain results in terms of the lock-and-key model and denaturation. This revision guide takes you through what enzymes are, how they work, how pH and temperature affect them, and how to answer “describe”, “explain”, and practical questions so you can pick up full marks.

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What is an enzyme?

An enzyme is a biological catalyst. That means it is a substance made by living organisms that speeds up a chemical reaction without being used up in the reaction. Enzymes are proteins: long chains of amino acids folded into a specific 3D shape. That shape is crucial, because it determines which reaction the enzyme catalyses.

Without enzymes, many reactions in your body would be far too slow to sustain life. For example, digestion of starch into sugars, the breakdown of hydrogen peroxide in cells, and the building of large molecules all depend on enzymes. In exam answers, always state that enzymes are proteins and biological catalysts, and that they speed up reactions and are not used up.

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The lock-and-key model

The lock-and-key model explains why each enzyme usually catalyses only one type of reaction (or a small group of very similar reactions). The enzyme has a region called the active site: a pocket or cleft on the surface where the reaction takes place. The molecule that fits into the active site and is changed in the reaction is called the substrate. The lock-and-key idea is that the shape of the active site is complementary to the shape of the substrate — like a key fitting a lock. Only the correct substrate (or substrates) fits well enough to bind and react.

Once the substrate is bound to the active site, the reaction happens and products are formed. The products then leave the active site, which is free to bind another substrate molecule. So one enzyme molecule can catalyse many substrate molecules in sequence. You may also see the induced-fit model: the active site changes shape slightly when the substrate binds, to fit the substrate more closely. For IGCSE, explaining lock-and-key is usually enough; use the terms active site, substrate, specific, and complementary shape in your answers.

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Effect of temperature on enzyme activity

Temperature has a big effect on the rate of an enzyme-controlled reaction. As temperature increases from low to moderate values, the rate of reaction increases. That is because the substrate and enzyme molecules move faster and collide more often, and more of these collisions have enough energy for the reaction to happen. So increasing temperature increases the rate, up to a point.

There is a temperature at which the rate is highest: the optimum temperature (for many human enzymes this is around 37°C). Above the optimum, the rate falls sharply. That is because the extra heat causes the enzyme’s 3D shape to change permanently. The active site is no longer the right shape for the substrate, so the substrate cannot bind properly. This permanent change of shape is called denaturation. Denatured enzymes do not return to their original shape when the temperature is lowered. In “explain” questions, you must link the high temperature to denaturation and to the active site no longer fitting the substrate.

A typical rate vs temperature graph rises to a peak (the optimum) and then falls. You should be able to label the optimum temperature and explain both the rising part (more collisions, more successful reactions) and the falling part (denaturation).

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Effect of pH on enzyme activity

pH measures how acidic or alkaline a solution is (pH 7 is neutral; below 7 is acidic; above 7 is alkaline). Each enzyme has an optimum pH at which it works fastest. Small changes in pH above or below the optimum reduce the rate of reaction. That is because the charges on the amino acids in the active site depend on pH. If the pH changes, these charges change, so the shape of the active site can change. The substrate then fits less well (or not at all), so the rate drops. At extreme pH values, the enzyme can be denatured: the shape change is so large that the active site no longer works, and the change may be permanent.

Different enzymes have different optimum pH values. For example, pepsin (in the stomach) works best in acidic conditions (low pH), while many enzymes in the blood or in the small intestine work best at neutral or slightly alkaline pH. In exams, you might be given a rate vs pH graph: it often has a peak at the optimum pH and falls on both sides. Always explain the fall in rate by referring to change in shape of the active site (and denaturation at extremes if appropriate).

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Denaturation in detail

Denaturation is the permanent change in the 3D structure of a protein (and so of an enzyme) so that it no longer functions. For enzymes, denaturation usually means the active site loses its correct shape, so the substrate can no longer bind and the reaction stops or slows greatly. Denaturation can be caused by high temperature or extreme pH (very acidic or very alkaline). It is permanent for most enzymes: cooling or bringing pH back to normal does not restore activity.

In your answers, avoid saying the enzyme “dies” or “is destroyed”. Use the correct term: the enzyme is denatured. Also avoid saying the enzyme “stops working” without explaining why: the active site has changed shape, so the substrate no longer fits.

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Planning and describing enzyme experiments

Many exam questions ask you to plan an experiment to investigate the effect of temperature or pH on enzyme activity (e.g. amylase and starch, or catalase and hydrogen peroxide). You need to identify and control variables clearly.

• Independent variable: the one you change (e.g. temperature or pH).
• Dependent variable: the one you measure (e.g. time for starch to disappear, or volume of oxygen produced in a given time).
• Control variables: all other factors that could affect the rate (e.g. same volume and concentration of enzyme, same volume and concentration of substrate, same reaction time, same pH when varying temperature). Keeping these the same makes the test fair.

You should also state how you measure the dependent variable (e.g. iodine test for starch; measuring cylinder for gas volume) and, if relevant, how you keep the temperature or pH constant (water bath; buffer solution).

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Exam tips and command words

• Define: Give a precise meaning (e.g. “An enzyme is a biological catalyst that speeds up reactions and is not used up.”).
• State: Give a short fact (e.g. “The optimum pH is 7.”).
• Describe: Say what happens without necessarily explaining why (e.g. “As temperature increases, the rate increases to a maximum and then decreases.”).
• Explain: Give the biological reason (e.g. “Above the optimum temperature, the enzyme denatures, so the active site no longer fits the substrate and the rate decreases.”).
• Interpret a graph: Identify the optimum; describe the trend on both sides; explain the shape using collision rate and denaturation where relevant.
• Plan an experiment: Identify independent, dependent, and control variables; state how you make it fair and how you measure the dependent variable.

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Common mistakes to avoid

• Saying the enzyme “dies” or “is destroyed” instead of denatured.
• Describing the effect of temperature or pH without linking it to the shape of the active site and (above optimum or at extreme pH) denaturation.
• Confusing optimum (the value at which rate is highest, e.g. “optimum pH is 7”) with “maximum” (the word “optimum” is the one examiners expect).
• In experiments: not stating control variables or how you keep them the same.

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Revision checklist

• Define enzyme, active site, substrate, denaturation, and optimum.
• Draw or interpret a graph of rate vs temperature and rate vs pH; label optimum and explain the shape on both sides.
• Describe and explain the effect of temperature and pH on enzyme activity (include denaturation above optimum / at extreme pH).
• Plan a fair experiment to investigate the effect of temperature or pH on enzyme activity (variables; how to measure; how to keep it fair).

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Next steps

Book a free trial with an IGCSE Biology tutor to practise enzyme graphs and practical questions, or explore Tutopiya’s learning portal for more revision resources.