Digestion, Absorption and Metabolism of Nutrients
The digestive sequence
Mouth - salivary amylase digests starch to maltose; lingual lipase acts minimally; chewing increases surface area. Stomach - hydrochloric acid denatures protein and activates pepsinogen to pepsin; churning forms chyme; rennin acts on milk in infants. Small intestine - pancreatic amylase, lipase, trypsin, chymotrypsin and carboxypeptidase; bile salts emulsify fat without containing any enzyme; brush-border maltase, sucrase, lactase and peptidases complete digestion. Large intestine - water and electrolyte absorption, bacterial synthesis of some B vitamins, faecal formation.
Absorption and fate
Monosaccharides and amino acids pass into the portal vein to the liver. Long-chain fatty acids and fat-soluble vitamins are reassembled into chylomicrons and enter lacteals, bypassing the liver at first.
Metabolism in brief
Glycolysis → TCA cycle → electron transport chain; glycogenesis and glycogenolysis; gluconeogenesis; β-oxidation and ketogenesis for fat; transamination and deamination of amino acids with urea formation.
Worked example
An adult reports bloating, gas and loose stools after drinking milk but tolerates curd. Reasoning: the enzyme lactase at the brush border is deficient, so lactose reaches the colon, where bacteria ferment it to gas and acids; the unabsorbed sugar draws water osmotically, causing diarrhoea. Curd is tolerated because fermentation has already broken down much of the lactose. Management is reduced lactose intake with adequate calcium from curd, paneer and greens.
Traps that cost marks
Protein digestion begins in the stomach, not the mouth. Bile emulsifies fat but is not an enzyme. Fat digestion is completed in the small intestine, not the stomach. Water absorption is mainly colonic. Do not write that all absorbed nutrients go first to the liver - fats enter lymph.