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Detailed notes on Enzymes for Cambridge IGCSE Biology, covering key concepts, explanations, examples, and exam-focused revision points.
Biological catalysts. Lock-and-key specificity. Effects of temperature and pH. Denaturation at extremes. The basis of all metabolism.
Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).
Biological catalyst, made of protein, specific to one substrate.
Cambridge definition (memorise verbatim):
"An enzyme is a biological catalyst (made of protein) that speeds up a chemical reaction without being used up. Each enzyme is SPECIFIC to one substrate."
Three keywords Cambridge marks:
Why enzymes matter. Most metabolic reactions in cells would happen FAR too slowly without catalysts. Enzymes lower the activation energy → reactions proceed at body temperature. They make life possible.
Naming conventions. Most enzyme names end in -ase. Substrate name + -ase:
Worked qualitative. Why doesn't an enzyme catalyse multiple different reactions? Because of its specific 3D ACTIVE SITE — only one substrate fits. Different reactions need different enzymes.
Substrate fits the active site like a key fits a lock. One enzyme = one substrate.
The model.
Why this explains specificity. Only ONE substrate has the right shape to fit a particular active site. The enzyme can't catalyse other reactions because the substrates don't fit.
Worked qualitative. Why doesn't amylase digest protein? Amylase's active site is shaped to fit STARCH molecules. Protein has a completely different shape — won't fit the active site → no binding → no reaction.
More modern: induced-fit model. A small refinement: the active site CHANGES SHAPE slightly when the substrate binds, fitting more snugly (like a glove around a hand). Cambridge IGCSE accepts both lock-and-key and induced-fit; lock-and-key is the standard.
Cambridge tip. Always say the substrate "fits" the active site, NOT "fits the enzyme". The active site is the specific binding region.
Rate rises with T up to optimum (~37°C). Above optimum, DENATURATION sets in — irreversible.
Sketch the curve. A typical enzyme rate vs temperature graph:
Why the curve is asymmetric.
Worked qualitative. Why does refrigeration preserve food? At ~4°C, enzymes (in food and in microbes) work very slowly → spoilage happens slowly → food lasts longer. Take it out and warm up → enzymes work normally → spoilage resumes.
Cambridge tip. Always mention the BIPHASIC behaviour. Below optimum: kinetic energy explanation. Above: denaturation explanation. Both halves needed for full marks.
Each enzyme has an optimum pH. Outside it, the active site changes shape → denatures.
Each enzyme has its own optimum pH matching where it works in the body.
| Enzyme | Where it works | Optimum pH |
|---|---|---|
| Amylase (salivary) | Mouth | ~7 (neutral) |
| Pepsin | Stomach | 2 (very acidic) |
| Pancreatic amylase | Small intestine | ~8 (slightly alkaline) |
| Trypsin | Small intestine | ~8 |
| Lipase | Small intestine | ~8 |
Curve. Like the temperature graph but symmetric (or close to it).
Why pH affects enzymes.
Worked qualitative. Why does the stomach use acid? Two reasons:
Worked qualitative. Why does pepsin stop working when food enters the small intestine? The pancreas releases bicarbonate to neutralise the acid. pH rises to ~8. At pH 8, pepsin is denatured (or close to it). Different enzymes (trypsin, etc.) take over, optimised for the alkaline environment.
Cambridge tip. Don't say "enzymes work best at neutral pH". Different enzymes have different optima. Always SPECIFY the enzyme.
Biological detergents, brewing, food industry, medicine.
1. Biological washing powders / detergents.
2. Brewing.
3. Food industry.
4. Medical.
Cambridge tip. When asked "give a use of enzymes outside the body", pick a clear example with a short explanation. "Biological washing powders contain protease that digests blood-stains in clothes" is better than "in clothes washing".
Verbatim phrases and definitions Cambridge mark schemes credit.
Enzymes appear every Paper 4 (8-10 marks): define, lock-and-key, draw temperature curve, draw pH curve, give uses. Examiner reports flag 'enzymes are killed' (use 'denatured'), symmetric temperature curves (it's asymmetric), and saying ALL enzymes work at pH 7.
Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (5.1); 0610/42 May/Jun 2024 — Q7 (enzymes, temperature, pH); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.
Step-by-step solutions to past-paper-style questions on enzymes, written exactly the way a tutor would explain them at the board.
Question
Give the Cambridge definition of an enzyme.
Step-by-step solution
Step 1
A biological catalyst — it speeds up the rate of a chemical reaction and is not changed by the reaction.
Step 2
Made of protein.
Step 3
Functions in all metabolic reactions (the chemical reactions in cells).
Step 4
Specific — each enzyme catalyses one type of reaction.
Answer
An enzyme is a protein that acts as a biological catalyst, speeding up a metabolic reaction without being changed by it. Each enzyme is specific to one substrate.
