Chapter 4 — Digestion and Absorption
Key Concepts Quick Recap
| Structure | Function |
|---|---|
| Oral cavity | Mastication (chewing), salivary amylase begins starch digestion |
| Oesophagus | Peristalsis — food transport from mouth to stomach; no digestion |
| Stomach | Churns food; HCl (pH 2) kills bacteria, activates pepsinogen → pepsin; protein digestion begins |
| Small intestine | Main digestion + absorption; bile, pancreatic juice, intestinal enzymes; villi for absorption |
| Large intestine | Water and electrolyte reabsorption; formation and elimination of faeces |
| Liver | Produces bile; detoxification; glycogen storage; produces plasma proteins |
| Pancreas | Produces pancreatic juice (trypsinogen, chymotrypsinogen, lipase, amylase) |
Digestive Enzymes Summary
| Enzyme | Source | Substrate → Product |
|---|---|---|
| Salivary amylase (ptyalin) | Salivary glands | Starch → Maltose |
| Pepsin | Stomach (chief cells) | Proteins → Peptides |
| HCl | Stomach (parietal/oxyntic cells) | Activates pepsinogen; kills bacteria; pH 2 |
| Trypsin | Pancreas | Proteins/peptides → smaller peptides |
| Chymotrypsin | Pancreas | Proteins → peptides |
| Pancreatic amylase | Pancreas | Starch → Maltose |
| Pancreatic lipase | Pancreas | Fats (after bile emulsification) → Glycerol + Fatty acids |
| Enterokinase | Intestinal mucosa | Trypsinogen → Trypsin (activation) |
| Sucrase | Intestinal mucosa | Sucrose → Glucose + Fructose |
| Maltase | Intestinal mucosa | Maltose → Glucose + Glucose |
| Lactase | Intestinal mucosa | Lactose → Glucose + Galactose |
I. Choose the Best Answer (MCQs)
1. HCl in the stomach is secreted by: (a) Chief cells (b) Parietal (oxyntic) cells (c) Mucous cells (d) G cells
- Parietal (oxyntic) cells: HCl + intrinsic factor (for Vitamin B12 absorption).
- Chief (peptic) cells: Pepsinogen (converted to pepsin by HCl).
- Mucous neck cells: Mucus (protects stomach lining from HCl).
- G cells: Gastrin hormone (stimulates HCl secretion).
2. Trypsinogen is activated to trypsin by: (a) HCl (b) Enterokinase (c) Pepsin (d) Bile
3. Bile is produced by: (a) Gall bladder (b) Pancreas (c) Liver (d) Duodenum
4. Maximum absorption of nutrients occurs in: (a) Stomach (b) Large intestine (c) Oesophagus (d) Small intestine
Surface area is enormously increased: plicae circulares → villi → microvilli → ~200 m² total surface.
- Digestion (pancreatic enzymes, intestinal enzymes).
- Absorption (villi, microvilli — brush border).
5. Fatty acids and glycerol after absorption are transported via: (a) Blood capillaries (b) Lymph vessels (lacteals) (c) Hepatic portal vein (d) Bile duct
6. Peristalsis is: (a) Enzymatic digestion of food (b) Rhythmic muscular contractions that move food through the GI tract (c) Absorption of nutrients (d) Secretion of bile
7. Which disease is caused by deficiency of Vitamin C? (a) Rickets (b) Night blindness (c) Scurvy (d) Beriberi
II. Short Answer Questions
Common Mistakes
- Saying bile is an enzyme — bile is not an enzyme; it emulsifies (physically breaks up) fats.
- Confusing where protein digestion begins (stomach — pepsin) vs carbohydrate digestion (mouth — amylase).
- Saying fatty acids go to blood — they go to lacteals (lymph); amino acids and glucose go to blood.
- Forgetting enterokinase activates trypsinogen (not pepsin or HCl).
Exam Checklist
- Enzymes table: source, substrate, product.
- Stomach cells: parietal (HCl), chief (pepsinogen), mucous, G cells (gastrin).
- Bile: liver → gall bladder → bile duct → duodenum; emulsifies fats; not an enzyme.
- Absorption: monosaccharides + amino acids → blood → hepatic portal vein; fats → lacteals.
- Liver roles: bile, glycogen storage, detox, plasma proteins, urea synthesis.