Cambridge O Level Biology · Syllabus 5090 · Human Nutrition
Chemical Digestion
What is Chemical Digestion?
The breakdown of large food molecules into small soluble molecules by enzyme-controlled reactions. Amylase, maltase, protease and lipase each act on one kind of substrate, breaking the bonds that hold the large molecule together so that the products are small enough and soluble enough to be absorbed through the wall of the intestine.
This definition is part of the Human Nutrition chapter in Cambridge O Level Biology.
Common mistakes with Chemical Digestion
- “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."
Questions students ask about Chemical Digestion
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.