Digestion
The Alimentary Canal
The alimentary canal is a continuous tube from mouth to anus. Each region is specialised for a stage of digestion.
| Region | Function |
|---|---|
| Mouth | Mechanical digestion (chewing); salivary amylase begins starch digestion |
| Oesophagus | Transports food to stomach by peristalsis |
| Stomach | Churns food; secretes pepsin (protease) and HCl (pH ≈ 2); kills bacteria |
| Small intestine | Enzymatic digestion completed; absorption of nutrients into blood |
| Large intestine | Absorption of water; formation of faeces |
| Rectum / Anus | Storage and egestion of faeces |
Associated organs:
- Liver → produces bile
- Gall bladder → stores bile; releases it into small intestine
- Pancreas → secretes amylase, protease (trypsin), lipase into small intestine
Peristalsis
Peristalsis is the wave-like muscular contraction that moves food through the gut.
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Circular muscle contracts → ████ ← food bolus pushed forward
Longitudinal muscle relaxes
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- Circular muscles contract behind the food bolus, narrowing the tube.
- Longitudinal muscles relax in front, widening it.
- The result is a coordinated wave that pushes food in one direction.
- Occurs throughout the entire alimentary canal.
Digestive Enzymes
| Enzyme | Produced in | Site of action | Substrate → Product | Optimum pH |
|---|---|---|---|---|
| Salivary amylase | Salivary glands | Mouth | Starch → maltose | ~7 |
| Pancreatic amylase | Pancreas | Small intestine | Starch → maltose | ~7–8 |
| Pepsin | Stomach wall | Stomach | Proteins → peptides | ~2 |
| Trypsin | Pancreas | Small intestine | Proteins → peptides | ~8 |
| Lipase | Pancreas | Small intestine | Lipids → fatty acids + glycerol | ~7–8 |
Summary
Amylase digests carbohydrates (starch). Proteases (pepsin, trypsin) digest proteins. Lipase digests lipids. End products: glucose, amino acids, fatty acids, glycerol: are small enough to be absorbed.
Bile
- Produced by: the liver
- Stored in: the gall bladder
- Released into: the duodenum (first part of small intestine) via the bile duct
Two roles of bile:
- Neutralises stomach acid: bile is alkaline (contains sodium hydrogen carbonate), raising pH in the small intestine to the optimum for pancreatic enzymes (~7–8).
- Emulsifies lipids: bile salts break large fat droplets into smaller droplets (an emulsion), massively increasing surface area for lipase to act on.
Emulsification vs Digestion
Bile does not chemically digest lipids: it only increases surface area. Lipase does the actual chemical breakdown.
Absorption in the Small Intestine
The small intestine is adapted for efficient absorption:
| Adaptation | How it helps |
|---|---|
| Villi (finger-like projections) | Greatly increase surface area for absorption |
| Microvilli (brush border on villi cells) | Further increase surface area |
| Rich blood supply (capillary network in each villus) | Maintains concentration gradient; carries absorbed nutrients away quickly |
| Lacteals (lymph vessels in each villus) | Absorb fatty acids and glycerol (reassembled into lipids) |
| Thin epithelium (one cell thick) | Short diffusion distance for nutrients |
| Long tube | More time and area for digestion and absorption |
Route of absorbed nutrients:
- Glucose and amino acids → capillaries → hepatic portal vein → liver
- Fatty acids and glycerol → lacteals → lymphatic system → blood
Exam Questions
Q: Describe how peristalsis moves food through the gut.
Circular muscles behind the food bolus contract, narrowing the tube and pushing food forwards. Longitudinal muscles in front relax, widening the tube. This wave of contraction passes along the gut, moving food in one direction.
Q: State the role of bile in digestion. Include two functions.
Bile neutralises the acid from the stomach, raising the pH of the small intestine to the optimum for pancreatic enzymes. Bile also emulsifies lipids: bile salts break large fat droplets into smaller droplets, increasing surface area for lipase to digest them.
Q: Explain why the small intestine is well adapted for absorption.
The inner lining has finger-like projections called villi, and each villus cell has microvilli, giving a very large surface area. Each villus has a capillary network to maintain a steep concentration gradient and carry absorbed nutrients away. The epithelium is only one cell thick, reducing the diffusion distance.
Q: Where is bile produced and where is it stored?
Bile is produced in the liver and stored in the gall bladder.
Exam Tip
Don’t confuse emulsification (bile breaking fat into droplets: physical) with digestion (lipase breaking fat into fatty acids and glycerol: chemical). Exam questions often ask you to distinguish between the two.