Nutrition & Digestion
Structures and Functions in Living Organisms · 9 question types
Digestive enzymes fall into three groups, each one catalysing the breakdown of a different nutrient:
| Enzyme group | What it digests | Into | Where it is made |
|---|---|---|---|
| Carbohydrases (amylase, maltase) | Carbohydrates | Simple sugars (e.g. glucose) | Salivary glands, pancreas, small intestine |
| Proteases (pepsin, trypsin) | Proteins | Amino acids | Stomach, pancreas, small intestine |
| Lipases | Lipids | Fatty acids and glycerol | Pancreas, small intestine |
Carbohydrase chain: starch → glucose
Starch is broken down in two steps:
- Amylase (in saliva, then in pancreatic juice in the duodenum) breaks into maltose (a disaccharide made of two glucose molecules).
- Maltase (made by the small intestine) then breaks maltose into individual glucose molecules, which can be absorbed.
Note: the amylase in saliva gets denatured as soon as it enters the acidic stomach (pH 2 destroys its shape), so most starch digestion actually happens in the duodenum after pancreatic amylase is added.
Proteases
- Pepsin in the stomach (optimum pH 2) breaks proteins into shorter chains called polypeptides.
- Trypsin and other proteases from the pancreas and small intestine (optimum pH 8) then finish the job, breaking polypeptides into individual amino acids.
Lipases
Lipase from the pancreas works in the small intestine, breaking into fatty acids and glycerol. Lipase needs the lipids to be in tiny droplets first, which is the job of bile (next section).
Matching digestive enzymes to substrates and products
Question: Complete a table of digestive enzymes, substrates and products, state what starch or protein is digested into, or describe how protein is digested in the alimentary canal (1–4 marks).
Asked in 7 of the 23 papers. One mark per row: amylase digests starch to maltose; maltase digests maltose to glucose; protease digests protein (or polypeptides) to amino acids, with peptides accepted; lipase digests lipids to fatty acids and glycerol. Amylase makes maltose, not glucose. The schemes list fatty acids and glycerol as alternatives, so either earns the lipase row, but write both. Describing protein digestion for 3 marks wants the enzyme (protease, pepsin or trypsin), where it acts (stomach, then duodenum, from the pancreas) and the product (amino acids); absorption is ignored. The same facts answer harder settings: a blocked pancreatic duct means less digestion of all three food groups, and insulin is injected because, being a protein, it would be digested by protease.