Chemical digestion by enzymes
Large, insoluble food molecules must be broken into small, soluble ones that can be absorbed. This is done by digestive enzymes:
Key idea
- Amylase breaks starch → maltose.
- Protease breaks proteins → amino acids.
- Lipase breaks fats → fatty acids + glycerol.
The role of bile
Bile is made in the liver and released into the duodenum. It has two jobs:
- It is alkaline, so it neutralises the acidic mixture from the stomach, giving the right pH for enzymes in the small intestine.
- It emulsifies fats — breaking large fat droplets into many small ones. This gives a larger surface area for lipase to work on, speeding up fat digestion.
Worked example
Pepsin (a protease in the stomach) works best at about pH 2. Why does the stomach contain hydrochloric acid? Answer: the acid gives the low pH (the optimum) at which pepsin is most active, so protein is digested efficiently.
Absorption
Digested food is absorbed in the small intestine, whose inner surface is folded into millions of villi. Villi are adapted for absorption because they:
- give a very large surface area;
- have a thin wall (one cell thick) for a short diffusion distance;
- have a rich network of blood capillaries and a lacteal.
Glucose and amino acids pass into the blood capillaries; fatty acids and glycerol pass into the lacteal.
Remember
- Bile is not an enzyme — it emulsifies fat and neutralises acid.
- Villi = large surface area + thin wall + good blood supply.