The Three Main Digestive Enzyme Groups
| Enzyme group | Substrate | Products | Produced by | Works in |
|---|---|---|---|---|
| Amylase (carbohydrase) | Starch | Maltose (then glucose) | Salivary glands, pancreas | Mouth, small intestine |
| Protease | Proteins | Amino acids | Stomach (pepsin), pancreas (trypsin) | Stomach, small intestine |
| Lipase | Lipids (fats/oils) | Glycerol + fatty acids | Pancreas | Small intestine |
Supplement: additional enzymes:
- Maltase: produced by the wall of the small intestine. Breaks maltose → glucose.
- Sucrase: breaks sucrose → glucose + fructose.
- Lactase: breaks lactose → glucose + galactose.
The Role of Bile
Bile is NOT an enzyme: it is a green-yellow alkaline fluid produced by the liver, stored in the gall bladder, and released into the duodenum (first part of the small intestine).
Bile has two functions:
- Emulsification of fats: bile breaks large fat droplets into many smaller droplets, increasing the surface area for lipase to act on. This speeds up fat digestion.
- Neutralisation of stomach acid: bile is alkaline and neutralises the hydrochloric acid from the stomach, creating the slightly alkaline pH (7–8) needed for pancreatic enzymes to work.
Key point: Emulsification is a physical change, not a chemical one: bile does not chemically break down fat molecules.
Why Digestion Is Necessary
Large food molecules must be digested because:
- They are too large to pass through the wall of the small intestine into the blood.
- They are insoluble: they cannot dissolve in blood plasma for transport.
- Digestion breaks them into small, soluble molecules (glucose, amino acids, fatty acids, glycerol) that can be absorbed and transported.
Types of digestion:
- Mechanical digestion: physical breakdown (teeth chewing, stomach churning) increases surface area for enzymes.
- Chemical digestion: enzymes hydrolyse bonds in food molecules, breaking them into smaller molecules.