Follow the food
Digestion breaks large food molecules into small soluble molecules that can be absorbed. Food passes through the mouth, oesophagus, stomach, small intestine, large intestine and rectum, then leaves through the anus.
Lesson 2.2 · AQA Combined Science
The digestive system
What each organ does
| Organ | Main job |
|---|---|
| Mouth and salivary glands | Chewing; saliva contains amylase. |
| Oesophagus | Muscular contractions move food to the stomach. |
| Stomach | Churns food; releases acid and protease. |
| Liver / gall bladder | Makes bile / stores bile. Bile enters the small intestine. |
| Pancreas | Makes amylase, proteases and lipases; releases them into the small intestine. |
| Small intestine | Digestion finishes; small soluble nutrients are absorbed. |
| Large intestine | Absorbs water from remaining material. |
| Rectum and anus | Store faeces, then release them. |
Absorption: a large surface
Villi are tiny projections lining the small intestine. They provide a large surface area. A thin surface gives a short diffusion path; a good blood supply carries absorbed nutrients away and maintains concentration gradients.
Food becomes useful molecules
Digested nutrients can build new carbohydrates, lipids and proteins. Some glucose is used in respiration. Undigested food is removed by egestion; this is different from excreting metabolic waste.
Remember
Digest food into small soluble molecules, then absorb them in the small intestine.
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