Curriculum fit: GCSE Biology and Combined Science (organisation: enzymes and digestion). US: high school life science (structure and function: proteins and digestion); also AP Biology.
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What Is It?
Enzymes are biological catalysts made of protein. They speed up the chemical reactions in living things without being used up. Each enzyme has an active site that fits a particular substrate, which is described by the lock and key model.
Digestive enzymes break large food molecules into small ones that can be absorbed into the blood.
Key Formulas and Ideas
- Amylase breaks starch into sugars. Protease breaks proteins into amino acids. Lipase breaks fats into fatty acids and glycerol.
- Enzyme activity depends on temperature and pH, and each enzyme has an optimum.
- Denaturing: a high temperature or the wrong pH changes the shape of the active site.
- Rate of reaction = amount of product or substrate changed ÷ time.
Worked Examples
Reading an Enzyme Graph
A graph of enzyme activity against temperature rises to a peak at 37 °C then falls sharply. Explain the fall.
- Above the optimum, the enzyme molecules gain too much energy.
- The active site changes shape (the enzyme denatures).
- The substrate no longer fits, so the rate falls.
✓ Answer The enzyme is denatured at high temperature
Calculating a Rate
An enzyme breaks down 12 cm³ of a substrate in 4 minutes. Find the mean rate.
- Rate = amount ÷ time.
- 12 ÷ 4 = 3.
✓ Answer 3 cm³ per minute
Bile
State two roles of bile in digestion.
- Bile neutralises stomach acid, giving alkaline conditions suitable for enzymes in the small intestine.
- It emulsifies fats into small droplets, increasing the surface area for lipase.
✓ Answer Neutralises acid and emulsifies fat
Common Mistakes to Avoid
- Saying enzymes are living or are 'killed' by heat. They are denatured.
- Thinking bile is an enzyme.
- Saying enzymes are used up in a reaction.
- Writing that low temperature denatures an enzyme (it only slows it).
Practice Questions
Frequently Asked Questions
Can a denatured enzyme work again?
Usually not. Once the active site has changed shape, the substrate no longer fits.
Where is bile made and stored?
It is made in the liver and stored in the gall bladder.
Keep Going With These Guides
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GCSEHigh SchoolAPRates of Reaction: Collision Theory and Factors That Affect It
Learn how temperature, concentration, surface area and catalysts change the rate of a reaction, and how to calculate a mean rate.
GCSEHigh SchoolDiffusion, Osmosis and Active Transport: Differences Explained
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GCSEHigh SchoolAPSources and Review
Reviewed by StudyMorrow editorial review on 5 Nov 2026.