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Learn4BI1Paper 2B · Paper 2 (all content)Structure and functions in living organismsDigestion and absorption
Structure and functions in living organisms · Subtopic 18 of 21

Digestion and Absorption

Digestion breaks large insoluble molecules into small soluble ones that can be absorbed. Physical digestion (teeth chewing, stomach churning) increases surface area. Chemical digestion uses enzymes: amylase (mouth, pancreas) turns starch into maltose; protease (pepsin in stomach, trypsin from pancreas) turns proteins into amino acids; lipase (pancreas) turns fats into fatty acids and glycerol. Bile from the liver neutralises stomach acid and emulsifies fats. Products are absorbed in the small intestine, whose villi give a huge surface area; water is absorbed mainly in the small intestine and colon.

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 45 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Describe the stages and organs of the digestive system.
  • Name digestive enzymes, where they act and their products.
  • Describe the role of bile and stomach acid.
  • Explain how villi are adapted for absorption.
Pause and think

Why can't a starch molecule get into your blood directly?

Section 2 of 14

Core idea

Start hereA nine-metre journey

From mouth to anus, your gut is about nine metres long. Along the way, a sandwich is broken down into molecules small enough to slip into your blood.

The big idea

Physical digestion increases surface area; enzymes break large molecules into small soluble ones; villi absorb them into the blood and lymph.

1. The digestive enzymes

Enzymes and their products

Large insoluble → small soluble.

Worked exampleFollowing a cheese sandwich

Bread (starch), cheese (protein, fat).

Goal
Where is each digested?
Strategy
Follow the food through the gut.
  1. mouth: salivary amylase on starch\text{mouth: salivary amylase on starch}
    Starch → maltose begins.
  2. stomach: pepsin on protein (pH 2)\text{stomach: pepsin on protein (pH 2)}
    Acid kills bacteria, suits pepsin.
  3. small intestine: pancreatic amylase, trypsin, lipase; bile\text{small intestine: pancreatic amylase, trypsin, lipase; bile}
    Most digestion; bile neutralises and emulsifies.
AnswerGlucose, amino acids, fatty acids and glycerol absorbed in the ileum.
✓ CheckWater mainly absorbed later ✓.

2. Absorption

  • Villi and microvilli: huge surface area.
  • Wall one cell thick: short diffusion distance.
  • Capillaries: carry glucose and amino acids away, keeping the gradient steep.
  • Lacteal: absorbs fatty acids and glycerol.
Common misconceptionBile Bile isn't an enzyme. It emulsifies fat (more surface area for lipase) and neutralises stomach acid.
Pause and think

Why do people who've had their gall bladder removed sometimes struggle with fatty meals?

Summary

  • Amylase, protease, lipase.
  • Bile: neutralise, emulsify.
  • Villi absorb.
Key words
Emulsify
Break fat into small droplets.
Villus
Finger-like projection of the small intestine wall.
Assimilation
Absorbed food being used by cells.
HistoryA window into the stomach. In 1822 a gunshot wound left Alexis St Martin with a permanent hole into his stomach. Doctor William Beaumont studied digestion through it for years.
Fun factTennis court. Thanks to villi and microvilli, the inside surface of your small intestine is roughly the area of a badminton court.
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