The digestive system
Why digestion?
Food molecules (starch, protein, fat) are too large and insoluble to be absorbed across the gut wall. Digestion breaks them down into small soluble molecules using enzymes.
Path of food
- Mouth — teeth chew, saliva (containing amylase) starts starch breakdown.
- Oesophagus — peristalsis pushes food to the stomach.
- Stomach — protease (pepsin) digests protein in acidic conditions (HCl, pH ~2).
- Small intestine (duodenum then ileum) — pancreatic juice and bile mix in; final digestion + absorption.
- Large intestine — water absorbed.
- Rectum / anus — faeces stored, egested.
Three key digestive enzymes
| Enzyme | Substrate | Product | Made in |
|---|---|---|---|
| Amylase (carbohydrase) | Starch | Maltose / sugars | Salivary glands, pancreas |
| Protease | Protein | Amino acids | Stomach, pancreas |
| Lipase | Lipids (fats) | Fatty acids + glycerol | Pancreas |
Each enzyme has an optimum pH. Pepsin in the stomach works at pH 2; pancreatic enzymes work at slight alkaline pH (~8) in the duodenum.
Bile
Bile is made in the liver, stored in the gall bladder, released into the duodenum.
Two roles:
- Emulsifies fats — large fat droplets are broken into many smaller droplets, increasing surface area for lipase to act on. This SPEEDS UP fat digestion but is not itself a chemical digestion.
- Neutralises stomach acid — bile is alkaline; raises pH so pancreatic enzymes can work.
Absorption
In the small intestine, the lining is folded into villi (and microvilli on each cell). Adaptations:
- Large surface area
- Thin (one-cell-thick) wall — short diffusion distance
- Rich blood supply removing absorbed molecules, maintaining concentration gradient
WJEC exam tip
When asked about bile, always mention BOTH its functions (emulsification and pH neutralisation). Examiners flag candidates who only state emulsification.
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