Food, Nutrition, Digestion and Enzymes
What this domain tests
Food and nutrition questions require you to link biomolecule structure to function, calculate energy content, and explain digestion as a sequence of physical and chemical events controlled by enzymes.
Biomolecules and energy
- Carbohydrates: monosaccharides (glucose), disaccharides (maltose, sucrose), polysaccharides (starch, glycogen, cellulose). Cellulose is structural fibre and cannot be digested by humans.
- Lipids: glycerol plus fatty acids; the most energy-dense nutrient at roughly 38 kJ per gram, about twice carbohydrate or protein (about 17 kJ per gram).
- Proteins: amino acids joined by peptide bonds; used for growth and repair and as enzymes.
- Vitamins and minerals: vitamin C for collagen and immune function, vitamin D and calcium for bone, iron for haemoglobin, iodine for thyroid hormones. Water and fibre are also essential.
Digestion and enzymes
- Physical digestion: chewing and churning; bile emulsifies fat but is not an enzyme.
- Chemical digestion: salivary amylase converts starch to maltose; pepsin in the stomach works near pH 2; trypsin from the pancreas works in the alkaline small intestine; lipase produces fatty acids and glycerol; maltase completes starch digestion to glucose.
- Enzymes are specific, reusable proteins with an active site. They are denatured - not 'killed' - by high temperature or the wrong pH. Human optimum is around 37 °C.
- Absorption occurs through villi, which have a rich blood supply and a lacteal for fats.
Worked example
A 40 g cereal bar contains 20 g carbohydrate, 10 g fat and 5 g protein. Energy = (20 × 17) + (10 × 38) + (5 × 17) = 340 + 380 + 85 = 805 kJ. Reasoning: multiply each mass by its energy value and add; the fat supplies nearly half the energy although it is only a quarter of the mass.
Traps
Saying an enzyme is 'killed' by heat; treating bile as an enzyme; placing pepsin in alkaline conditions; forgetting a boiled-enzyme control; mixing up Benedict's and Biuret results.