Enzymes and Digestion — AQA GCSE Biology
Enzymes are biological catalysts that speed up reactions, including the breakdown of food in digestion.
What are enzymes?
Enzymes are proteins that act as biological catalysts — they speed up reactions without being used up. Each enzyme has a specific shape with an active site that fits only a particular substrate — the "lock and key" model.
Factors affecting enzyme activity
- Temperature — rate increases with temperature up to an optimum (around 37 °C in humans). Too hot and the active site changes shape — the enzyme denatures and stops working.
- pH — each enzyme has an optimum pH. The wrong pH also denatures the enzyme.
Note: a denatured enzyme is not "killed" — its active site has changed shape so the substrate no longer fits.
Digestive enzymes
| Enzyme | Breaks down | Into | Made in |
|---|---|---|---|
| Carbohydrase (e.g. amylase) | starch | sugars (glucose) | salivary glands, pancreas, small intestine |
| Protease | proteins | amino acids | stomach, pancreas, small intestine |
| Lipase | lipids | fatty acids + glycerol | pancreas, small intestine |
The small, soluble products are absorbed into the blood.
Bile
Bile (made in the liver) neutralises stomach acid and emulsifies fats, increasing the surface area for lipase to work.
Exam tips
- Describe the lock and key model and the role of the active site.
- Explain denaturing as a change in the active site's shape (from heat or wrong pH).
- Learn the three enzymes, what they break down and the products.
- Link bile to neutralising acid and emulsifying fats.