Digestion and absorption
The gut, digestive enzymes, bile, and how the small intestine is adapted for absorption.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 7.2.1Describe the functions of the main regions of the alimentary canal.
- 7.2.2Describe the roles of digestive enzymes and bile, and the adaptations of the small intestine.
6 minute read
Digestion and absorption
Food molecules are far too large to cross a cell membrane. Digestion breaks them into small, soluble molecules that can be absorbed.
Two kinds of digestion
Physical (mechanical) digestion — chewing by the teeth, churning in the stomach, and emulsification by bile — breaks food into smaller pieces. Nothing chemical changes, but the surface area increases, which speeds up the next step. Chemical digestion uses enzymes to break the bonds within molecules.
The three enzyme groups
| Enzyme | Substrate | Products |
|---|---|---|
| Amylase | starch | maltose, then glucose |
| Protease (e.g. pepsin) | protein | amino acids |
| Lipase | fats | fatty acids + glycerol |
Bile
Bile is made in the liver, stored in the gall bladder, and released into the small intestine. It has two jobs, and neither is enzymatic:
- It emulsifies fats — breaking large fat droplets into many small ones, greatly increasing surface area for lipase to work on.
- It is alkaline, neutralising the acidic mixture arriving from the stomach so that intestinal enzymes work at their optimum pH.
Absorption in the small intestine
The small intestine is superbly adapted:
- Very long, giving time for absorption.
- Villi and microvilli give an enormous surface area.
- Villi have a very thin wall (one cell thick), giving a short diffusion distance.
- A rich blood supply carries absorbed molecules away, maintaining a steep concentration gradient.
- Each villus contains a lacteal for absorbing fatty acids and glycerol.
Think of it like this
Bile emulsifying fat is like breaking one big block of butter into a thousand crumbs. There is exactly as much butter, but vastly more surface for the enzyme to attack — which is why emulsification is physical, not chemical.
Worked examples
Method, step by step
Explain how bile increases the rate of fat digestion, even though it contains no enzymes.
- 1Bile emulsifies fats, breaking large droplets into many smaller droplets.
- 2This greatly increases the total surface area of fat exposed.
- 3Lipase can then act on a much larger surface, so digestion is faster.
Bile emulsifies fat into small droplets, increasing the surface area available for lipase to act on, so fat is digested faster. Bile also neutralises stomach acid so lipase is at its optimum pH.
Common misconceptions
- Calling bile an enzyme. It contains no enzymes; it emulsifies physically and neutralises chemically.
- Saying bile "digests fat". Lipase digests fat; bile only increases the surface area so lipase works faster.
- Confusing where enzymes work. Pepsin needs acid (stomach, pH 2); most other enzymes need the alkaline small intestine.
In the exam
- A very common 3-mark question asks how the small intestine is adapted. Give surface area (villi/microvilli), thin wall (short diffusion distance) and good blood supply (maintains the gradient) — three distinct points.
- If asked the function of bile, always give both roles: emulsification and neutralisation. One alone rarely earns full marks.