Biology Chemistry Physics Controlled Assessments Long Answer Questions 11-14 Science Big Moments in Science Linkup


Animals eat food through the mouth (ingestion), break down the food (digestion) and release the waste products of digestion through the anus (egestion)

Digestion is the breakdown of large, insoluble food chunks into small, soluble nutrients. This is important so that the soluble nutrients can be absorbed into the blood and circulated to all body cells.

The digestive system (Alimentary canal) is a spceialised tube for the breakdown of food. The breakdown happens outside the cells in this massive tube.

There are 2 types of digestion that happen in the digestive system:

Physical (mechanical) digestion- break down of food into smaller chunks by the chewing action of teeth in the mouth. This:

  • makes the food small enough to swallow,
  • mixes it up with saliva and
  • increases the surface area for enzymes to act on it later.

Chemical digestion- breakdown of food using:

  • Enzymes- protein molecules that speed up chemical reactions- in the mouth, stomach and small intestine
  • Stomach acid- Hydrochloric acid in the stomach
  • Bile- A liquid released by the liver

Digestion of starch

Starch breakdown starts in the mouth- saliva contains the enzyme Amylase which breaks down starch to glucose.

Starch breakdown continues in the small intestine- The pancreas releases more Amylase to breakdown starch to glucose

Digestion of proteins

Protein digestion begins in the stomach, helped by hydrochloric acid. The acid kills germs taken in with food and activates enzymes that split proteins into amino acids. Protein digestion continues in the small intestine, helped by more protein digesting enzymes from the pancreas

Digestion of Fats and Oils

fat digestion starts in the small intestine. Bile from the liver is poured onto the fats, bile emulsifies fats (breaks them down to small droplets). This increases the surface area for enzymes to breakdown the fats into fatty acids and Glycerol.

Nutrient Broken down to...
Starch Sugar (glucose)
Protein Amino acids
Fats/oils Fatty acids and Glycerol



The end products of digestion (glucose, amino acids, fatty acids and glycerol) are absorbed (taken into the blood) in the last half of the small intestine (ileum).

This part is specialised for aborption in the following ways:

  • It is highly folded = large surface area
  • Has millions of finger like projections (villi and microvilli) on its inside = also for increasing the surface area
  • Has a dense network of capillaries supplying it with blood= to take away the nutrients to the rest of the body


Finally, water is absorbed from the waste in the large intestine. This leaves semi-solid waste (faeces) that are stored in the rectum until you can go to the toilet.