Lesson overview
Chemical digestion: Where does each enzyme break food down?
Hang two bags of starch in warm water, one with amylase, and test the water outside them for starch and sugar; then find the pH at which pepsin and trypsin clear cloudy egg white. Match each enzyme to its food, its product and its place along the gut.
Suggested sequence
- Lesson 1 · Through the wall of the gut. Chemical digestion: why large insoluble molecules must become small soluble ones, and amylase breaking starch into maltose. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Three enzymes, three foods. Amylase, protease and lipase: the food each one digests, its products, where it is made and where it works. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- Lesson 3 · Acid and alkali. Hydrochloric acid in the stomach, pepsin and trypsin at their optimum pH, and bile neutralising the acid. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 9 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 9 questions, 15 min, lesson closed.
Learning goals
Part 1: Through the wall of the gut
- Describe chemical digestion as the breakdown of large insoluble molecules into small soluble molecules (7.4.1)
- State the role of chemical digestion in producing small soluble molecules that can be absorbed (7.4.2)
- Supplement: describe the digestion of starch in the digestive system: amylase breaks down starch to maltose; maltase breaks down maltose to glucose on the membranes of the epithelium lining the small intestine (S7.4.6)
Part 2: Three enzymes, three foods
- Describe the functions of enzymes: amylase breaks down starch to simple reducing sugars, proteases break down protein to amino acids and lipase breaks down fats and oils to fatty acids and glycerol (7.4.3)
- State where, in the digestive system, amylase, protease and lipase are secreted and where they act (7.4.4)
Part 3: Acid and alkali
- Describe the functions of hydrochloric acid in gastric juice, limited to killing harmful microorganisms in food and providing an acidic pH for optimum enzyme activity (7.4.5)
- Supplement: describe the digestion of protein by proteases in the digestive system: pepsin breaks down protein in the acidic conditions of the stomach; trypsin breaks down protein in the alkaline conditions of the small intestine (S7.4.7)
- Supplement: explain that bile is an alkaline mixture that neutralises the acidic mixture of food and gastric juices entering the duodenum from the stomach, to provide a suitable pH for enzyme action (S7.4.8)
Misconceptions to listen for
- “Chemical digestion breaks food into smaller pieces.” — That is physical digestion, as in chewing: the pieces get smaller, but the molecules stay the same. Chemical digestion breaks the molecules themselves, so new, small soluble molecules are made.
- “Amylase got out of bag C and made starch in the water outside.” — Amylase breaks starch down; it never makes it. Amylase is also a protein, a large molecule, so it cannot get through whole tubing either.
- “Sugar got out of bag C and turned back into starch in the water.” — Nothing in the water outside could join sugar molecules back into long starch chains. The blue-black colour means starch itself was there.
- “Starch molecules are small enough to pass through the tubing.” — If they were, iodine would have turned blue-black outside bag A too, and it never did. Something must be different about bag C: a hole lets the large starch molecules out.
- “Glucose must be digested, because all food has to be digested.” — Glucose is already a small soluble molecule: it is what starch is digested into. It is absorbed as it is.
- “Vitamin C must be digested, because it is a nutrient.” — Being a nutrient does not decide it; size does. Vitamin C is a small soluble molecule, so it is absorbed without digestion.
- “Fat need not be digested, because it is a liquid at body temperature.” — Being a liquid is not what counts: fat molecules are large and do not dissolve in water. Lipase must break them into fatty acids and glycerol first.
- “Amylase breaks starch all the way down into glucose.” — Amylase stops at maltose, two glucose units joined together. Maltase, on the membranes of the epithelium lining the small intestine, splits maltose into glucose.
- “Maltase in the saliva splits maltose into glucose in the mouth.” — Maltase is not poured into the gut in a juice at all: it stays on the cell membranes of the epithelium lining the small intestine, so maltose is split only there.
- “Amylase passes out, because it is dissolved in the water.” — Being dissolved is not enough: a molecule must be small enough for the holes. Amylase is a protein, a large molecule, so it stays inside.
Scope
Chemical digestion as the breakdown of large insoluble molecules into small soluble molecules that can be absorbed (7.4.1, 7.4.2), through a Visking-tubing model gut; amylase, protease and lipase, their substrates, products and where they are secreted and act (7.4.3, 7.4.4); hydrochloric acid in gastric juice (7.4.5); Supplement: maltase on the epithelium of the small intestine (7.4.6), pepsin and trypsin at their optimum pH (7.4.7) and bile neutralising the acid entering the duodenum (7.4.8). Absorption and villi are in a later module, enzyme action and its graphs in the enzymes module. Results in the experiment come from a teaching model.