The stomach, acid and bileSpec 7.3.6, 7.3.7, 7.4.5, 7.4.8
In short
The stomach churns food and mixes it with gastric juice containing protease and hydrochloric acid. The acid kills harmful microorganisms in food and provides the optimum pH for the stomach protease. Bile, made in the liver and stored in the gall bladder, emulsifies fats and neutralises the acidic mixture entering the duodenum.
The stomach
The wall of the stomach is muscular. It contracts and relaxes to churn the food. This is physical digestion: it breaks the food into smaller pieces and mixes it with gastric juice, which contains protease and hydrochloric acid.
Hydrochloric acid in gastric juice has two functions:
- It kills harmful microorganisms in food.
- It provides an acidic pH, which is the optimum pH for the protease enzyme in the stomach to work.
Bile
Bile is made in the liver, stored in the gall bladder and released into the duodenum.
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean- Bile emulsifies fats and oils. It breaks large drops into many tiny droplets, which increases the surface area for lipase to act on (physical digestion, so there is no chemical change).
- Bile is alkaline. It neutralises the acidic mixture of food and gastric juice that enters the duodenum from the stomach. This provides a suitable pH for the enzymes in the small intestine to work.
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanBile is not an enzyme and does not chemically digest fat. It emulsifies fat so that lipase can work faster.
Quick check
Name the process: the removal of undigested food as faeces.
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Egestion.
Which type of tooth is used for tearing food?
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Canine.
What part of a tooth contains nerves and blood vessels?
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The pulp.
Give two functions of hydrochloric acid in the stomach.
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It kills harmful microorganisms in food and provides an acidic pH for optimum enzyme activity.
Supplement How does bile help the digestion of fats?
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It emulsifies them, which increases the surface area for lipase.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026