2.7Unit 2 — Multicellular Organisms
Absorption of Materials — SQA National 5 Biology — SQA National 5 Biology Revision Notes & Topical Questions
5 min read 2 parts Lesson 7 of 7
Section 1 of 2Common features of absorption surfaces
- All specialised exchange and absorption surfaces share three fundamental adaptations to maximise the rate of diffusion:
- 1. Large surface area — Folded or branching architecture (e.g. millions of alveoli or villi) provides a vast area across which molecules can diffuse simultaneously.
- 2. Thin walls (short diffusion distance) — Epithelial barriers are only one cell thick, minimising the distance that gases and dissolved nutrients must travel.
- 3. Extensive capillary blood network — Dense blood capillary supply continuously transports absorbed materials away, maintaining a steep concentration gradient.
Section 2 of 2Alveoli in lungs and Villi in small intestine
| Absorption Organ | Specialised Structure | Absorbed Substances | Specific Structural Adaptations |
|---|---|---|---|
| Lungs | Alveoli (air sacs) | Oxygen diffuses into blood; Carbon dioxide diffuses into lungs | Moist inner lining allows gases to dissolve; thin squamous epithelium (one cell thick); surrounded by dense pulmonary capillary network |
| Small Intestine | Villi (finger-like projections) | Glucose, amino acids, fatty acids, glycerol | Each villus has a central lacteal and a surrounding blood capillary network; lined with single-cell-thick epithelium with microvilli |
- Fate of digested products in a villus:
- Blood capillaries: Absorb water-soluble breakdown products — glucose and amino acids — which are transported directly to the liver via the hepatic portal vein.
- Central lacteal: Part of the lymphatic system; absorbs water-insoluble fat digestion products — fatty acids and glycerol.
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