Open the app
  1. Home
  2. Lessons
  3. GCSE & IGCSE Biology
  4. The human digestive system

The human digestive system

🎬 The doodle video for this lesson is coming soon. Subscribe on YouTube to see it first.

Digestion breaks large insoluble molecules into small soluble ones that can be absorbed into the blood.

The alimentary canal

Peristalsis and enzymes

Peristalsis: waves of muscle contraction in the gut wall push food along.
Amylase digests starch to maltose, and maltase digests maltose to glucose.
Proteases digest proteins to amino acids.
Lipases digest lipids to fatty acids and glycerol.

Bile and the villi

Bile is made in the liver and stored in the gall bladder. It is alkaline, so it neutralises stomach acid, and it emulsifies fats into small droplets, giving lipase a bigger surface area.
The small intestine is lined with villi: they give a huge surface area, have walls one cell thick, have microvilli, and contain a dense network of capillaries (and a lacteal that absorbs fatty acids and glycerol).
Worked example

Explain why emulsifying fat speeds up its digestion.

  1. Bile breaks large fat drops into many small droplets.
  2. Small droplets have a larger total surface area.
  3. So lipase can act on more fat at once.

Answer: Emulsifying increases the surface area of fat for lipase to work on.

Key idea

Food passes mouth, oesophagus, stomach, small intestine (duodenum and ileum), large intestine (colon and rectum). Amylase and maltase give glucose, proteases give amino acids, lipases give fatty acids and glycerol. Bile neutralises acid and emulsifies fat. Villi absorb digested food.

The interactive lesson includes the diagrams for this topic.

Check you have got it

Answer 6 quick questions with instant marking. If you get one wrong, GCSE-ready shows you why and gives you another go. It is free, and you do not need an account.

Practise this lesson free (6 questions)All GCSE & IGCSE Biology lessons