Enzymes
What enzymes do
Biological catalysts — proteins that speed up reactions without being used up. Essential for digestion, respiration and synthesis.
Lock-and-key model
Each enzyme has a specific active site that fits one substrate, like a key in a lock — this makes enzymes specific.
Factors affecting rate
- Temperature – rate increases up to the optimum (~37 °C in humans). Too hot → the enzyme denatures (active site changes shape) and stops working.
- pH – each enzyme has an optimum pH; the wrong pH also denatures it (e.g. pepsin works in the stomach's acid, ~pH 2).
Digestive enzymes
| Enzyme | Made in | Breaks down | Into |
|---|---|---|---|
| Amylase (carbohydrase) | salivary glands, pancreas | starch | sugars |
| Protease | stomach, pancreas | proteins | amino acids |
| Lipase | pancreas | lipids | fatty acids + glycerol |
Bile (from the liver) emulsifies fat and neutralises stomach acid — it's not an enzyme.
Exam tip
"Denatured" means the active site changes shape so the substrate no longer fits — the enzyme is not "killed". Each enzyme is specific to one substrate.