Digestion and Absorption
Digestion and Absorption covers the alimentary canal, digestive glands, enzymes, motility and absorption. Questions often ask where an enzyme acts, what activates it, and which segment absorbs a nutrient.
Core rules
- Gut wall layers: mucosa, submucosa, muscularis and serosa.
- Mouth: salivary amylase converts starch to maltose at near-neutral pH.
- Stomach: HCl converts pepsinogen to pepsin, which digests protein.
- Small intestine: pancreatic amylase, lipase and trypsinogen are delivered; enterokinase activates trypsinogen. Bile emulsifies fat but has no enzyme. Brush-border enzymes complete digestion.
- Absorption: glucose and amino acids enter blood capillaries; fats form chylomicrons and enter lacteals. Vitamin B12 binds intrinsic factor and is absorbed in the terminal ileum.
Worked example
A patient has the terminal ileum removed. Which deficiency is most predictable?
Reasoning: vitamin B12 binds intrinsic factor and is absorbed only in the terminal ileum. Fat-soluble vitamins and bile salts also have ileal absorption, but B12 is the classic deficiency. Answer: vitamin B12 deficiency, with possible megaloblastic anaemia.
Traps that cost marks
- Bile has no digestive enzyme; it only emulsifies fat.
- Pepsinogen is activated by HCl, not by pepsin.
- Trypsinogen is activated by enterokinase.
- Salivary amylase stops working in stomach acid.
- Glucose absorption uses sodium-dependent co-transport, not simple diffusion alone.