Cambridge IGCSE Biology 0610: Enzymes – Revision Notes

For full explanations and worked investigations, read the enzymes study guide first. These notes are for quick recall in the final weeks.

They cover topic 5, Enzymes (sub-topic 5.1), of the Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028. Outcomes 1 to 5 are Core. Outcomes 6 to 9 are Supplement content, so they are Extended only (Papers 2 and 4). Enzyme-rate investigations can also appear on Paper 5 and Paper 6.

Links: 0610 course hub · printable checklist · enzymes practice questions · biological molecules revision notes · diagnostic: Core or Extended.

Definitions – Core

Term	Meaning
Catalyst	A substance that increases the rate of a chemical reaction and is not changed by the reaction
Enzyme	A protein that functions as a biological catalyst; involved in all metabolic reactions
Active site	The region of an enzyme where the substrate fits
Substrate	The molecule an enzyme acts on
Product	The molecule(s) made by the reaction
Optimum	The temperature or pH at which the enzyme’s activity is highest
Denatured	The enzyme’s shape, including its active site, has changed so it no longer works

Why enzymes matter (5.1.3): without them, metabolic reactions would be too slow to sustain life.

Core vs Extended: what each tier writes

Idea	Core (describe)	Extended (explain)
Enzyme action	Active site shape is complementary to the substrate; products form	Substrate enters active site → enzyme-substrate complex → products released; enzyme unchanged
Specificity	(not required)	Only a substrate with a complementary shape fits the active site and forms a complex
Temperature	Activity rises to an optimum, then falls; high temperature denatures	Kinetic energy, frequency of effective collisions, shape and fit, denaturation
pH	Activity highest at the optimum pH; extremes denature	Shape of active site changes, substrate fits less well, denaturation

Answer templates – Extended

Enzyme action (four terms)

1. Substrate fits into the active site (complementary shapes).
2. Enzyme-substrate complex forms.
3. Substrate converted to products.
4. Products leave; enzyme unchanged, reused.

Temperature below the optimum

More kinetic energy → molecules move faster → more frequent effective collisions → more enzyme-substrate complexes per second → faster rate.

Temperature above the optimum

Enzyme denatured → active site changes shape → substrate no longer fits (not complementary) → fewer complexes → rate falls.

pH away from the optimum

Active site changes shape → substrate fits less well → fewer complexes → rate falls; extreme pH → denatured.

Graph shapes to recognise

Temperature                       pH
rate                              rate
 |        /\                       |       /\
 |       /  \                      |      /  \
 |      /    \                     |     /    \
 |    /       \                    |    /      \
 |__/__________\___ temp           |__/________\___ pH
         optimum:                         optimum:
   gradual rise, steep fall          falls on both sides

On a temperature graph, the fall after the optimum is usually steeper than the rise. Each enzyme has its own optimum pH: for example, pepsin works in the acidic stomach and trypsin in the alkaline small intestine.

Investigations – method in steps

Amylase and starch (temperature)

1. Iodine drops in a spotting tile.
2. Warm starch and amylase separately to the test temperature.
3. Mix, start timing.
4. Sample into iodine at fixed intervals.
5. End point: iodine stays orange-brown.
6. Rate = 1000 ÷ time.

Catalase and hydrogen peroxide (pH)

1. Buffer sets pH.
2. Collect oxygen in a gas syringe or upturned measuring cylinder.
3. Measure volume in a fixed time.
4. Rate = volume ÷ time.

Planning checklist

• Independent variable: temperature or pH (5 values, for example 20, 30, 40, 50, 60 °C).
• Dependent variable: time to end point, or volume of gas in a fixed time.
• Controlled variables: enzyme volume and concentration, substrate volume and concentration, pH (when changing temperature), temperature (when changing pH).
• Control: boiled and cooled enzyme.
• Reliability: repeat each value and calculate a mean; discard anomalous results.
• Precision: sample more often, or test smaller intervals near the optimum.

Worked reminders: rate calculations

• End point after 125 s: rate = 1000 ÷ 125 = 8.0 arbitrary units.
• 24 cm³ of oxygen in 3 minutes: rate = 24 ÷ 3 = 8.0 cm³ per minute.
• A shorter time to the end point means a faster rate.
• Round every calculated rate in a table to the same number of decimal places, for example 1 d.p.: 1000 ÷ 70 = 14.28…, so write 14.3.

