Enzymes
Enzymes control almost every chemical reaction in living organisms. This is a short topic with a small number of ideas, but it is examined heavily — partly in its own right and partly because it reappears in digestion, respiration, photosynthesis and biotechnology.
Almost all the marks depend on using four keywords precisely: active site, substrate, complementary, and denatured.
1. What an enzyme is
An enzyme is a protein that functions as a biological catalyst.
A catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction.
Two consequences follow, and both are examinable:
- Because enzymes are not used up, a small amount can catalyse a large number of reactions repeatedly.
- Because enzymes are proteins, they can be denatured — which is why temperature and pH matter so much.
Enzymes control metabolism — all the chemical reactions taking place in a living organism.
2. How enzymes work
Each enzyme has a region called the active site. The molecule it acts on is the substrate.
The sequence:
- The substrate has a shape complementary to the active site
- The substrate enters the active site, forming an enzyme–substrate complex
- The reaction takes place
- The products are released, and the enzyme is free to be used again
Enzymes are specific
Because the substrate must be complementary to the active site, each enzyme catalyses only one reaction or one type of reaction. This is the lock and key model — one key fits one lock.
So amylase breaks down starch and nothing else. If protease is added to starch, nothing happens, because starch is not complementary to protease’s active site, so no enzyme–substrate complex can form.
“Complementary” does not mean “the same shape.” Think of a jigsaw: two pieces fit together precisely, but their shapes are not identical — they are opposite. Writing that the substrate is “the same shape as the active site” is marked wrong.
Don’t stop at “not complementary.” If a question asks why an enzyme cannot break down a particular substance, follow the reasoning through: not complementary → cannot bind → no enzyme–substrate complex forms → no reaction. Each link can be a mark.
3. The effect of temperature
The graph rises to a peak, then falls sharply.
Below the optimum — as temperature increases, rate increases:
Molecules gain more kinetic energy → they move faster → more collisions between enzyme and substrate → more enzyme–substrate complexes form → the reaction goes faster.
At the optimum: the rate is at its maximum. For most human enzymes the optimum is about 37 °C — body temperature.
Above the optimum — the rate falls sharply:
The enzyme is denatured. The active site changes shape, so it is no longer complementary to the substrate, the substrate can no longer bind, and no enzyme–substrate complex forms.
“Kinetic energy” is the keyword below the optimum. Any temperature question is asking you to reach for it. Saying only “the enzyme works faster when it’s warmer” describes rather than explains.
Denatured does not mean “killed.” An enzyme is a molecule, not an organism. It is also not reversible — cooling a denatured enzyme does not restore it.
At low temperatures, the enzyme is not denatured — it is simply inactive or working slowly, because there is too little kinetic energy. Warming it up restores activity. This asymmetry is the single most common misunderstanding in the topic:
| Temperature | What happens | Reversible? |
|---|---|---|
| Too low | Enzyme inactive — few collisions | Yes |
| Optimum | Maximum rate | — |
| Too high | Enzyme denatured — active site changes shape | No |
4. The effect of pH
Each enzyme has an optimum pH at which it works fastest.
Moving away from the optimum in either direction — more acidic or more alkaline — denatures the enzyme: the active site changes shape and the substrate can no longer bind.
This is where pH differs from temperature. For temperature, only an increase past the optimum denatures the enzyme; a decrease merely slows it. For pH, both directions denature it.
Different enzymes have different optima, which suits where they work:
| Enzyme | Where it works | Approximate optimum pH |
|---|---|---|
| Pepsin (a protease) | Stomach — acidic | About pH 2 |
| Amylase | Mouth and small intestine | About pH 7 |
| Trypsin (a protease) | Small intestine — alkaline | About pH 8 |
5. The enzymes you should know
| Enzyme | Substrate | Products |
|---|---|---|
| Amylase | Starch | Maltose (a sugar) |
| Protease | Protein | Amino acids |
| Lipase | Fats and oils | Fatty acids and glycerol |
Note the naming convention: enzyme names usually end in -ase, and the first part usually names the substrate. Lactase acts on lactose; protease acts on protein; pectinase acts on pectin.
Named proteases worth knowing: pepsin in the stomach, trypsin in the small intestine.
6. Investigating enzymes
The standard experiment follows amylase breaking down starch, tested with iodine solution at intervals.
- Iodine turns blue-black while starch is still present
- When the mixture stops turning blue-black, the starch has been fully broken down
- The time taken for this is the measure of enzyme activity — shorter time means faster reaction
Typical variables:
| Independent | Temperature, or pH |
| Dependent | Time taken for the iodine to stop turning blue-black |
| Control | Concentration and volume of enzyme, concentration and volume of substrate, pH (if testing temperature), temperature (if testing pH) |
Method points that score: use a thermostatically controlled water bath to keep temperature constant; use a buffer solution to keep pH constant; repeat the experiment and identify anomalous results.
Read carefully whether the graph shows rate or time taken. They are inverses of each other, so the curves are opposite shapes. A short time means a fast rate. Misreading this reverses your whole answer.
7. Mistakes that cost marks
Saying the substrate is “the same shape” as the active site. It is complementary.
Saying the enzyme is “killed.” It is denatured.
Saying low temperature denatures an enzyme. Low temperature makes it inactive, and that is reversible.
Explaining a temperature increase without “kinetic energy.”
Saying denaturation is reversible. It is not.
Stopping at “the active site changes shape.” Continue: so the substrate can no longer bind and no enzyme–substrate complex forms.
Forgetting that both high and low pH denature.
Saying enzymes are used up. Catalysts are not changed by the reaction.
Saying enzymes are “made of protein” as though incidental. That they are proteins is exactly why they denature.
Frequently asked questions
What is an enzyme? A protein that acts as a biological catalyst, speeding up a reaction without being changed by it.
What is the active site? The region of the enzyme where the substrate binds. Its shape is complementary to the substrate.
What does denatured mean? The enzyme’s active site has changed shape, so the substrate can no longer bind. It is permanent.
Why do enzymes work faster as temperature increases, up to a point? Molecules gain kinetic energy, so there are more collisions and more enzyme–substrate complexes form.
What is the difference between an enzyme being inactive and being denatured? Inactive (too cold) is reversible — warm it and it works again. Denatured (too hot, or wrong pH) is permanent.
Why is each enzyme specific? Only a substrate complementary to the active site can bind, so each enzyme catalyses only one reaction.
Why does pepsin have a different optimum pH from trypsin? Pepsin works in the acidic stomach (about pH 2); trypsin works in the alkaline small intestine (about pH 8).
Quick revision checklist
- I can define an enzyme and a catalyst
- I know enzymes are proteins, and why that matters
- I can describe enzyme action using active site, substrate, complementary and enzyme–substrate complex
- I can explain the lock and key model and why enzymes are specific
- I can explain the effect of increasing temperature using kinetic energy and collisions
- I can explain denaturation as the active site changing shape so the substrate cannot bind
- I know low temperature is reversible and denaturation is not
- I know both high and low pH denature enzymes
- I can name amylase, protease and lipase with substrates and products
- I know pepsin and trypsin and their optimum pH values
- I can describe the iodine/starch/amylase investigation and its variables
- I can tell a rate graph from a time taken graph
These notes cover topic 5 of the Cambridge IGCSE Biology (0610) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across many one-to-one IGCSE Biology lessons, with particular attention to the errors students make most often and the wording examiners reward.
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