How Does the Human Digestive System Work?

The human digestive system breaks large, insoluble food molecules into small, soluble molecules that can be absorbed into the bloodstream. It is an organ system: several organs work together to digest and absorb food.

The journey of food

Food is chewed in the mouth, increasing its surface area. It then travels down the oesophagus to the stomach, where it is mixed with digestive juices. Digestion continues in the small intestine, where the soluble products are absorbed into the blood. Water is absorbed from the remaining material in the large intestine.

Digestive enzymes

Enzymes are biological catalysts that speed up digestion. Each enzyme has an active site with a particular shape, so it catalyses a specific reaction:

• Carbohydrases break carbohydrates into simple sugars. Amylase breaks down starch.
• Proteases break proteins into amino acids.
• Lipases break lipids, or fats, into glycerol and fatty acids.

These products are small and soluble, allowing them to be absorbed. The body uses them to build new carbohydrates, lipids and proteins. Some glucose is used in respiration.

What does bile do?

Bile is made in the liver and stored in the gall bladder. It helps digestion in two ways:

1. It is alkaline, so it neutralises hydrochloric acid arriving from the stomach. This produces suitable conditions for enzymes in the small intestine.
2. It emulsifies fat into small droplets. This increases the fat's surface area, so lipase breaks it down more quickly.

Worked example: digesting starch

Starch in bread is broken down by amylase into simple sugars. These sugars are absorbed through the wall of the small intestine into the bloodstream.

Common exam mistake: Do not say that bile is an enzyme or that it chemically digests fat. Bile emulsifies fat; lipase digests it.

Scope assumed: AQA GCSE Biology 8461 Higher Tier, separate science. The digestive-system content explained here is also assessed at Foundation Tier and is not biology-only content.