Question
Describe how an enzyme acts on its substrate, using the lock-and-key model.
Step-by-step solution
Step 1
The enzyme has a region called the active site with a specific shape.
Step 2
The substrate has a shape complementary to the active site, so it fits in — like a key in a lock.
Step 3
An enzyme–substrate complex forms, and the reaction takes place.
Step 4
The products are released, and the unchanged enzyme is free to act on another substrate molecule.
Answer
The substrate fits into the enzyme's complementary active site, forming an enzyme–substrate complex; the reaction occurs and the products are released, leaving the enzyme unchanged to be used again.
Examiner tip
The marking points are: active site; complementary shape; enzyme–substrate complex; products formed; enzyme unchanged.
Question
Why does pepsin (a stomach enzyme) work best at pH 2 but amylase (a salivary enzyme) works best at about pH 7?
Step-by-step solution
Step 1
Each enzyme has an OPTIMUM pH at which its active site has the best shape and works fastest.
Step 2
Pepsin works in the stomach, where hydrochloric acid makes conditions very acidic (about pH 2), so its optimum is pH 2.
Step 3
Amylase works in the mouth, where the pH is near neutral (about pH 7), so its optimum is pH 7.
Step 4
Away from the optimum pH, the active site changes shape (denatures), so the substrate no longer fits and activity falls.
Answer
Each enzyme has an optimum pH that matches where it works in the body — pepsin pH 2 (stomach), amylase pH 7 (mouth). Outside the optimum, the active site changes shape and activity falls.
Question
Explain why each enzyme can catalyse only one type of reaction.
Step-by-step solution
Step 1
An enzyme's active site has a particular shape.
Step 2
Only a substrate with a shape complementary to the active site can fit into it.
Step 3
Other substrates have the wrong shape and cannot fit, so no enzyme–substrate complex forms with them.
Step 4
Therefore each enzyme catalyses only the one reaction whose substrate fits its active site — it is specific.
Answer
The active site has a specific shape, and only the one substrate with a complementary shape fits in to form an enzyme–substrate complex. Other substrates do not fit, so the enzyme is specific.
Examiner tip
The key phrase is 'complementary shape and fit of the active site with the substrate' (syllabus 5.1.7).
Question
Describe and explain how the activity of an enzyme changes as the temperature rises from 0 °C to 60 °C.
Step-by-step solution
Step 1
Below the optimum: as temperature rises, the enzyme and substrate molecules gain kinetic energy and move faster, so there are more frequent effective collisions between them — the rate increases.
Step 2
At the optimum (about 37 °C for human enzymes), the rate is at its maximum.
Step 3
Above the optimum: the high temperature breaks the bonds that hold the enzyme's shape, so the active site changes shape and is no longer complementary to the substrate.
Step 4
The substrate no longer fits, fewer enzyme–substrate complexes form, and the rate falls sharply. The enzyme is denatured — this is permanent (irreversible).
Answer
Up to the optimum, the rate rises because molecules gain kinetic energy and collide more often. Above the optimum, the active site loses its complementary shape (denatures), so the substrate no longer fits and the rate falls sharply and permanently.
Examiner tip
Both halves are needed: kinetic energy / effective collisions below the optimum, and denaturation / change of active-site shape above it. The curve is asymmetric.
Question
An enzyme breaks down starch. The time taken to digest the starch fully was: 20 °C → 200 s; 30 °C → 100 s; 40 °C → 60 s; 50 °C → no digestion even after 600 s. Calculate the rate at 40 °C and explain the result at 50 °C.
Step-by-step solution
Step 1
Rate =time1, so at 40 °C rate =601=0.017s−1 (or '1 reaction per 60 s').
Step 2
From 20 to 40 °C the time falls (200 → 60 s), so the rate increases — the molecules have more kinetic energy and collide more often.
Step 3
At 50 °C no digestion occurred at all, which is not just 'slow' — it means the enzyme has stopped working.
Step 4
The high temperature has denatured the enzyme: the active site has changed shape, so starch can no longer bind, and no product forms even after a long time.
Answer
Rate at 40 °C =1/60≈0.017s−1. The rate rises up to 40 °C (more kinetic energy, more collisions), but at 50 °C the enzyme is denatured — its active site has lost its shape — so no digestion occurs at all.
Examiner tip
'No reaction at all' signals denaturation, not merely a slower reaction. Use rate = 1/time and link the trend to kinetic energy then denaturation.
High-scoring sample answers for enzymes on the Cambridge IGCSE 0610 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
State what is meant by a catalyst. (1 mark)
Model answer
A catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction.
Why this scores
One mark for 'speeds up a reaction and is not used up/changed'. Both ideas help secure the mark.
State two factors that affect the activity of an enzyme. (2 marks)
Model answer
Temperature and pH.
