Cambridge O Level Biology · Syllabus 5090 · Human Nutrition
Physical Digestion
What is Physical Digestion?
The breakdown of food into smaller pieces without any chemical change to the food molecules themselves. It is carried out by chewing in the mouth, by the churning of the stomach and by the emulsification of lipid droplets by bile, and its purpose is to increase the surface area of the food that is available to digestive enzymes.
This definition is part of the Human Nutrition chapter in Cambridge O Level Biology.
Common mistakes with Physical Digestion
- “Physical digestion changes the food molecules.” Why it is wrong Chewing a piece of bread leaves the starch molecules in it entirely unchanged. Only the size of the pieces has changed. Write instead "Physical digestion breaks food into smaller pieces without changing the food molecules, increasing the surface area for 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."
Questions students ask about Physical 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.