Chemical digestion — Cambridge IGCSE Biology
How digestive enzymes break large insoluble molecules into small soluble ones, where they are made and act, and the digestion of starch and protein.
Revision notes
2 short notes, in the order of the specification. Each one 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.
Digesting starch and protein in detail
SupplementStarch is digested in two steps: amylase from the salivary glands and pancreas breaks starch down to maltose, then maltase on the membranes of the small intestine epithelium breaks maltose down to glucose. Protein is digested by proteases: pepsin works in the acidic stomach, and trypsin works in the alkaline small intestine.
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Quick check questions
What is chemical digestion?
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The breakdown of large insoluble molecules into small soluble molecules.
What does lipase break down fats and oils into?
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Fatty acids and glycerol.
Where is amylase secreted?
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Salivary glands and pancreas.
Supplement Which enzyme breaks maltose down to glucose, and where is it found?
Show answer
Maltase, on the membranes of the epithelium lining the small intestine.
Supplement Why does trypsin work in the small intestine but pepsin does not?
Show answer
Trypsin works in alkaline conditions and pepsin works in acidic conditions; bile makes the small intestine alkaline.
Frequently asked questions
What enzymes break down proteins?
Proteins are broken down into amino acids by enzymes called proteases. Proteases are secreted by the stomach and the pancreas, and they act in the stomach and the small intestine. Amino acids are small and soluble, so they can be absorbed into the blood. Learn the substrate, product and both places for each enzyme.
What enzymes digest carbohydrates?
Starch is digested by amylase, secreted by the salivary glands and the pancreas, which breaks it down to simple reducing sugars in the mouth and small intestine. The pancreas releases its amylase into the duodenum, so it is secreted there but acts in the small intestine. The small, soluble sugars can then be absorbed into the blood.
What enzymes does the pancreas produce?
The pancreas produces amylase, protease and lipase and releases them into the duodenum. Amylase breaks starch into simple reducing sugars, protease breaks protein into amino acids, and lipase breaks fats and oils into fatty acids and glycerol. The pancreas secretes these enzymes, but they act in the small intestine.
Why do large food molecules need to be digested?
Large food molecules need to be digested because they are insoluble and cannot pass through the wall of the gut into the blood. Chemical digestion by enzymes breaks them into small soluble molecules, such as simple reducing sugars, amino acids, fatty acids and glycerol. These small molecules can then be absorbed into the blood.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026