Digestion solves a size and solubility problem
Food contains large biological molecules that cannot cross cell membranes or the gut epithelium efficiently. Mechanical digestion breaks food into smaller pieces and increases surface area, while chemical digestion hydrolyses polymers into small soluble units.
Ingestion brings food into the body; digestion breaks it down; absorption moves products across the intestinal epithelium; assimilation incorporates absorbed molecules into cells and tissues; egestion removes undigested material. Keeping these processes separate prevents many definition errors.
The alimentary canal provides changing conditions
In the mouth, chewing mixes food with saliva. Salivary amylase begins starch hydrolysis under near-neutral conditions. The bolus moves by peristalsis through the oesophagus to the stomach.
The stomach churns food and secretes hydrochloric acid, producing an acidic environment that helps denature proteins and supports pepsin activity. Mucus protects the stomach lining, and proteases begin substantial protein digestion.
Chewing and the start of starch digestion.
Acid, mixing and protein digestion.
Most chemical digestion and absorption.
Water and ion recovery; faeces formation.
Pancreatic enzymes and bile act in the small intestine
Pancreatic amylase continues starch digestion, proteases digest polypeptides, and pancreatic lipase hydrolyses triglycerides. Bicarbonate helps neutralise acidic chyme arriving from the stomach, creating conditions suitable for intestinal and pancreatic enzymes.
Bile contains bile salts that emulsify fat into small droplets, increasing surface area for lipase. Bile is not an enzyme: it does not hydrolyse lipid bonds. It is produced by the liver and stored and concentrated in the gall bladder.
| Substrate | Enzyme group | Absorbable product |
|---|---|---|
| Starch | Amylases then disaccharidases | Monosaccharides |
| Proteins | Proteases and peptidases | Amino acids |
| Triglycerides | Lipases | Fatty acids and monoglycerides/glycerol |
| Nucleic acids | Nucleases and related enzymes | Nucleotides/components |
Villi are built for rapid absorption
The small intestine has folds, villi and microvilli that create a very large surface area. Each villus has a thin epithelium, a capillary network and a central lacteal. Rich blood flow maintains concentration gradients for many absorbed nutrients.
Glucose and amino acids enter epithelial cells through transport proteins, including sodium-linked co-transport, and then pass into capillaries. Products of lipid digestion enter epithelial cells, are rebuilt into triglycerides and packaged into particles that move into lacteals before joining the bloodstream through lymph.
Monosaccharides, amino acids, water-soluble products.
Most packaged lipid products enter lymph first.
Short diffusion and transport distance.
Folds, villi and microvilli increase uptake.
Absorption is not the end of the story
Absorbed nutrients reach tissues and are assimilated. Amino acids may build proteins, glucose may be respired or stored as glycogen, and lipids may form membranes, hormones or energy stores. The liver helps regulate and process many absorbed substances carried in hepatic portal blood.
The large intestine absorbs much of the remaining water and ions. Gut microbes metabolise some undigested material and produce compounds that can be absorbed. Egestion removes material that never entered the body's internal tissues.
The ideas to carry forward
- Mechanical digestion increases surface area; chemical digestion hydrolyses molecules.
- Bile emulsifies fats but does not enzymatically digest them.
- Villi combine large area, thin barriers and good transport routes.
- Absorption enters the body; assimilation uses the absorbed material.
Answer first. Then reveal the marking logic.
01Why does emulsification increase lipid-digestion rate without breaking triglyceride bonds?2 marks · show the biological link
Answer: It divides fat into smaller droplets, increasing the surface area available for lipase action.
02Why do most lipid products enter lacteals rather than blood capillaries directly?2 marks · show the biological link
Answer: They are reassembled and packaged into particles that are too large for ordinary capillary entry and therefore move through lymphatic lacteals.
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