The big idea: The food you eat is too large to enter your cells. The job of the digestive system is to break big food molecules into small ones, then absorb them into the blood.
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.
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.
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 for two reasons:
(1) it kills most bacteria swallowed with food, and
(2) 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.
The pancreas is an exocrine gland here: its exocrine cells secrete pancreatic fluid into the small intestine. This fluid contains digestive enzymes — amylase (digests starch), protease (digests protein) and lipase (digests fat) — plus 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: On Paper 1A a one-mark question often asks you to 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.
On Paper 3 a Describe or Outline question can ask how gastric acid secretion is controlled, why the stomach keeps a low pH, or to name an enzyme secreted by the exocrine pancreas.
A favourite Paper 2 Explain asks 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 |