Quick Summary
Digestion converts large food molecules into small absorbable units. In humans, digestion occurs in the alimentary canal (mouth → oesophagus → stomach → small intestine → large intestine) with help from accessory glands (liver, pancreas, salivary glands). Absorption mainly occurs in the small intestine via villi.
Human Digestive System
Alimentary Canal
| Organ | Key Function |
|---|---|
| Mouth | Mechanical grinding; salivary amylase begins starch digestion |
| Oesophagus | Peristalsis — moves food to stomach |
| Stomach | Churning; HCl (activates pepsinogen, kills bacteria); pepsin digests proteins |
| Small intestine | Main site of digestion and absorption; duodenum, jejunum, ileum |
| Large intestine | Water and electrolyte absorption; faeces formation |
| Rectum / Anus | Faeces storage and elimination |
Digestive Enzymes
| Source | Enzyme | Substrate | Product |
|---|---|---|---|
| Salivary glands | Salivary amylase (ptyalin) | Starch | Maltose |
| Stomach (chief cells) | Pepsinogen → Pepsin | Proteins | Peptones/peptides |
| Stomach (parietal cells) | HCl | Activates pepsinogen | — |
| Pancreas | Trypsin, chymotrypsin | Proteins | Peptides |
| Pancreas | Pancreatic amylase | Starch | Maltose |
| Pancreas | Pancreatic lipase | Fats | Fatty acids + glycerol |
| Intestinal glands | Maltase, lactase, sucrase | Disaccharides | Monosaccharides |
| Intestinal glands | Peptidases (erepsin) | Peptides | Amino acids |
Digestion Summary
Carbohydrates: Starch → (salivary + pancreatic amylase) → Maltose → (maltase) → Glucose
Proteins: Proteins → (pepsin, trypsin, chymotrypsin) → Peptides → (peptidases) → Amino acids
Fats: Fats → (bile emulsification) → fat droplets → (pancreatic lipase) → Fatty acids + Glycerol
Absorption in Small Intestine
Villi: Finger-like projections; increase surface area ~600-fold
Each villus contains:
- Blood capillaries: Absorb glucose and amino acids → portal vein → liver
- Lacteals: Lymph capillaries; absorb fatty acids/glycerol (as chylomicrons) → lymph system
Mechanisms of absorption:
- Glucose and amino acids: active transport (against concentration gradient, requires ATP)
- Water and electrolytes: osmosis and diffusion
- Fats: diffusion (as chylomicrons via lacteals)
Role of Liver
- Produces bile (stored in gallbladder)
- Bile emulsifies fats (no digestive enzymes)
- Regulates blood glucose (glycogen synthesis/breakdown)
- Detoxification
- Produces plasma proteins
Disorders of Digestion
| Disorder | Description |
|---|---|
| Jaundice | Yellowing due to excess bilirubin (liver dysfunction) |
| Vomiting | Reverse peristalsis; medulla oblongata controls vomiting centre |
| Diarrhoea | Excess water loss in faeces; intestinal infection or food poisoning |
| Constipation | Infrequent, hard stools; insufficient water/fibre |
| Peptic ulcer | Erosion of stomach/duodenal lining by HCl; often due to H. pylori |
Important Questions
One Mark
- Name the enzyme secreted by chief cells of the stomach.
- What is the role of bile salts in digestion?
- Where does maximum absorption of digested food occur?
- Name the structures that absorb fat in the small intestine.
Short Answer (3 marks)
- Describe the process of fat digestion and absorption in the small intestine.
- What are villi? How do they aid absorption?
- Name the enzymes secreted by the pancreas and state the substrate of each.
Long Answer (5 marks)
- Describe the digestion of carbohydrates from the mouth to the small intestine.
- Write a detailed account of the structure and functions of the small intestine in digestion and absorption.
Common Mistakes
- Bile contains NO digestive enzymes — it only emulsifies fats; digestion is done by pancreatic lipase.
- Fats are absorbed via lacteals (lymph), NOT directly into blood capillaries.
- Pepsinogen is the inactive precursor — HCl converts it to active pepsin; students often say HCl digests protein (wrong — pepsin does).
- Large intestine does NOT digest food — it only absorbs water and electrolytes.
Board Exam Tips
- Enzyme table (source, enzyme, substrate, product) is a guaranteed 5-mark question.
- "Explain fat digestion and absorption" is a standard 3-mark or 5-mark question.
- Villi structure and function with diagram appears regularly.
- NEET: Digestion enzymes, role of bile, and absorption mechanisms contribute 2–3 questions per paper.
Frequently Asked Questions
Pancreatic juice contains: trypsinogen (→ trypsin, digests proteins), chymotrypsinogen, pancreatic amylase (starch → maltose), pancreatic lipase (fats → fatty acids + glycerol), and nucleases. It is alkaline (NaHCO₃) to neutralise acidic chyme.