Explain how the small intestine is adapted to absorb the products of digestion efficiently.

AQA A-Level Biology (7402) — 3.3.3 Digestion and Absorption · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

The small intestine has finger‑like villi on its inner wall, which increase the surface area for absorption. Each villus is covered with a single layer of epithelial cells, so the distance for diffusion is very short. Villi contain a dense capillary network that quickly removes absorbed molecules, maintaining a concentration gradient. In addition, villi have lacteals – lymphatic vessels – that absorb fatty acids and glycerol from digested fats.

Examiner tips

  • Mention villi and their surface‑area advantage first; then detail the thin epithelium, rich capillary supply, and lacteals for fats.
  • Use the exact terms: villi, capillaries, lacteals, one‑cell‑thick epithelium.
  • Show the logical sequence: structure → surface area → diffusion distance → transport.
  • Keep the answer concise – 5 marks only.

Common mistakes

  • Confusing villi with microvilli or omitting the lacteals; students often describe only surface area and forget the capillaries and lymphatics.
  • Using vague terms like ‘many cells’ instead of ‘one‑cell‑thick epithelium’ or ‘rich blood supply’.

Mark scheme (5 marks)

  1. The small intestine has villi (finger-like projections) on its inner wall
  2. Villi increase the surface area (for absorption)
  3. Villi have a rich / dense blood supply (capillary network) to maintain a concentration gradient / carry absorbed molecules away quickly
  4. Villi have a thin wall / epithelium (one cell thick) so the distance for diffusion is short
  5. Villi contain lacteals to absorb fatty acids and glycerol (lipids / fats) into the lymphatic system

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