Digestion is the breakdown of large food molecules into small, soluble molecules. It happens in two ways: physical (mechanical) digestion and chemical digestion by enzymes.
Physical digestion
Teeth cut and grind food; the stomach churns it; and bile emulsifies fats into tiny droplets. Physical digestion does not change the food chemically but increases its surface area so enzymes work faster.
Chemical digestion
Enzymes are biological catalysts that speed up the breakdown of food. Each enzyme is specific to one type of substrate.
| Site | Enzyme | Substrate → Product |
|---|---|---|
| Mouth | Salivary amylase | Starch → maltose |
| Stomach | Pepsin | Protein → polypeptides |
| Duodenum | Pancreatic lipase | Fats → fatty acids + glycerol |
| Ileum | Maltase | Maltose → glucose |
Other enzymes complete the job. In the duodenum, trypsin (a protease from the pancreas) breaks polypeptides into shorter peptides, and in the ileum peptidases finish this by producing amino acids. Sucrase and lactase in the ileum digest the sugars sucrose and lactose. In this way each large molecule is broken down step by step by a series of specific enzymes released from glands along the canal.
Key idea
The three final products of digestion are: carbohydrates → glucose; proteins → amino acids; lipids → fatty acids and glycerol.
Conditions for enzymes
Stomach enzymes such as pepsin work best in acidic conditions provided by hydrochloric acid. Enzymes in the small intestine work in slightly alkaline conditions, so bile and pancreatic juice neutralise the acid arriving from the stomach.
Example
Chewing bread releases salivary amylase, which begins turning starch into maltose. If you hold bread in your mouth, it slowly tastes sweeter as maltose forms.
Remember
Bile is not an enzyme — it emulsifies fat (physical), giving lipase a larger surface area to work on.