# Human Physiology — Digestion and Absorption (IB)
The digestive system breaks down macromolecules into absorbable monomers. IB Biology (Topic 6.1) requires understanding of enzyme action, the structure of the digestive tract, absorption mechanisms, and the role of the liver and pancreas.
1. Overview of Digestion
Digestion involves mechanical (physical breakdown) and chemical (enzymatic hydrolysis) processes.
- Mouth: Teeth mechanically break food; salivary amylase begins starch digestion (starch → maltose); food mixed into a bolus
- Oesophagus: Peristalsis moves food to stomach
- Stomach: Churning (mechanical); HCl (pH ~2) denatures proteins and kills bacteria; pepsin (protease) digests proteins into polypeptides
- Small intestine (duodenum + ileum): Pancreatic enzymes complete digestion; bile emulsifies fats; absorption through villi
- Large intestine: Absorbs water and mineral ions; bacteria produce vitamin K and biotin
2. Digestive Enzymes
| Enzyme | Source | Substrate | Products | Optimum pH |
|---|---|---|---|---|
| Salivary amylase | Salivary glands | Starch | Maltose | ~7 |
| Pepsin | Stomach (chief cells) | Proteins | Polypeptides | ~2 |
| Pancreatic lipase | Pancreas | Triglycerides | Fatty acids + glycerol | ~8 |
| Pancreatic amylase | Pancreas | Starch | Maltose | ~8 |
| Trypsin | Pancreas | Polypeptides | Amino acids | ~8 |
Endopeptidases (e.g., pepsin, trypsin) cut bonds within the polypeptide chain. Exopeptidases remove amino acids from the ends of chains.
3. The Role of Bile
- Produced by the liver, stored in the gall bladder
- Released into the duodenum
- Emulsifies fats: breaks large fat globules into smaller droplets → increases surface area for lipase
- Alkaline: neutralises stomach acid, providing optimal pH for intestinal enzymes
- Bile is NOT an enzyme — it does not chemically digest food
4. Absorption in the Small Intestine
The ileum is adapted for maximum absorption:
Villi and Microvilli
- Villi: finger-like projections increasing surface area ~600×
- Microvilli: tiny projections on epithelial cell surface (brush border) → further increase surface area
- Single epithelial cell layer: short diffusion distance
- Rich blood capillary network: maintains concentration gradient; carries away absorbed nutrients
- Lacteal: lymph vessel in each villus; absorbs fatty acids and glycerol (as chylomicrons)
Transport Mechanisms
| Substance | Mechanism |
|---|---|
| Glucose | Co-transport with Na⁺ (secondary active transport) |
| Amino acids | Co-transport with Na⁺ |
| Fatty acids & glycerol | Diffusion into epithelial cells → reassembled as triglycerides → packaged with proteins as chylomicrons → enter lacteal |
| Fructose | Facilitated diffusion |
| Water | Osmosis |
| Minerals/vitamins | Active transport or facilitated diffusion |
5. Assimilation
After absorption, nutrients are assimilated — used by cells:
- Glucose: respiration (energy), stored as glycogen (liver/muscles)
- Amino acids: protein synthesis, deamination in liver (excess → urea)
- Fatty acids/glycerol: cell membranes, energy storage, insulation
Worked Example
Question: Explain the role of villi in absorption. (4 marks)
Solution: Villi are finger-like projections in the ileum that greatly increase the surface area for absorption. The epithelium is only one cell thick, providing a short diffusion distance. Each villus contains a dense network of blood capillaries that maintains a steep concentration gradient by continuously carrying absorbed nutrients away. Each villus also contains a lacteal (lymph vessel) for absorbing fats. The epithelial cells have microvilli (brush border) that further increase the surface area.
Practice Questions
- State the substrates and products of pepsin, lipase, and amylase. (3 marks)
- Explain the role of bile in fat digestion. (3 marks)
- Describe how glucose is absorbed in the ileum. (3 marks)
- Explain why the stomach has an acidic pH. (2 marks)
Answers
- Pepsin: substrate = proteins; products = polypeptides. Lipase: substrate = triglycerides; products = fatty acids + glycerol. Amylase: substrate = starch; products = maltose.
Want to check your answers and get step-by-step solutions?
Summary
- Digestion is mechanical (teeth, churning) and chemical (enzymes hydrolyse bonds).
- Key enzymes: amylase (starch), protease/pepsin/trypsin (proteins), lipase (fats).
- Bile emulsifies fats and neutralises acid — it is NOT an enzyme.
- Villi and microvilli maximise surface area; single-cell thick epithelium; rich blood supply.
- Glucose absorbed by Na⁺ co-transport; fats enter lacteals as chylomicrons.