Digestion
Year 7 ๐ซ Physiology & Health The digestive system: organs, enzymes, and absorption.
๐ซ The Digestive System
Digestion breaks large insoluble food molecules into small soluble ones for absorption into the blood.
- Mouth โ mechanical digestion (teeth); salivary amylase starts starch digestion; pH ~7
- Oesophagus โ food transported by peristalsis (muscular contractions)
- Stomach โ pepsin (protease) digests proteins; hydrochloric acid (pH 2) kills bacteria and activates pepsin
- Duodenum โ pancreatic enzymes added; bile from liver emulsifies fats; pH raised to ~8
- Ileum โ main site of nutrient absorption via villi; huge surface area
- Large intestine โ absorbs water from undigested food; forms faeces
- Rectum/anus โ egestion (expulsion) of faeces
๐งช Digestive Enzymes
Enzymes are biological catalysts that speed up the breakdown of specific nutrients.
| Enzyme | Produced in | Substrate โ Product | Optimum pH |
|---|---|---|---|
| Salivary amylase | Salivary glands | Starch โ maltose | ~7 |
| Pepsin | Stomach (as pepsinogen) | Protein โ peptides | ~2 |
| Pancreatic amylase | Pancreas | Starch โ maltose | ~7โ8 |
| Pancreatic lipase | Pancreas | Lipids โ fatty acids + glycerol | ~7โ8 |
| Trypsin (protease) | Pancreas | Proteins โ amino acids | ~8 |
| Maltase | Small intestine | Maltose โ glucose | ~7 |
Role of Bile
Bile (made in liver, stored in gall bladder, released into duodenum) emulsifies fats โ breaks large fat droplets into small droplets, increasing surface area for lipase. Also neutralises stomach acid.๐ฌ Absorption in the Ileum
Villi are finger-like projections in the ileum wall that maximise absorption.
- Large surface area โ millions of villi (and microvilli on each villus cell)
- One cell thick epithelium โ short diffusion distance into blood/lymph
- Rich blood supply (capillaries) โ maintains steep concentration gradient
- Lacteals (lymph vessels) โ absorb fatty acids and glycerol (reassembled as triglycerides)
- Glucose and amino acids โ absorbed into blood capillaries โ portal vein โ liver
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