Worked reminder: describing a graph with data

Suppose a lipase graph rises from 1.0 units at 10 °C to a peak of 7.5 units at 45 °C, then falls to 0 at 65 °C. A full “describe” answer:

1. Overall trend: as temperature increases from 10 °C to 45 °C, the rate increases.
2. Peak with values: the highest rate is 7.5 units at 45 °C, so the optimum is about 45 °C.
3. Second trend: above 45 °C the rate falls, reaching 0 at 65 °C.

Quote at least one pair of values. “It goes up then down” is too vague for more than one mark. Then, if asked to explain, switch to the templates above: kinetic energy and collisions for the rise, denaturation for the fall.

Evaluating an enzyme investigation

When a question asks for improvements or sources of error, choose points that fit the method given:

• End point is hard to judge (for example, a gradual iodine colour change): compare each well against a colour standard, or use a white tile under good light.
• Sampling interval is too long: test every 10 s instead of every 30 s.
• Temperature drifts: use a thermostatically controlled water bath, and check the temperature with a thermometer during the run.
• Gas escapes before collection: fit the bung quickly, or add the substrate from a syringe through the bung.
• Too few values near the optimum: add extra temperatures or pH values close to the peak.
• Only one result per value: repeat three times and calculate a mean.

Must-know distinctions

• Cold vs hot: cold enzymes are slow but not denatured; hot enzymes above the optimum are denatured.
• Complementary vs identical: the active site is complementary to the substrate, not the same shape.
• Denatured vs killed: enzymes are proteins, not organisms. Never write “killed” or “died”.
• Catalyst vs reactant: a catalyst is not used up; a reactant is.
• Temperature vs pH explanations (Extended): kinetic energy and collisions belong in temperature answers only.
• Rate vs time: high time = low rate.

Quick self-test

1. Define a catalyst.
2. What type of biological molecule is an enzyme?
3. Why do living organisms need enzymes?
4. State what happens to an enzyme at a temperature well above its optimum.
5. An amylase investigation reaches its end point in 40 s. Calculate the rate using 1000 ÷ time.
6. Catalase releases 24 cm³ of oxygen in 3 minutes. Calculate the rate in cm³ per minute.
7. Why is the rate low at 5 °C? (Core answer.)
8. (Extended) Use kinetic energy and collisions to explain why an enzyme works faster at 30 °C than at 15 °C.
9. (Extended) Explain why lipase does not break down starch.
10. (Extended) Name the structure formed when a substrate binds to the active site.
11. A catalase rate falls from 9.0 to 2.5 cm³ per minute when the pH changes. Calculate the percentage decrease, to 2 significant figures.
12. What is the purpose of a boiled-enzyme control?

Answers

1. A substance that increases the rate of a chemical reaction and is not changed by the reaction.
2. A protein.
3. They speed up metabolic reactions to a rate necessary to sustain life.
4. It is denatured: the active site changes shape and the substrate no longer fits.
5. 1000 ÷ 40 = 25 arbitrary units.
6. 24 ÷ 3 = 8.0 cm³ per minute.
7. The temperature is well below the optimum, so the enzyme works slowly. It is not denatured.
8. At 30 °C the molecules have more kinetic energy and move faster, so there are more frequent effective collisions between enzyme and substrate, and more enzyme-substrate complexes form each second.
9. Starch does not have a shape complementary to lipase’s active site, so it cannot fit and no enzyme-substrate complex forms.
10. An enzyme-substrate complex.
11. (9.0 − 2.5) ÷ 9.0 × 100 = 72.2…, so 72%.
12. It shows that the change only happens with active enzyme, so the enzyme caused the reaction.

Where marks are usually lost

• Writing “the enzyme dies” or “is killed” instead of “denatured”.
• Saying the active site and substrate are “the same shape” instead of complementary.
• Saying an enzyme is denatured at a low temperature.
• Giving the optimum as an exact value when the data only show it lies between two tested values.
• (Extended) Writing “more collisions” without “more frequent” or “effective”, or missing out the enzyme-substrate complex.
• (Extended) Using kinetic energy in a pH explanation.
• Describing a graph without quoting values from it when the question gives data.
• Not warming the enzyme and substrate separately before mixing, so the reaction starts at the wrong temperature.
• Inverting the rate: using time ÷ 1000, or saying a longer time means a faster reaction.
• Forgetting a control (boiled enzyme) when planning an investigation.

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.