BileSpec 4.2.2.1
In short
Bile is an alkaline substance made in the liver, stored in the gall bladder and released into the small intestine. It neutralises hydrochloric acid from the stomach and emulsifies fat into small droplets, which increases the surface area of the fat. Both effects increase the rate of fat breakdown by lipase. Bile is not an enzyme.
Bile is made in the liver and stored in the gall bladder. It is released into the small intestine.
Bile does two jobs:
- It is alkaline, so it neutralises hydrochloric acid that comes from the stomach. This gives the enzymes in the small intestine the alkaline conditions they work best in.
- It emulsifies fat, breaking it into small droplets. This increases the surface area of the fat.
The alkaline conditions and the large surface area increase the rate of fat breakdown by lipase.
Common mistake:Bile is not an enzyme and it does not chemically break down fat. It emulsifies fat so that lipase can work faster.
Written and checked against the AQA GCSE Combined Science (8464) specification · Updated October 2026
Frequently asked questions
Why do enzymes denature at high temperatures?
At high temperatures the shape of the enzyme's active site changes, so the substrate no longer fits and the reaction cannot be catalysed. The enzyme is denatured, and this cannot be changed back. Below the optimum, the rate rises with temperature because particles move faster and collide more often. Enzymes are not killed, because they are not alive.
How are alveoli adapted for gas exchange?
Millions of alveoli give a very large surface area for diffusion. Their walls are one cell thick, so the diffusion distance is short. A network of capillaries carries oxygen away and brings carbon dioxide, and constant ventilation brings fresh air. Both keep a steep concentration gradient, so oxygen diffuses into the blood and carbon dioxide diffuses out quickly.
Why is the left ventricle thicker than the right?
The left ventricle has a thicker muscular wall than the right because it pumps blood all the way around the body at a higher pressure. The right ventricle only pumps blood to the lungs. Both are part of the double circulatory system, in which blood passes through the heart twice on each complete circuit.
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