# Digestion and Absorption
Digestion breaks large insoluble molecules into small soluble ones that can be absorbed. A-Level requires understanding of enzyme specificity, the role of the small intestine, and absorption mechanisms.
1. Types of Digestion
- Mechanical: teeth (chewing), stomach churning → increases surface area
- Chemical: enzymes hydrolyse bonds
2. Digestive Enzymes
| Enzyme | Substrate | Products | Where |
|---|---|---|---|
| Amylase | Starch | Maltose | Mouth, pancreas |
| Maltase | Maltose | Glucose | Small intestine (brush border) |
| Sucrase | Sucrose | Glucose + Fructose | Small intestine |
| Lactase | Lactose | Glucose + Galactose | Small intestine |
| Pepsin | Proteins | Polypeptides | Stomach (pH 2) |
| Trypsin | Polypeptides | Smaller peptides | Small intestine (pH 8) |
| Endopeptidases | Proteins | Peptide fragments | Hydrolyse internal peptide bonds |
| Exopeptidases | Peptides | Amino acids | Remove terminal amino acids |
| Dipeptidases | Dipeptides | Amino acids | Brush border |
| Lipase | Triglycerides | Fatty acids + Glycerol | Small intestine |
Bile
- Made in liver, stored in gall bladder
- Emulsifies fats (increases SA for lipase)
- Neutralises acid (alkaline; optimum pH for intestinal enzymes)
3. Small Intestine Adaptations
| Feature | How It Helps |
|---|---|
| Very long | Large surface area |
| Villi | Increase surface area |
| Microvilli (brush border) | Further increase SA |
| Single epithelium | Short diffusion distance |
| Rich blood supply | Maintains concentration gradient |
| Lacteal (in each villus) | Absorbs fatty acids and glycerol |
4. Absorption of Glucose (Co-transport)
- Na⁺/K⁺ ATPase pumps Na⁺ out of epithelial cell into blood (active transport)
- Low Na⁺ inside cell → Na⁺ diffuses from gut lumen into cell via co-transport protein
- Glucose carried in with Na⁺ (against glucose gradient)
- Glucose diffuses out of cell into blood via facilitated diffusion
This is an example of indirect active transport (secondary active transport).
5. Practice Questions
- Distinguish between endopeptidases and exopeptidases.
- Describe the mechanism of glucose absorption by co-transport.
- Explain the role of bile in fat digestion.
- List four adaptations of the small intestine for absorption.
- Why is the Na⁺/K⁺ ATPase essential for glucose absorption?
Want to check your answers and get step-by-step solutions?
Summary
- Digestion: mechanical (teeth, churning) + chemical (enzymes)
- Enzymes: amylase, protease, lipase; specific to substrates
- Bile: emulsifies fats, neutralises acid
- Small intestine: villi, microvilli, single epithelium, rich blood supply
- Glucose absorption: Na⁺-glucose co-transport (requires Na⁺/K⁺ ATPase)