2.5.11Structure and Functions in Living Organisms: Part 1
Small Intestine: Structure and Adaptations — Edexcel International GCSE Science Double Award (Modular) - Biology Unit 1 Revision Notes & Topical Questions
5 min read 6 parts Lesson 24 of 32
Section 1 of 6Absorption in the small intestine
What absorption is
- What absorption is Absorption is the movement of small digested molecules from the gut into the blood or lymph.
- Glucose and amino acids enter the blood; fatty acids and glycerol enter the lymph.
How molecules cross
- How molecules cross Small soluble molecules move mainly by diffusion,
- and sometimes by active transport when they must be taken up against a concentration gradient.
Water
- Water Water is absorbed by osmosis, mostly in the small intestine, with the remainder taken up in the large intestine.
Section 2 of 6Adaptations of the whole organ
Small intestine
Very long
- Very long The long tube gives food plenty of time to be digested and absorbed.
Folded lining with villi
- Folded lining with villi The inner surface is highly folded and covered in millions of finger-like villi, hugely increasing the surface area.
Muscular wall
- Muscular wall Peristalsis mixes food with enzymes and keeps it moving along the canal.
Enzymes in the lining
- Enzymes in the lining Cells of the wall make enzymes such as maltase that complete digestion right next to the absorbing surface.
Section 3 of 6Adaptations of a villus
| Feature | How it helps absorption |
|---|---|
| Large surface area (villi and microvilli) | More space for molecules to cross, so absorption is faster |
| Wall only one cell thick | Short diffusion distance, so molecules cross quickly |
| Dense network of blood capillaries | Carries glucose and amino acids away, keeping a steep concentration gradient |
| Central lacteal (lymph vessel) | Carries fatty acids and glycerol away in the lymph |
| Microvilli on surface cells | Further increase the surface area of each cell |
Section 4 of 6How a glucose molecule is absorbed
1High concentration in gut
- High concentration in gut After digestion the lumen contains a high concentration of glucose.
2Diffusion into villus
- Diffusion into villus Glucose diffuses across the thin villus wall into the cells.
3Into the capillary
- Into the capillary It passes into a blood capillary and is quickly swept away by the circulating blood.
4Gradient maintained
- Gradient maintained Removal keeps the concentration inside the villus low,
- so diffusion continues; the blood delivers glucose to cells for respiration.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Absorption | The movement of digested food molecules from the gut into the blood or lymph. |
| Villus | A tiny finger-like projection of the small intestine wall that increases surface area. |
| Microvilli | Microscopic projections on the surface of villus cells that increase the area further. |
| Lacteal | A lymph vessel in the centre of a villus that absorbs fatty acids and glycerol. |
| Concentration gradient | A difference in concentration between two regions, which drives diffusion. |
Section 6 of 6Exam technique
1
Saying fatty acids and glycerol enter the blood capillaries. They enter the lacteal and are carried in the lymph.
2
Giving 'large surface area' as the only adaptation. Link each feature to its effect: thin wall means short diffusion distance, capillaries mean steep gradient.
3
Mixing up digestion and absorption. Digestion breaks food down; absorption moves the products into the body.
4
Writing that villi are in the stomach or large intestine. They line the small intestine.
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