Cambridge IGCSE Biology 0610: Enzymes – Study Guide

This study guide teaches topic 5, Enzymes (sub-topic 5.1), of the Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028. Learning outcomes 1 to 5 are Core. Outcomes 6 to 9 are Supplement content, so they are Extended only: they ask you to explain what Core candidates only describe. Core content is tested on Papers 1 and 3, Core and Supplement content on Papers 2 and 4, and rates of enzyme-catalysed reactions are a listed context for the practical papers (Paper 5 or Paper 6).

Keep the 0610 course hub and the printable checklist open as you work. Afterwards, use the enzymes revision notes to condense the topic and the enzymes practice questions to test it.

What this topic covers

0610 outcome	What you must be able to do	Tier
5.1.1	Describe a catalyst: increases the rate of a chemical reaction and is not changed by it	Core
5.1.2	Describe enzymes as proteins involved in all metabolic reactions, acting as biological catalysts	Core
5.1.3	Describe why enzymes are important: a reaction rate necessary to sustain life	Core
5.1.4	Describe enzyme action: active site complementary to the substrate, products formed	Core
5.1.5	Investigate and describe the effect of temperature and pH, with optimum temperature and denaturation	Core
5.1.6	Explain enzyme action: active site, enzyme-substrate complex, substrate, product	Extended only
5.1.7	Explain specificity: complementary shape and fit of active site and substrate	Extended only
5.1.8	Explain the effect of temperature: kinetic energy, shape and fit, frequency of effective collisions, denaturation	Extended only
5.1.9	Explain the effect of pH: shape and fit, denaturation	Extended only

Enzymes are proteins, so this topic builds on the biological molecules study guide. You will use it again in digestion: see the human nutrition study guide.

5.1.1–5.1.3 Catalysts and enzymes

A catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction. Because it is not changed, the same catalyst molecule can be used again and again.

Enzymes are proteins that act as biological catalysts. They are involved in all metabolic reactions: the chemical reactions that happen in living organisms, such as respiration, photosynthesis and building large molecules from small ones.

Why do organisms need them? At the temperatures found in living things, most metabolic reactions would happen far too slowly without a catalyst. Enzymes speed these reactions up to a rate necessary to sustain life. Without enzymes, for example, digestion would not release nutrients fast enough to keep you alive.

5.1.4 How an enzyme works – Core description

Each enzyme has a region called the active site. The molecule the enzyme acts on is its substrate.

• The shape of the active site is complementary to the shape of the substrate.
• The substrate fits into the active site.
• The reaction takes place and products are formed.
• The products leave, and the enzyme is unchanged.

Lipase is a good example. Its substrate is fat. Lipase breaks fats and oils down into fatty acids and glycerol, and the same lipase molecule can then act on another fat molecule.

5.1.6–5.1.7 Enzyme action and specificity – Extended only

Extended candidates must explain the process using four terms: active site, enzyme-substrate complex, substrate, product.

1. The substrate collides with the enzyme and fits into the active site, because their shapes are complementary.
2. An enzyme-substrate complex forms.
3. The substrate is converted into products.
4. The products are released from the active site. The enzyme is unchanged and can bind another substrate molecule.

Specificity means each enzyme catalyses only one reaction, or one type of reaction. The reason is shape. Only a substrate with a shape complementary to the active site can fit and form an enzyme-substrate complex. Lipase, for example, has no effect on starch: starch has a different shape, does not fit lipase’s active site, and so no enzyme-substrate complex forms.

Use the word complementary, not “the same shape”. The substrate and active site fit together like a key in a lock.

5.1.5 Temperature and pH – Core description

Temperature

As temperature rises from a low value, enzyme activity increases. It reaches a maximum at the optimum temperature. Above the optimum, activity falls quickly. At high temperatures the enzyme is denatured: the shape of its active site changes, and it stops working.

Many human enzymes have an optimum near body temperature, about 37 °C. Enzymes from other organisms can have very different optimum temperatures.

Two points Core answers must get right:

• At low temperatures the enzyme is not denatured. It is working slowly, and it works faster again if warmed.
• Denaturation is caused by temperatures above the optimum. A heat-denatured enzyme does not start working again when it is cooled.

pH

Each enzyme has an optimum pH at which its activity is highest. Moving the pH away from the optimum, in either direction, reduces activity. Far from the optimum, the enzyme is denatured.

Different enzymes have different optimum pH values. Protease in the stomach (pepsin) works in acidic conditions. Protease in the small intestine (trypsin) works in alkaline conditions. The shape of the graph is a peak at the optimum, falling on both sides.

