Starch, proteins and fats are far too large to cross the wall of the gut. Digestive enzymes, made in glands along the alimentary canal, break them into small soluble molecules that can be absorbed. Where each enzyme works depends on pH, and the stomach and small intestine have very different pH values. Maltase, pepsin, trypsin and the neutralising role of bile are Extended content.
Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules, which can then be absorbed. 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. Hydrochloric acid in gastric juice kills harmful microorganisms in food and provides an acidic pH for the stomach protease.
Choose a region of the gut in the sim and press Run digestion. In the mouth, green starch chains are cut into pairs (maltose); in the stomach, protein bead chains are cut into shorter chains; in the duodenum fats are broken down too. Watch the pH and ENZYMES HERE readouts change.
| Enzyme | Substrate | Products |
|---|---|---|
| Amylase | starch | simple reducing sugars (maltose) |
| Proteases | protein | amino acids |
| Lipase | fats and oils | fatty acids and glycerol |
Say "proteases break down protein to amino acids", not "to peptides" or "to glucose". And amylase produces simple reducing sugars (maltose), not glucose directly.
An enzyme is often secreted in one place and acts in another: pancreatic juice is made in the pancreas but works in the duodenum. Step through mouth, stomach, duodenum and ileum in the sim and note the pH of each.
| Enzyme | Secreted by | Acts in |
|---|---|---|
| Amylase | salivary glands; pancreas | mouth; duodenum (small intestine) |
| Protease | stomach wall; pancreas | stomach; duodenum (small intestine) |
| Lipase | pancreas | duodenum (small intestine) |
Hydrochloric acid is not an enzyme and does not digest protein itself. Its jobs are to kill microorganisms and to provide the optimum pH for the protease.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the sim, what pH is shown for the stomach?
Choose the stomach and press Run digestion. What happens to the pink rods (microorganisms)?
Which enzymes does the sim list for the duodenum?
In the mouth, what does amylase turn the green starch chains into?
Extended. Choose the ileum in the sim: maltase sits on the membranes of the cells lining the small intestine and turns maltose pairs into single glucose units, which are absorbed straight away.
Pepsin and trypsin are both proteases, but each has a different optimum pH. Pepsin would be denatured or inactive in the alkaline duodenum, and trypsin in the acid stomach.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Which describes chemical digestion?
Which enzyme is secreted by the pancreas and acts only in the small intestine?
Extended: what is the role of bile in chemical digestion?
Extended: where does maltase break down maltose to glucose?
This skill is now recorded on your profile. Keep the chain going.