Cambridge O Level Biology · Syllabus 5090 · Human Nutrition
Bile
What is Bile?
An alkaline liquid produced by the liver and stored in the gall bladder before being released into the duodenum. It contains no enzymes. Bile emulsifies lipids, breaking large fat droplets into many small ones, which greatly increases the total lipid surface area available to lipase and so increases the rate of lipid digestion.
This definition is part of the Human Nutrition chapter in Cambridge O Level Biology.
Common mistakes with Bile
- “Bile is an enzyme.” Why it is wrong Bile contains no enzyme, catalyses no reaction and breaks no chemical bond. Write instead "Bile is an alkaline liquid made by the liver. It contains no enzymes."
- “Bile chemically digests fat.” Why it is wrong Bile changes the size of the droplets, not the molecules. That is physical digestion by definition. Write instead "Bile emulsifies lipid, increasing its surface area so that lipase digests it faster. Lipase does the chemical digestion."
- “Enzymes are made where they act.” Why it is wrong Pancreatic amylase is made in the pancreas and acts in the small intestine; bile is made in the liver and acts in the duodenum. Questions ask for both columns precisely because they differ. Write instead "Amylase is produced by the salivary glands and the pancreas, and acts in the mouth and the small intestine."
Questions students ask about Bile
Why is bile not an enzyme?
Bile contains no enzymes, catalyses no reaction and breaks no chemical bonds. What it does is emulsify lipids: it breaks large fat droplets into many small ones. Because only the size of the droplets changes and the molecules themselves are unaltered, this is physical digestion. The chemical digestion of lipid is carried out by lipase, which breaks lipid molecules into fatty acids and glycerol.