What are the main parts of the digestive system, and how are they involved in digestion?

The main parts of the digestive system are the...

- mouth

- oesophagus

- stomach

- liver

- gall bladder

- pancreas

- small intestine

- large intestine

There are three main enzymes involved in digestion:

Amylase:

- produced by the salivary glands in the mouth, the pancreas, and the small intestine

- catalyses the breakdown of starch into sugars

Lipase:

- produced in the pancreas and small intestine

- catalyses the breakdown of lipids (fats and oils) into fatty acids and glycerol

Protease:

- produced in the stomach, pancreas, and small intestine

- catalyses the breakdown of proteins into amino acids

The process of digestion is as follows:

In the mouth:

1. In the mouth, the salivary glands secrete salivary amylase. This enzyme begins to break down large starch molecules into smaller sugar molecules.

2. The food is swallowed, and travels down the oesophagus into the stomach.

In the stomach:  

3. The stomach produces hydrochloric acid. This establishes a low pH, which is the optimum pH for the activity of the protease enzymes. These enzymes catalyse the breakdown of proteins to amino acids.

The role of the liver, gall bladder, and pancreas:

4. The liver produces bile, which is stored in the gall bladder.
5. The pancreas produces pancreatic amylase, protease, and lipase.

In the small intestine:

5. Bile is released from the gall bladder into the small intestine. The bile neutralises the acid that was added to the food in the stomach, and establishes alkaline conditions in the small intestine, which allow the enzymes in the small intestine to work most effectively.
6. Amylase, protease, and lipase from the pancreas break down large, insoluble molecules into small, soluble molecules (see section on digestive enzymes).
7. The small, soluble molecules are absorbed by cells in the lining of the small intestine.

In the large intestine:
8. Water is absorbed from undigested food, to produce faeces.