Unit 8
Human Biology
Chapter 8.1
Digestive SystemClass 12 Biology – Digestive System Notes PDF
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Introduction
Digestion includes mechanical breakdown and enzyme-controlled chemical digestion. The major organs are mouth, pharynx, oesophagus, stomach, small intestine and large intestine, assisted by salivary glands, liver, gall bladder and pancreas.
1. Human Alimentary Canal
Fig. 1 – Labelled Human Digestive System
| Part | Main Role |
|---|---|
| Mouth | Ingestion, mastication and beginning of carbohydrate digestion. |
| Oesophagus | Moves food to stomach by peristalsis. |
| Stomach | Stores and mixes food; begins major protein digestion. |
| Small intestine | Completes most digestion and absorbs most nutrients. |
| Large intestine | Absorbs water and electrolytes; forms faeces. |
2. Digestion in the Mouth
Teeth mechanically break food into smaller pieces. The tongue mixes food with saliva and helps form a bolus for swallowing. Saliva moistens food and contains salivary amylase, which begins digestion of starch.
Fig. 2 – Main Types of Human Teeth
3. Stomach
The stomach is a muscular sac that stores food, churns it and mixes it with gastric juice. Gastric glands secrete hydrochloric acid, enzymes and mucus.
- Hydrochloric acid: creates an acidic environment and helps activate pepsin.
- Pepsin: begins protein digestion.
- Mucus: protects the stomach lining.
Fig. 3 – Stomach and Its Regions
4. Small Intestine
The small intestine consists of duodenum, jejunum and ileum. It receives bile and pancreatic juice, completes most chemical digestion and is the principal site of nutrient absorption.
Role of Bile
Bile contains bile salts that emulsify large fat droplets into smaller droplets, increasing surface area for lipase action. Bile is produced by the liver and stored in the gall bladder.
Fig. 4 – Intestinal Villus
5. Major Digestive Glands
Salivary Glands
Produce saliva containing mucus and salivary amylase.
Liver
Produces bile, processes absorbed nutrients and performs many metabolic functions.
Pancreas
Releases pancreatic enzymes and bicarbonate into the duodenum.
Intestinal Glands
Contribute enzymes and secretions that help complete digestion.
Fig. 5 – Liver, Gall Bladder and Pancreas
6. Chemical Digestion of Food
| Nutrient | Main Digestion | Final Absorbable Products |
|---|---|---|
| Carbohydrates | Amylases and disaccharidases break polysaccharides/disaccharides | Monosaccharides such as glucose |
| Proteins | Pepsin, pancreatic proteases and peptidases | Amino acids and small absorbable peptides |
| Fats | Bile emulsification followed by lipase action | Fatty acids and monoglycerides/glycerol-related products |
Fig. 6 – Overview of Digestion
7. Absorption and Assimilation
Most nutrient absorption occurs through the villi of the small intestine. Glucose and amino acids enter blood capillaries; many lipid digestion products enter intestinal cells and are transported largely through lacteals before reaching the circulation.
Large Intestine
The large intestine absorbs much of the remaining water and electrolytes, compacts undigested material and stores faeces before defecation.
8. Quick Revision & Exam Points
- Draw and label the human digestive system.
- Describe digestion in the mouth.
- Explain the functions of the stomach.
- Write the role of liver, bile and pancreas in digestion.
- Describe digestion in the small intestine.
- Draw and label an intestinal villus.
- Differentiate absorption and assimilation.
- Write the functions of the large intestine.
- Mouth begins starch digestion.
- Peristalsis moves food through oesophagus.
- Stomach begins major protein digestion.
- Liver produces bile.
- Bile emulsifies fats.
- Pancreas provides digestive enzymes and bicarbonate.
- Small intestine completes most digestion.
- Villi increase absorptive surface area.
- Large intestine absorbs water.
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