Chemical digestion and digestive enzymesSpec B7.3.2, B7.3.4, B7.3.5, B7.3.6
In short
Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules, carried out by enzymes, so they can be absorbed into the blood. Amylase breaks starch into simple reducing sugars, protease breaks protein into amino acids, and lipase breaks fats and oils into fatty acids and glycerol. Pancreatic enzymes act in the small intestine.
Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules. It is carried out by enzymes.
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanThe small soluble molecules made by chemical digestion can be absorbed into the blood. Large insoluble molecules cannot pass through the wall of the gut.
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean| Enzyme | Breaks down | Into | Secreted by (made in) | Acts in |
|---|---|---|---|---|
| Amylase | Starch | Simple reducing sugars | Salivary glands and pancreas | Mouth and small intestine |
| Protease | Protein | Amino acids | Stomach and pancreas | Stomach and small intestine |
| Lipase | Fats and oils | Fatty acids and glycerol | Pancreas | Small intestine |
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanSupplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanSupplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanSupplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanEnzymes made in the pancreas flow into the duodenum, so the pancreas is where they are secreted but the small intestine is where they act. Quote both places if a question asks where an enzyme is secreted and where it acts.
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanLearn each enzyme as a set of three: substrate, product, and where it is secreted and acts. Mark schemes ask for the product by name.
Quick check
What is chemical digestion?
Show answer
The breakdown of large insoluble molecules into small soluble molecules.
Supplement What does lipase break down fats and oils into?
Show answer
Fatty acids and glycerol.
Supplement Where is amylase secreted?
Show answer
Salivary glands and pancreas.
Written and checked against the Cambridge IGCSE Combined Science (0653) specification · Updated October 2026