1.3Cells, Organ Systems and Ecosystems
Digestion and the Digestive System in Humans — WJEC GCSE Biology Revision Notes & Topical Questions
5 min read 4 parts Lesson 3 of 6
Section 1 of 4The alimentary canal and digestive enzymes
The alimentary canal and digestive enzymes
Digestion
The chemical and physical breakdown of large, insoluble food molecules into small, soluble molecules that can be absorbed across the intestinal wall into the bloodstream.
Main classes of digestive enzymes
| Enzyme Class | Specific Example | Substrate | End Products | Site of Production |
|---|---|---|---|---|
| Carbohydrase | Amylase | Starch | Maltose and Glucose | Salivary glands and Pancreas |
| Protease | Pepsin / Trypsin | Proteins | Amino acids | Stomach (pepsin) and Pancreas (trypsin) |
| Lipase | Pancreatic lipase | Lipids (fats/oils) | Fatty acids and Glycerol | Pancreas |
Role of Bile
- Produced in the liver and stored in the gallbladder.
- It is alkaline and neutralises acidic stomach chyme entering the duodenum, providing optimum pH for pancreatic enzymes.
- Emulsifies fatsBreaks large fat globules into tiny droplets, vastly increasing the surface area for lipase action.
Section 2 of 4Absorption in the small intestine
- The ileum (small intestine) is adapted for maximum nutrient absorption:
- Extremely long with deeply folded walls lined with millions of villi and microvilli, providing a massive surface area.
- Villi epithelium is only one cell thick, providing a minimal diffusion distance.
- Rich network of blood capillaries absorbs glucose and amino acids directly.
- Central lacteals absorb fatty acids and glycerol into the lymphatic system.
Section 3 of 4Key terms
| Term | Meaning |
|---|---|
| Peristalsis | Waves of muscular contraction and relaxation that push food along the alimentary canal. |
| Emulsification | The mechanical breakdown of large fat droplets into tiny droplets by bile salts. |
| Villus | A microscopic finger-like projection in the small intestine that increases surface area for absorption. |
| Lacteal | A central lymphatic vessel in an intestinal villus that absorbs fatty acids and glycerol. |
Section 4 of 4Exam technique
Avoid these mistakes
Stating bile is an enzyme — bile contains no enzymes; it is an alkaline emulsifying fluid that mechanically breaks fat globules.
Saying digestion finishes in the large intestine — digestion and nutrient absorption are completed in the small intestine; the large intestine absorbs only water.
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