Why this scores
Two marks for temperature and pH. (Substrate concentration is also acceptable.)
Using the lock-and-key model, explain why an enzyme is specific to one substrate. (3 marks)
Model answer
An enzyme has an active site with a particular shape. Only a substrate whose shape is complementary to the active site can fit into it to form an enzyme–substrate complex. Other substrates have a different shape and cannot fit, so the enzyme catalyses only the one reaction — it is specific.
Why this scores
Three marks: active site with a specific shape; only the complementary substrate fits / enzyme–substrate complex; other substrates do not fit, so it is specific.
Explain why the activity of an enzyme decreases at temperatures above its optimum. (4 marks)
Model answer
Above the optimum temperature, the extra heat energy breaks the bonds that hold the enzyme molecule in its three-dimensional shape. This causes the active site to change shape, so it is no longer complementary to the substrate. The substrate can no longer fit into the active site, so fewer (or no) enzyme–substrate complexes form and the rate of reaction falls. The enzyme is said to be denatured, and this change is permanent.
Why this scores
Four marks: bonds break / shape changes; active site no longer complementary; substrate cannot fit / fewer complexes; denatured (permanent). 'Killed' is not credited — enzymes are not alive.
Describe and explain how the rate of an enzyme-controlled reaction changes as the temperature increases from 0 °C to 60 °C. (5 marks)
Model answer
As the temperature increases from 0 °C towards the optimum, the rate increases, because the enzyme and substrate molecules gain kinetic energy and move faster, leading to more frequent collisions between them and so more enzyme–substrate complexes forming. At the optimum temperature (about 37 °C for human enzymes) the rate is at its maximum. Above the optimum, the rate falls sharply, because the heat breaks the bonds holding the enzyme's shape, so the active site changes shape and is no longer complementary to the substrate. By about 60 °C the enzyme is fully denatured and the reaction stops.
Why this scores
Five marks across: rate rises with temperature; due to kinetic energy / more collisions; optimum gives maximum rate; above optimum the active site changes shape; denatured so rate falls. Describing AND explaining is required.
Amylase breaks down starch. Describe an investigation to find the effect of pH on the activity of amylase. (6 marks)
Model answer
Set up a series of test tubes, each with the same volume and concentration of starch solution and the same volume of amylase. Add a different buffer solution to each tube to give a range of known pH values, for example pH 4, 6, 7, 8 and 10. Keep them all in a water bath at the same temperature. At regular time intervals, remove a drop from each tube and add it to iodine solution on a spotting tile: while starch is present the iodine stays blue-black, and when the starch has all been digested it stays orange-brown. Record the time taken for each tube to stop turning the iodine blue-black, and calculate the rate as 1÷time. To make it a fair test, only the pH is changed; the temperature, volumes and concentrations are kept constant, and each pH is repeated to find a mean. The pH giving the shortest time (fastest rate) is the optimum pH for amylase.
Why this scores
Up to 6 marks: range of pH using buffers; same volumes/concentrations of starch and amylase; constant temperature; using iodine to follow starch disappearance; measuring time / calculating rate; repeats and identifying the optimum. The iodine method is the expected way to follow the reaction.
Definitions to memorise and the exact keywords mark schemes credit for enzymes answers — sharpened from recent examiner reports for the 2026 0610 sitting.
A protein that acts as a biological catalyst, speeding up a metabolic reaction without being changed. Each enzyme is specific to one substrate.
The molecule that an enzyme acts on. The substrate fits into the enzyme's active site.
The region of an enzyme where the substrate binds. Has a specific shape complementary to one substrate.
The structure formed when a substrate fits into an enzyme's active site, where the reaction takes place.
The condition at which an enzyme works fastest.
Permanent change in the shape of an enzyme (especially the active site) caused by extreme heat or pH, so the substrate no longer fits.
The idea that an enzyme's active site has a specific shape that fits its substrate (like a lock and key). Explains enzyme specificity.
The traps other students keep falling into on enzymes questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.
Why it happens
Casual everyday wording.
How to avoid it
Enzymes aren't alive — they're proteins. They're DENATURED, not killed. Cambridge marks 'denatured'.
Why it happens
Bell curves look natural.
How to avoid it
The curve is ASYMMETRIC: a gentle rise to the optimum, then a steep DROP after. Cold doesn't denature; heat does.
Why it happens
Most lab examples (catalase, amylase) work near pH 7.
How to avoid it
Each enzyme has its OWN optimum pH. Pepsin pH 2 (stomach). Pancreatic amylase/trypsin about pH 8. Different enzymes, different optimum pHs.
Why it happens
Cold reduces activity to nearly zero.
How to avoid it
Cold INACTIVATES enzymes (slows them) but does NOT denature them. Warm them again and activity returns. Heat denaturation is IRREVERSIBLE.
The things students keep getting wrong in this sub-topic, answered.