Biology 20 · Digestion
One tube, several environments
The digestive tract is a tube running through you, and what is inside it is technically still outside your body until it is absorbed. Each section maintains a different pH and a different set of enzymes, and the sequence is not arbitrary.
- 1. The words, first
- 2. The tract, section by section
- 3. The enzymes, and where each works
- 4. Absorption
- 5. What costs marks
The words, first
The idea: Digestion, absorption and assimilation are three different steps, and questions distinguish them.
| Word | What it means |
|---|---|
| Mechanical digestion | Breaking food into smaller pieces without chemical change — chewing, churning, emulsifying. Increases surface area for enzymes. |
| Chemical digestion | Breaking bonds with enzymes, turning polymers into monomers. |
| Absorption | Monomers crossing the gut wall into the blood or lymph. |
| Peristalsis | Waves of smooth muscle contraction moving food along. It works against gravity. |
| Sphincter | A ring of muscle controlling passage between sections. |
| Bolus / chyme | A swallowed lump of food; chyme is the acidic soup leaving the stomach. |
| Accessory organ | Liver, gall bladder, pancreas and salivary glands — they empty into the tract but food never passes through them. |
| Bile | Made by the liver, stored in the gall bladder. Contains no enzymes; it emulsifies fat. |
| Emulsify | Break fat into small droplets, multiplying the surface area lipase can act on. |
| Villus / microvillus | Finger-like projections of the small intestine lining. Together they raise the absorbing area enormously. |
| Optimum pH | The pH at which an enzyme works fastest. Pepsin's is about 2; pancreatic enzymes need about 8. |
The tract, section by section
The idea: Each section does something the next one depends on. Chewing gives enzymes surface area; acid gives pepsin its pH; bicarbonate gives pancreatic enzymes theirs.
Mouth. Mechanical digestion by chewing; salivary amylase starts on starch. Bread turns sweet if you hold it long enough — that is maltose appearing.
Oesophagus. No digestion; peristalsis moves the bolus down, which is why swallowing works upside down.
Stomach. HCl holds the pH near 2, which kills most swallowed microbes, denatures proteins and gives pepsin its optimum. Churning is mechanical. Almost nothing is absorbed here.
Small intestine. Where most digestion is finished and nearly all absorption happens. The pancreas delivers enzymes and bicarbonate to neutralize the arriving acid — both are needed, since pancreatic enzymes cannot work at pH 2.
Large intestine. Water and salts reclaimed; resident bacteria produce vitamin K and some B vitamins, which are absorbed here.
The enzymes, and where each works
The idea: Enzyme, substrate, product, location and pH. A question can ask for any one of them, so learn them as a set.
| Enzyme | Acts on | Where | pH |
|---|---|---|---|
| Salivary amylase | Starch → maltose | Mouth | Neutral |
| Pepsin | Protein → peptides | Stomach | ≈ 2 |
| Pancreatic amylase | Starch → maltose | Small intestine | ≈ 8 |
| Trypsin | Protein → peptides | Small intestine | ≈ 8 |
| Lipase | Fat → fatty acids + glycerol | Small intestine | ≈ 8 |
| Maltase, peptidases | Finish the job | Intestinal lining | ≈ 8 |
Bile is the exception. It contains no enzymes at all. It emulsifies fat — a mechanical effect — and that is what makes lipase effective, because enzymes can only reach the outside of a fat globule.
Absorption
The idea: Surface area is the whole design. The small intestine is long, folded, lined with villi, and each villus is covered in microvilli.
Where each product goes. Sugars and amino acids pass into the capillaries of each villus and travel to the liver by the hepatic portal vein. Fatty acids and glycerol are reassembled and enter the lacteal, a lymph vessel, joining the bloodstream later.
The liver's role. Everything absorbed into the blood reaches the liver first. It stores glucose as glycogen, deaminates excess amino acids, converts the resulting ammonia to urea, and detoxifies. Urea is made here, not in the kidney — the kidney only removes it.
Surface area turns up everywhere. Villi in the gut, alveoli in the lungs, the nephron in the kidney: any structure whose job is exchange is folded.
What costs marks
The idea: Four precise points that markers look for.
- Calling bile an enzyme. It emulsifies; it does not break bonds.
- Saying the liver produces urine or the kidney produces urea. Urea is made in the liver.
- Saying food passes through the liver or pancreas. They are accessory organs.
- Placing protein digestion in the mouth. It starts in the stomach with pepsin.