The big idea: The food you eat is too large to enter your cells. The digestive system breaks big food molecules into small ones, then absorbs them into the blood.
A one-way journey
- Food travels along the gut in one direction: mouth → oesophagus → stomach → small intestine → large intestine.
- It is pushed along by peristalsis — waves of muscle contraction.
- Along the way, enzymes digest the food and the small intestine absorbs the products.
- Digestion
- The breakdown of large food molecules into small, soluble ones that can be absorbed.
- Absorption
- The movement of the small, soluble products of digestion out of the gut and into the blood (or lymph).
- Peristalsis
- Waves of muscle contraction in the gut wall that push food along the digestive tract.
- Enzyme
- A protein that speeds up a specific reaction — here, the breakdown of a particular food molecule.
- Villi
- Tiny finger-like folds lining the small intestine that greatly increase the surface area for absorption (one fold = a villus).
| Region of the gut | What happens there | Key feature to remember |
|---|---|---|
| Mouth and oesophagus | Food is chewed, then pushed down by peristalsis | Peristalsis = waves of muscle that move food along |
| Stomach | Stores food; secretes hydrochloric acid and protease (pepsin); begins protein digestion | Very low pH (acidic) — about pH 1.5–2 |
| Small intestine | Pancreatic and intestinal enzymes finish digestion; the products are absorbed | Long, folded, covered in villi — built for absorption |
| Large intestine | Reabsorbs water and mineral ions; forms and stores faeces | Mostly water reabsorption, no enzyme digestion |
Two jobs, two ends: Think of the gut as a tube with two jobs: break food down (digestion, mostly in the stomach and small intestine) and take the pieces in (absorption, mostly in the small intestine).
The large intestine does neither of these — it mainly reabsorbs water and forms faeces.
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Three things keep the gut working: peristalsis moves the food, stomach acid prepares it, and enzymes break it down. Each is controlled — the gut does not run at random.
Let's take them one at a time.
Peristalsis — and how it is controlled: Peristalsis is produced by circular muscle in the gut wall contracting behind the food and relaxing in front of it, so a wave squeezes the food along.
How peristalsis is controlled
- This muscle is smooth (involuntary) muscle, so peristalsis is not under conscious control.
- It is controlled by the autonomic nervous system — specifically the network of nerves in the gut wall (the enteric nervous system).
- Stretch caused by food triggers the nerves, which set off the muscle waves automatically.
Stomach acid — low pH on purpose: The stomach lining secretes hydrochloric acid (HCl), giving the stomach a very low pH (about 1.5–2). This low pH is kept on purpose.
Why the stomach keeps a low pH
- It kills most bacteria swallowed with food.
- It gives the stomach's protein-digesting enzyme, pepsin, its optimum pH — pepsin only works well in acidic conditions.
- The acid also denatures (unfolds) proteins, exposing more bonds for pepsin to attack.
- Acid secretion is switched on by nerves and the hormone gastrin when food enters the stomach.
| Question about stomach acid | Answer |
|---|---|
| What is it? | Hydrochloric acid (HCl), secreted by cells in the stomach lining |
| What controls its secretion? | Both nerves and the hormone gastrin trigger the cells to release acid when food arrives |
| Why keep the pH so low? | It kills most ingested bacteria AND gives the enzyme pepsin its optimum (acidic) pH |
| What does it do to protein? | The acid denatures (unfolds) proteins, and pepsin works best at this low pH to break them into shorter chains |
| What lowers acid secretion? | Antacids neutralise existing acid; proton-pump inhibitors (a class of drug) reduce how much acid is made |
Enzymes — and the pancreas: Most digestion is finished in the small intestine using enzymes from the pancreas. Here the pancreas is an exocrine gland: its exocrine cells secrete pancreatic fluid into the small intestine.
What pancreatic fluid contains
- Amylase — digests starch
- Protease — digests protein
- Lipase — digests fat
- An alkali (hydrogencarbonate) that neutralises the stomach acid
- Each enzyme breaks one food type into its small, absorbable products.
| Food molecule | Broken down by | Absorbable products |
|---|---|---|
| Starch (a carbohydrate) | Amylase (from salivary glands and pancreas) | Glucose (a small sugar) |
| Protein | Protease — e.g. pepsin in the stomach, then pancreatic protease | Amino acids |
| Triglyceride (a fat) | Lipase (from the pancreas) | Fatty acids and glycerol |
Stomach
- Acidic (low pH ≈ 1.5–2) from hydrochloric acid
- Acid kills bacteria and unfolds (denatures) proteins
- Enzyme pepsin (a protease) begins protein digestion
- Acid + enzyme secretion triggered by nerves and gastrin
Small intestine
- Alkaline — pancreatic fluid neutralises the acid
- Pancreatic enzymes: amylase, protease, lipase
- Digestion is finished here
- Products are absorbed through the villi
A memory hook: Peristalsis = muscle waves (involuntary, nerve-controlled). Stomach = acid + pepsin (protein). Pancreas + small intestine = the rest of the enzymes, then absorption.
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How this is tested: Digestion is tested on Paper 1A, Paper 3 and Paper 2, from one-mark recall up to Explain questions.
The question types
- Paper 1A, 1 mark: describe the control of peristalsis or identify the type and source of its nervous control. The answer is involuntary smooth muscle, controlled by the autonomic / enteric nervous system.
- Paper 3, Describe or Outline: how gastric acid secretion is controlled, why the stomach keeps a low pH, or name an enzyme secreted by the exocrine pancreas.
- Paper 2 Explain (a favourite): how the small intestine is adapted to absorb digestion products. Give villi (surface area), a thin wall (short diffusion distance) and a rich blood supply (steep gradient).
IB-style question — explain the adaptations for absorption
Explain how the structure of the small intestine is adapted to absorb the products of digestion. [3]
How to score all three marks
- Large surface area. The lining is folded into many villi (and microvilli), which greatly increase the surface area available for absorption.
- Short diffusion distance. Each villus has a thin, one-cell-thick wall, so absorbed molecules only have to cross a short distance to reach the blood.
- Steep concentration gradient. A dense network of blood capillaries quickly carries absorbed glucose and amino acids away, keeping a steep concentration gradient so absorption continues. (Award 1 mark per distinct structure-linked-to-function point, max 3.)
Final answer
Villi give a large surface area; the thin wall gives a short diffusion distance; and a rich blood supply maintains a steep concentration gradient — together these maximise absorption.
Why this scores full marks: Each point links a structure to its function ('villi → surface area', 'thin wall → short distance', 'capillaries → steep gradient').
An 'Explain' loses marks if you just list the structures without saying how each one helps absorption.
| Adaptation of the small intestine | How it helps absorption |
|---|---|
| Villi (and tiny microvilli) | Hugely increase the surface area for absorbing nutrients |
| Thin (one-cell) wall lining the villus | Short diffusion distance — nutrients cross quickly into the blood |
| Dense network of blood capillaries | Carries absorbed glucose and amino acids away, keeping a steep concentration gradient |
| A lacteal (small lymph vessel) inside each villus | Absorbs the products of fat (triglyceride) digestion |
| Very long, folded intestine | Provides time and surface area for digestion to finish and absorption to occur |