The digestive system: organs, enzymes and absorption
TutorAura guide · Updated
The digestive system breaks food down into small, soluble molecules that can be absorbed into the blood and used by the body. It's an organ system: several organs working together. Food is broken down physically (chewing, churning) and chemically (by enzymes), then absorbed, mostly in the small intestine.
Key takeaways
- Mouth → oesophagus → stomach → small intestine → large intestine → rectum → anus.
- The liver, gall bladder and pancreas help but food doesn't pass through them.
- Enzymes turn large insoluble molecules into small soluble ones.
- Villi in the small intestine absorb the products of digestion.
The organs and their jobs
| Organ | Job |
|---|---|
| Mouth | Teeth break food into smaller pieces; salivary glands release amylase |
| Oesophagus | Muscular tube that moves food to the stomach |
| Stomach | Churns food; makes protease (pepsin) and hydrochloric acid |
| Liver | Makes bile |
| Gall bladder | Stores bile and releases it into the small intestine |
| Pancreas | Makes amylase, protease and lipase |
| Small intestine | Makes enzymes; absorbs soluble food molecules into the blood |
| Large intestine | Absorbs water from undigested food, forming faeces |
| Rectum and anus | Store and release faeces |
Why food must be digested
Starch, proteins and fats are large, insoluble molecules. They're too big to pass through the wall of the small intestine. Digestion breaks them into small, soluble molecules that can be absorbed:
- Starch → sugars (by amylase)
- Proteins → amino acids (by proteases)
- Lipids → fatty acids and glycerol (by lipase)
The body then uses these to build new carbohydrates, lipids and proteins, and some glucose is used in respiration.
Stomach acid and bile
Hydrochloric acid in the stomach kills many bacteria in food and gives the acidic pH that the stomach protease works best at.
Bile is alkaline. Released into the small intestine, it neutralises the acid from the stomach (enzymes there work best in slightly alkaline conditions) and emulsifies fat: it breaks large fat drops into small droplets, increasing the surface area for lipase to act on.
Villi: adapted for absorption
The small intestine is lined with millions of tiny finger-like projections called villi. They're adapted for fast absorption:
- They give a very large surface area, and their cells have microvilli that increase it further.
- Their walls are one cell thick, giving a short diffusion distance.
- They have a good blood supply, which carries absorbed molecules away and keeps the concentration gradient steep.
Absorption happens by diffusion and by active transport. See diffusion, osmosis and active transport.
Food tests
| Test for | Reagent | Positive result |
|---|---|---|
| Reducing sugars (e.g. glucose) | Benedict's solution, heated in a water bath | Blue → green, yellow or brick-red |
| Starch | Iodine solution | Orange-brown → blue-black |
| Protein | Biuret reagent | Blue → purple |
| Lipids | Ethanol, then water (emulsion test) | A cloudy white layer forms |
Some specifications use Sudan III, which stains lipids red. Wear eye protection, and take care with hot water and ethanol near flames.
How the system works together
Digestion shows the levels of organisation in biology: specialised cells (such as the cells lining the villi) form tissues (such as muscle and glandular tissue), which form organs (such as the stomach), which form the organ system. Muscular tissue churns food, glandular tissue releases enzymes and acid, and epithelial tissue lines the organs.
Common mistakes
- Saying food passes through the liver or pancreas (it doesn't; they release substances into the gut).
- Saying bile digests fat. It emulsifies fat; lipase digests it.
- Saying absorption happens in the stomach or large intestine (most food is absorbed in the small intestine; the large intestine absorbs water).
Practice questions
- Which organ makes bile, and where is it stored?
- Give two ways villi are adapted for absorption.
- What colour does Biuret reagent turn if protein is present?
- Why does the stomach contain hydrochloric acid?
Answers: 1. Made in the liver, stored in the gall bladder. 2. Any two of: large surface area, walls one cell thick, good blood supply, microvilli. 3. Purple. 4. To kill bacteria and to provide the acidic pH the stomach protease needs.
Frequently asked questions
What are the main organs of the digestive system?
The mouth, oesophagus, stomach, small intestine and large intestine, with the liver, gall bladder and pancreas releasing substances that help digestion.
Where is food absorbed?
Most digested food is absorbed in the small intestine, through the villi. The large intestine mainly absorbs water.
What does bile do?
Bile neutralises stomach acid and emulsifies fats into small droplets, giving lipase a larger surface area to work on. It isn't an enzyme.
Sources
- GCSE Biology (8461) specification: subject content. AQA.
- Cambridge IGCSE Biology 0610 syllabus for 2026, 2027 and 2028. Cambridge International.
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