Enzymes and digestion
What are enzymes?
Enzymes are biological catalysts — they speed up chemical reactions in living organisms without being consumed. They are proteins with a specific 3D shape.
Key concept — the active site: Each enzyme has a uniquely shaped region called the active site. Only a substrate with the correct complementary shape can bind (the lock and key model). This makes enzymes specific — each enzyme catalyses only one type of reaction.
Effect of temperature on enzyme activity
- Below optimum: low kinetic energy → fewer successful collisions between enzyme and substrate → slow rate.
- At optimum temperature (~37°C for human digestive enzymes): maximum rate of reaction.
- Above optimum: the enzyme denatures — heat breaks the bonds holding the protein in shape; the active site changes shape; the substrate can no longer fit.
Effect of pH on enzyme activity
- Each enzyme has an optimum pH (e.g. pepsin in stomach works best at pH 2; amylase in saliva at pH 7).
- Too acidic or too alkaline: bonds in the protein break → enzyme denatures.
The digestive system — key organs and their roles
Pathway: Mouth → Oesophagus → Stomach → Small intestine → Large intestine → Rectum → Anus
| Organ | Function |
|---|---|
| Mouth/salivary glands | Mechanical digestion (chewing); amylase secreted (breaks starch → maltose) |
| Oesophagus | Muscular tube; peristalsis moves food to stomach |
| Stomach | Hydrochloric acid (pH 2, kills bacteria, denatures proteins); pepsin (protease) breaks proteins → peptides |
| Liver | Produces bile; bile stored in gall bladder |
| Pancreas | Produces amylase, protease and lipase; secretes them into small intestine |
| Small intestine | Final digestion by pancreatic enzymes; absorption of nutrients into blood via villi |
| Large intestine | Absorbs water from undigested food; forms faeces |
The three key digestive enzymes
| Enzyme | Substrate | Product | Where produced | Where acts |
|---|---|---|---|---|
| Amylase | Starch | Maltose (sugars) | Salivary glands, pancreas | Mouth, small intestine |
| Protease (e.g. pepsin, trypsin) | Proteins | Amino acids | Stomach (pepsin), pancreas | Stomach, small intestine |
| Lipase | Lipids (fats) | Fatty acids + glycerol | Pancreas | Small intestine |
Bile — not an enzyme, but essential
Bile is produced by the liver, stored in the gall bladder, and released into the small intestine via the bile duct.
Bile does two things:
- Emulsifies fats (breaks large fat droplets into small droplets) → greatly increases surface area for lipase to work on.
- Neutralises stomach acid → raises pH from 2 to ~7 in the small intestine → optimum pH for pancreatic enzymes.
Absorption in the small intestine
The small intestine is adapted for absorption via villi and microvilli:
- Very large surface area (villi + microvilli = area of a tennis court).
- Thin walls (one cell thick).
- Rich blood supply (capillaries and lacteals in each villus).
- Glucose and amino acids → absorbed into capillaries → blood → liver → body.
- Fatty acids + glycerol → reassembled into lipids → absorbed into lacteals (lymph).
AI-generated · claude-opus-4-7 · v3-ccea-combined-science