5.1.8–5.1.9 Explaining temperature and pH – Extended only

Temperature: build the answer in two halves

Below the optimum (rate rising):

• As temperature increases, enzyme and substrate molecules gain kinetic energy and move faster.
• There are more frequent effective collisions between enzyme and substrate molecules.
• More enzyme-substrate complexes form per unit time, so the rate increases.

Above the optimum (rate falling):

• The enzyme is denatured: the shape of the active site changes.
• The substrate no longer fits the active site; the shapes are no longer complementary.
• Fewer enzyme-substrate complexes form, so the rate falls, and eventually stops.

The syllabus lists four ideas: kinetic energy, shape and fit, frequency of effective collisions, and denaturation. A full answer on a temperature graph uses all four.

pH

A pH away from the optimum changes the shape of the active site. The substrate fits less well, or not at all, so fewer enzyme-substrate complexes form. At extreme pH values the enzyme is denatured. Note that kinetic energy is not part of the pH explanation.

Investigating enzyme activity

The practical papers expect you to plan, carry out and evaluate investigations like these.

Temperature: amylase and starch

1. Put drops of iodine solution in the wells of a spotting tile.
2. Place a tube of starch solution and a tube of amylase solution in a water bath at the chosen temperature for a few minutes, so both reach that temperature.
3. Mix them and start a stopwatch.
4. Every 30 seconds, transfer one drop of the mixture to a fresh well of iodine.
5. The end point is the first well where the iodine stays orange-brown: all the starch has been broken down.
6. Repeat at other temperatures.

Keep constant: volume and concentration of starch, volume and concentration of amylase, pH (for example with a buffer solution), and the sampling interval. Control: amylase that has been boiled and cooled, mixed with starch. The iodine should keep turning blue-black, showing that active enzyme is needed.

Worked example 1: rate from time

A student records the time for the starch to disappear.

Temperature / °C	Time / s	Rate = 1000 ÷ time (arbitrary units)
10	450	2.2
20	240	4.2
30	150	6.7
40	90	11.1
50	180	5.6
60	no end point within 600 s	0

How the rates were found, for 30 °C: 1000 ÷ 150 = 6.67, which rounds to 6.7.

Conclusion. The rate rises from 10 °C to 40 °C and then falls. The optimum is about 40 °C. At 60 °C the amylase was denatured, so no starch was broken down.

Evaluation. Samples every 30 s mean each time could be up to 30 s too long. Testing every 10 s would be more precise. The true optimum could be anywhere between 30 °C and 50 °C, so the next step is to test at smaller intervals in that range, for example every 2 °C.

pH: catalase and hydrogen peroxide

Catalase breaks down hydrogen peroxide into water and oxygen. You can measure the volume of oxygen collected in a gas syringe, or in an upturned measuring cylinder full of water, over a fixed time. Buffer solutions set the pH. Keep temperature, volumes and concentrations constant. Wear eye protection: hydrogen peroxide is an irritant.

Worked example 2: rate from volume

Volumes of oxygen collected in 2 minutes:

pH	Volume of oxygen / cm³	Rate / cm³ per minute
5	6.0	3.0
6	11.0	5.5
7	18.0	9.0
8	13.0	6.5
9	5.0	2.5

Rate = volume ÷ time. At pH 6: 11.0 ÷ 2 = 5.5 cm³ per minute.

In this data set the optimum is near pH 7. The rate at pH 7 is 9.0 ÷ 3.0 = 3 times the rate at pH 5.

Common errors

• Saying enzymes are “killed”. Enzymes are proteins, not living things; they are denatured.
• Saying the active site has “the same shape” as the substrate. It is complementary.
• Using denaturation to explain a slow rate at a low temperature.
• Saying the enzyme is “used up”. A catalyst is unchanged by the reaction.
• (Extended) Writing “more collisions” without “more frequent” or “effective”, and leaving out the enzyme-substrate complex.
• (Extended) Bringing kinetic energy into a pH answer.
• Reading the optimum from a table as the highest value tested without saying it is about that value.
• Calculating rate as time ÷ 1000 instead of 1000 ÷ time. A shorter time means a faster rate.

Next steps

• Condense this page with the enzymes revision notes.
• Test yourself with the enzymes practice questions.
• Revisit the biological molecules revision notes if protein structure feels shaky.
• Find your gaps with the free 0610 diagnostic: Core or Extended.

Official syllabus

Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028 (Version 3), Cambridge International. Topic 5, Enzymes, sub-topic 5.1.