Biology · Both
Enzymes & Digestion
How enzymes work, factors affecting enzyme activity, and the main digestive enzymes and organs.
Enzymes are biological catalysts – proteins that speed up reactions in living cells without being used up. Digestion relies on enzymes breaking large food molecules into smaller ones that can be absorbed.
How enzymes work
Enzymes have an **active site** with a specific shape.
The **substrate** fits into the active site (lock and key model).
When the reaction is done, products leave and the enzyme can work again.
Enzymes are **specific** – each type usually works on one substrate.
Factors affecting enzymes
• **Temperature** – rate rises as temperature rises, until the enzyme denatures (active site shape changes) if too hot
• **pH** – each enzyme has an optimum pH; extremes denature the enzyme
• **Substrate concentration** – more substrate → faster rate until all active sites are busy
Optimum temperature for many human enzymes ≈ 37°C.
Stomach enzyme (pepsin) works best in acidic conditions; intestinal enzymes prefer nearer neutral/alkaline.
Worked example
Why does enzyme activity fall at high temperatures?
Solution
The enzyme is **denatured** – the active site changes shape so the substrate no longer fits.
Digestive enzymes
• **Amylase** – starch → sugars (mouth and small intestine)
• **Protease** (e.g. pepsin, trypsin) – proteins → amino acids (stomach and small intestine)
• **Lipase** – lipids (fats) → fatty acids and glycerol (small intestine)
Bile (from the liver, stored in gall bladder) emulsifies fats – increases surface area for lipase – and neutralises acid from the stomach.
Practice questions
Try these without looking at the answers first.
Q1.What does amylase break down?
Show answer
Starch (into sugars)
Q2.What happens to an enzyme when it is denatured?
Show answer
Active site shape changes / it no longer works
Q3.Where is bile produced?
Show answer
Liver