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Topic 7 · 7.4–7.5

Chemical digestion and absorption

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe chemical digestion and its role; state the functions of amylase, proteases and lipase and where they are secreted and act.
  • Describe the functions of hydrochloric acid in gastric juice.
  • Describe the digestion of starch and protein in detail, and explain the role of bile in neutralising acid EXTENDED.
  • State where nutrients and water are absorbed; explain villi and microvilli; describe a villus EXTENDED.

📚The biology

Chemical digestion

Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules. Only small soluble molecules can be absorbed into the blood.

Three reactions. A chain of glucose units, starch, is broken by amylase into pairs, maltose, which maltase breaks into single glucose units. A chain of amino acids, protein, is broken by protease into separate amino acids (pepsin in the stomach, trypsin in the small intestine). A fat molecule is broken by lipase into fatty acids and glycerol.
Amylase: starch → simple reducing sugars. Proteases: protein → amino acids. Lipase: fats and oils → fatty acids and glycerol.
A four-row chart. Mouth: made by the salivary glands, amylase, about pH 7. Stomach: made by the stomach wall, protease (pepsin) and hydrochloric acid, about pH 2. Duodenum: made by the pancreas, amylase, protease (trypsin) and lipase, alkaline. Ileum: made by the lining of the small intestine, maltase on the cell membranes, alkaline.
Enzymes are made in one place and act in another: pancreatic enzymes, for example, act in the duodenum.

The hydrochloric acid in gastric juice kills harmful microorganisms in food and provides the acidic pH at which the stomach protease works best.

EXTENDED In detail:

  • EXTENDED Starch: amylase breaks starch down to maltose; maltase then breaks maltose down to glucose on the membranes of the epithelium lining the small intestine.
  • Protein: pepsin breaks down protein in the acidic conditions of the stomach; trypsin breaks down protein in the alkaline conditions of the small intestine.
  • Bile is alkaline: it neutralises the acidic mixture of food and gastric juice entering the duodenum, providing a suitable pH for the enzymes there (and it emulsifies fats, 7a).

Absorption

The small intestine is where nutrients are absorbed. Most water is also absorbed there; some is absorbed in the colon.

A villus: a finger-like projection of the small intestine lining, about 1 millimetre high. Its outer wall, the epithelium, is one cell thick and covered in microvilli. Inside, a network of capillaries absorbs glucose and amino acids and drains to the hepatic portal vein, and a central lacteal absorbs fatty acids and glycerol.
EXTENDED Millions of villi, each covered with microvilli, give the small intestine a huge internal surface area.
  • EXTENDED Villi and microvilli greatly increase the internal surface area, so absorption is faster.
  • The wall of each villus is one cell thick: a short distance for diffusion.
  • A dense network of capillaries absorbs glucose and amino acids and carries them away, keeping a steep concentration gradient; the blood goes to the liver in the hepatic portal vein.
  • The lacteal, part of the lymphatic system, absorbs fatty acids and glycerol.

✏️Worked example

(a) Name the enzyme that digests fat, the organ that secretes it, and the products. [3] (b) EXTENDED Explain why trypsin would work poorly if bile were not released into the duodenum. [2] (c) EXTENDED Explain how the structure of a villus increases the rate of absorption of glucose. [3]

(a) Lipase; the pancreas; fatty acids and glycerol.

(b) The food arriving from the stomach is acidic. Bile is alkaline and neutralises it; without bile the pH in the duodenum would stay low, far from trypsin’s alkaline optimum.

(c) Villi and microvilli give a large surface area; the wall is one cell thick, so the diffusion distance is short; a good blood supply carries glucose away, maintaining a steep concentration gradient.

Check it. Every feature in (c) matches one of the diffusion factors from topic 3: surface area, distance, concentration gradient.
“Bile digests fat.” Bile contains no enzymes. It emulsifies fat and neutralises acid; lipase does the digesting.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Which enzyme breaks down protein? A: amylase. B: lipase. C: protease. D: maltase.
C.
2. (Theory.) Explain why starch must be digested before it can be absorbed. [2]
Starch molecules are large and insoluble, so they cannot pass through the wall of the small intestine into the blood; digestion produces small soluble molecules (glucose) that can.
3. (Theory.) State two functions of hydrochloric acid in the stomach. [2]
Kills harmful microorganisms in food; provides an acidic pH for the optimum activity of the stomach protease.
4. (Theory.) Name the organ that secretes amylase in each place: (a) the mouth, (b) the duodenum. [2]
(a) Salivary glands. (b) Pancreas.
5. (Theory.) Name the region where most water is absorbed, and one other region where some is absorbed. [2]
Small intestine; colon.
6. (Theory.) EXTENDED Describe the complete digestion of starch to glucose, naming both enzymes and where each acts. [3]
Amylase (from the salivary glands and the pancreas) breaks starch down to maltose, in the mouth and duodenum. Maltase breaks maltose down to glucose on the membranes of the epithelium lining the small intestine.
7. (Theory.) EXTENDED Pepsin and trypsin are both proteases. Explain why they work in different parts of the gut. [2]
Pepsin has an acidic optimum pH and works in the stomach; trypsin has an alkaline optimum and works in the small intestine.
8. (Theory.) EXTENDED Name the vessel in a villus that absorbs fatty acids and glycerol, and the vessels that absorb glucose. [2]
The lacteal; the capillaries.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • Royal Society of Biology — modelling digestion and absorption with Visking tubing
  • BBC Bitesize — enzymes in digestion