Food is made up of large, insoluble molecules — starch, proteins and fats — that are too big to pass through the wall of the small intestine into the blood.
Digestion breaks these large molecules into small, soluble ones that CAN be absorbed.
There are two types of digestion:
MECHANICAL DIGESTION — physical breakdown (chewing, churning). Increases surface area for enzymes.
CHEMICAL DIGESTION — enzymes break chemical bonds to produce smaller molecules.
The Mouth
Digestion begins in the mouth.
Teeth physically grind food into smaller pieces — increasing surface area for enzymes to work on.
Salivary glands produce SALIVA, which contains:
Amylase — begins the chemical digestion of starch → maltose (a sugar).
Mucus — lubricates food to make swallowing easier.
The tongue shapes food into a BOLUS (a soft ball) that is swallowed.
The OESOPHAGUS is a muscular tube that carries food from the mouth to the stomach using waves of muscle contractions called PERISTALSIS.
The Stomach
The stomach is a muscular bag that churns food (mechanical digestion) and carries out chemical digestion.
Glandular tissue in the stomach wall produces GASTRIC JUICE, which contains:
Hydrochloric acid (HCl) — makes the stomach very acidic (pH ~2). This KILLS most bacteria in food and provides the optimum pH for the enzyme pepsin.
Pepsin (a PROTEASE enzyme) — breaks proteins into shorter chains of amino acids.
Food stays in the stomach for several hours, slowly being mixed into a liquid called CHYME.
Chyme is released in small amounts into the small intestine through the pyloric sphincter.
The Small Intestine — Digestion and Absorption
The small intestine is the main site of both DIGESTION and ABSORPTION.
The PANCREAS produces digestive enzymes that are released into the small intestine:
The LIVER produces BILE, which is stored in the GALL BLADDER and released into the small intestine. Bile:
Emulsifies fats — breaks large fat droplets into many small ones, increasing surface area for lipase.
Is alkaline — neutralises the stomach acid, creating the correct pH (neutral/alkaline) for pancreatic enzymes to work.
ABSORPTION — digested molecules (glucose, amino acids, fatty acids, glycerol) pass through the wall of the small intestine into the blood.
The small intestine is adapted for absorption with VILLI — finger-like folds that massively increase surface area. Villi have:
A large surface area for fast absorption.
Thin walls — only one cell thick — so the diffusion distance is very short.
A rich blood supply — maintains the concentration gradient by constantly removing absorbed molecules.
The Large Intestine, Rectum and Anus
By the time food reaches the large intestine, most nutrients have already been absorbed.
The LARGE INTESTINE absorbs water from the remaining undigested material.
Too little water absorbed → diarrhoea (watery faeces).
Too much water absorbed → constipation (hard, dry faeces).
The RECTUM stores faeces (made of undigested fibre, dead cells, bacteria and bile pigments which give it the brown colour).
Faeces are expelled through the ANUS.
⚠️ Common Mistake
Bile is NOT an enzyme and does NOT chemically digest fats. It EMULSIFIES them — a physical process that breaks large fat droplets into smaller ones to give lipase more surface area to work on. Students often say 'bile digests fat' — it does not. LIPASE chemically digests fat.
📌 Key Note
Mouth: amylase digests starch. Stomach: pepsin digests proteins, HCl creates pH 2. Small intestine: all three enzymes from pancreas + bile from liver. Large intestine: absorbs water.
🎯 Matching Activity — Match the Organ to its Role in Digestion
Match each organ to what it does. — drag the symbols on the right to match the component names on the left.
Mouth
Drop here
Stomach
Drop here
Pancreas
Drop here
Liver
Drop here
Small intestine
Drop here
Large intestine
Drop here
Absorbs water from undigested material — faeces formed
HCl + pepsin — proteins digested — chyme produced
Produces bile — stored in gall bladder — emulsifies fats in small intestine
Produces amylase, protease and lipase — released into small intestine
Main site of digestion and absorption — villi absorb nutrients
Bile emulsifies fats — breaking large droplets into smaller ones, greatly increasing the surface area for lipase. The alkaline conditions created by bile neutralising stomach acid provide the optimum pH for pancreatic enzymes. Villi and microvilli on the intestinal lining further maximise surface area for absorption. Students should be able to explain how the small intestine is adapted for efficient absorption (large surface area, thin walls, rich blood supply).
🧪 Required Practical
🔬 RP4 — Food tests: iodine solution tests for starch (blue-black = positive), Benedict's solution tests for glucose (brick red = positive), Biuret reagent tests for protein (purple = positive), ethanol emulsion test for fat (cloudy white = positive).
Know the method, variables, equipment and how to analyse results.
🎯 Test Yourself
Question 1 of 5
1. What is the role of bile in digestion?
2. Why does the small intestine have villi?
3. Where are proteins first chemically digested?
4. What does the large intestine absorb?
5. Which enzyme is produced by the salivary glands?
⭐ How Well Do You Understand This Topic?
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