Nutrition, digestion and enzymes
What this domain tests
Balanced diet and deficiency diseases, the alimentary canal, mechanical and chemical digestion, enzyme properties and the effects of temperature and pH, and absorption in the villi. Data questions giving an enzyme's rate against temperature or pH are common and always ask for an explanation of the graph's shape, not just its reading.
Core rules
- A balanced diet supplies carbohydrates and fats for energy, proteins for growth and repair, vitamins and minerals, water and roughage. Roughage is not a nutrient; it adds bulk and keeps peristalsis working.
- Enzymes are protein catalysts, specific to a substrate and reused. They act by forming an enzyme-substrate complex at the active site. Rate rises to an optimum near 37 degrees Celsius in humans, then falls sharply as the enzyme denatures and the active site loses shape.
- Each enzyme has an optimum pH: salivary amylase about pH 7 (starch to maltose), pepsin about pH 2 (protein to peptides), pancreatic trypsin and lipase about pH 8.
- Bile is made in the liver, stored in the gall bladder, emulsifies fats and neutralises acidic chyme so pancreatic enzymes can work.
- Absorption happens mainly in the ileum. Villi provide a large surface area, a one-cell-thick epithelium, a dense capillary network and a central lacteal. Glucose and amino acids enter capillaries; fatty acids and glycerol re-form fats and enter lacteals.
Worked example
A patient's pancreatic duct is blocked by a tumour. Predict the effect on digestion. Pancreatic juice cannot reach the duodenum, so trypsin, pancreatic amylase and lipase are missing. Protein and starch are only partly digested by stomach and salivary enzymes. Fat digestion is worst affected because lipase is almost entirely pancreatic. Undigested fat appears in the faeces and the patient loses weight because fat-soluble vitamins A and D are poorly absorbed.
Traps that cost marks
Saying amylase digests starch in the stomach (acid denatures it). Writing that enzymes are used up or that they are killed, rather than denatured. Confusing absorption into blood or lymph with assimilation into cells. Claiming the stomach absorbs most nutrients.