Three enzymes, three substrates
Chemical digestion uses enzymes to break large, insoluble molecules into small, soluble ones that can be absorbed across the wall of the small intestine. Amylase converts starch to maltose. Protease (pepsin) converts protein to amino acids. Lipase converts fat to fatty acids and glycerol. Each enzyme is specific — its active site is complementary in shape to one substrate only, so amylase cannot digest protein and lipase cannot digest starch.
Hydrochloric acid does two jobs
Hydrochloric acid (HCl) is secreted by the stomach wall, giving the stomach contents a pH of about 2. It kills harmful bacteria taken in with food, and it provides the optimum pH for pepsin, the stomach protease. Pepsin is the standard exception to the idea that enzymes work best at pH 7 — it is most active in strong acid, whereas the pancreatic enzymes acting in the small intestine prefer pH 7–8.
(Extended) Maltase and bile
Maltase is produced in, and acts in, the wall of the small intestine, converting maltose to glucose — small enough to be absorbed. Bile is made in the liver, stored in the gall bladder and released into the duodenum. Bile is alkaline: it neutralises the hydrochloric acid arriving from the stomach, raising the pH to about 7–8, the optimum for the pancreatic enzymes. Bile also emulsifies fat, which is physical digestion.
Drawn from real examiner reports.
Bile is not an enzyme
Bile catalyses nothing. It emulsifies fat — breaking large fat globules into small droplets, increasing the surface area available to lipase. That is physical digestion. Lipase is the enzyme that chemically digests fat to fatty acids and glycerol. Writing that bile digests fat, or that bile is an enzyme, scores zero.
Bile: candidates wrote "bile digests fat" or "bile is an enzyme" — bile is not an enzyme; it emulsifies fat (breaks it into smaller droplets to increase surface area). (s23 Paper 41 Q1b i)
Fat digestion gives two products
A fat (triglyceride) is a glycerol backbone carrying three fatty acid chains. Lipase releases both products — fatty acids and glycerol — and Cambridge awards a separate mark for each, so "fatty acids" alone loses a mark. The same precision applies to protein: the product is amino acids, and nothing else should be added.
Products of fat digestion: candidates gave "fatty acids only," omitting glycerol; full answer requires both fatty acids AND glycerol. (s23 Paper 33 Q3b i)
Amylase digests starch, not "carbohydrate"
"Carbohydrate" is too broad to earn the mark — maltose and glucose are carbohydrates too, and amylase digests neither. The required substrate is starch, a polysaccharide. The product is maltose, not glucose; glucose only appears once maltase breaks maltose down in the small intestine (Extended).
Substrate for amylase: "carbohydrate" given instead of the specific substrate starch. (s23 Paper 33 Q3b i)
Pepsin and trypsin get swapped
Both are proteases that digest protein to amino acids, but they work in different places. Pepsin is produced in the stomach wall and acts in the stomach, at pH about 2. Trypsin is produced in the pancreas and acts in the small intestine, at pH about 8. Examiners flag this swap in report after report.
Pepsin vs. trypsin: pepsin is secreted in the stomach, trypsin by the pancreas; these were swapped by many. (s22 Paper 41 Q2b; s23 Paper 41 Q1d)
Not every enzyme prefers pH 7
Pepsin has an optimum pH of about 2 — it needs the acidic stomach to work. The pancreatic enzymes acting in the small intestine work best at about pH 7–8. So "all enzymes work best at pH 7" is wrong, and a question asking why the stomach is acidic is asking about pepsin specifically, not about enzymes in general.
All enzymes work best at pH 7 — incorrect; pepsin has optimum pH ~2. (s23 Papers 11/21 Q9–10)
Maltose is digested in the small intestine
"The stomach" is the common wrong answer. Maltase is produced in, and acts in, the wall of the small intestine — that is where maltose becomes glucose. The stomach handles protein digestion by pepsin; no carbohydrate digestion happens there, because the acidic pH inactivates any swallowed salivary amylase.
Site of maltose digestion: "stomach" was a common wrong answer; maltose is digested in the small intestine by maltase. (s23 Paper 22 Q9; w22 Papers 21/22/23 Q14)
Physical digestion is not only in the mouth
Chewing in the mouth is one site, but the stomach churns food and bile emulsifies fat in the duodenum — both are physical digestion too. Be careful what physical digestion does, as well: it increases the surface area for enzymes to act on. Breaking large molecules into small ones is chemical digestion.
Physical digestion said to occur only in the mouth; and described as breaking large molecules into small ones rather than increasing surface area. (s23 Papers 11/21 Q14-15)
Enzyme tables: use five columns
Retrieve every row as enzyme → substrate → product → produced in → acts in. Amylase: starch → maltose, produced in the salivary glands and pancreas, acting in the mouth and small intestine. Site of production and site of action often differ, and either can be marked.
HCl questions carry two marks
"Explain the role of hydrochloric acid in the stomach" always wants both roles: it kills harmful bacteria in food, and it provides the optimum acidic pH for pepsin. One role scores half. Avoid "activates pepsin" — the mark-scheme wording is "provides the optimum pH".
Name the exact molecule
Vague nouns lose marks here. Write starch, not "carbohydrate". Write hydrochloric acid or HCl, never "acid", "lactic acid" or "amino acid". Write fatty acids and glycerol, not "fatty acids". Use the plural amino acids for the products of protein digestion.
Check whether it says chemical or physical
If the question says chemical digestion, your answer needs an enzyme breaking bonds to produce smaller molecules. If it says physical, the answer is chewing, churning or emulsification increasing surface area. Bile belongs in the physical column; lipase in the chemical one.
Chemical digestion is the breakdown of large, insoluble food molecules into small, soluble molecules using enzymes, so they can be absorbed across the wall of the small intestine.
This is different from physical digestion (e.g., chewing, stomach churning, bile emulsification), which increases the surface area of food but does NOT break chemical bonds or change the type of molecule.
Define chemical digestion.
Complete the table below.
| Enzyme | Substrate | Product(s) |
|---|---|---|
| Amylase | ||
| Pepsin | ||
| Lipase |
