Explain why large food molecules must be broken down before they can be absorbed into the blood.

Eduqas GCSE Biology — 1.3.2 Digestion and absorption · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Large food molecules are too big to pass through the wall of the small intestine, so they cannot be absorbed directly into the blood.
Digestion breaks these large, insoluble molecules into smaller, soluble ones.
The smaller, soluble molecules can then pass through the intestinal wall into the bloodstream.
For example, starch is broken down into glucose, proteins into amino acids, and fats into fatty acids and glycerol.

Examiner tips

  • Use the word ‘break down’ and mention ‘soluble’ to show understanding of digestion’s purpose.
  • Give at least one specific example of a large molecule and its smaller product.
  • Explain the barrier (intestinal wall) and the route (into blood).

Common mistakes

  • Failing to mention that the intestinal wall is the barrier. Not providing a specific example of a large molecule and its breakdown product. Using vague terms like ‘small’ without specifying ‘soluble’ or ‘absorbed’.

Mark scheme (4 marks)

  1. Large molecules cannot pass through the wall of the small intestine / gut wall
  2. Digestion breaks large, insoluble molecules into smaller, soluble molecules
  3. Small / soluble molecules can pass through the wall of the small intestine into the blood
  4. Named example of a large molecule broken down to a named smaller molecule (e.g. starch broken down to glucose / proteins broken down to amino acids / fats broken down to fatty acids and glycerol)

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