The stomach, acid and bileSpec B7.3.7
In short
Stomach acid kills microorganisms and gives protease its optimum pH; bile emulsifies fats and neutralises acid so enzymes in the small intestine work.
The stomach
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanHydrochloric acid in gastric juice has two functions:
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean- 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.
Quick check
Name the process: the removal of undigested food as faeces.
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Egestion.
Supplement 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.
Written and checked against the Cambridge IGCSE Combined Science (0653) specification · Updated October 2026