Explain why proteins must be chemically digested before they can be used by the body, and describe the role of gastric juice in this process.

Cambridge International IGCSE Biology (0610) — 7.4 Chemical digestion · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Proteins are large, insoluble molecules that cannot pass through the intestinal wall, so they must be broken down into smaller, soluble units before absorption.
Chemical digestion cleaves peptide bonds, producing amino acids and small peptides that can be absorbed.
Gastric juice contains the protease pepsin, which hydrolyses proteins.
The hydrochloric acid in gastric juice creates an acidic pH (≈pH 2) that optimises pepsin activity and also kills many harmful microorganisms in food.

Examiner tips

  • Use the exact terms ‘proteins’, ‘amino acids’, ‘pepsin’, ‘hydrochloric acid’, and ‘acidic pH’.
  • Show the cause–effect link: large → cannot absorb; digestion → small → absorbable.
  • Mention both roles of HCl: optimal pH and antimicrobial action.
  • Keep the answer concise, no extra wording.

Mark scheme (4 marks)

  1. Proteins are large / insoluble molecules that cannot be absorbed through the wall of the intestine
  2. Chemical digestion breaks proteins down into amino acids (smaller, soluble molecules) so they can be absorbed
  3. Gastric juice contains protease enzymes (pepsin) which break down protein
  4. Hydrochloric acid in gastric juice provides an optimum (acidic) pH for pepsin / protease activity AND/OR kills harmful microorganisms in food

Key terms in this question

gastric juice

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