Proteins are polymers made from monomers called amino acids. Explain how a protein polymer is formed from amino acid monomers, and explain how the protein can be broken down again to release individual amino acids.

AQA A-Level Biology (7402) — 3.1.1 Monomers and Polymers · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

A protein polymer is formed when many amino‑acid monomers join together by peptide bonds. The carboxyl group of one amino acid reacts with the amino group of the next, releasing a molecule of water – a dehydration synthesis reaction. The resulting long chain is the protein.

The protein can be broken down again by hydrolysis. In digestion, enzymes add water across the peptide bonds, breaking them and releasing the individual amino acids.

Examiner tips

  • Use the terms "peptide bond", "dehydration synthesis" and "hydrolysis". Show the water molecule is removed in synthesis and added in hydrolysis. Mention enzymes (e.g. pepsin, trypsin) only if you have time – not required for full marks.
  • common_mistakes
  • :
  • Forgetting to mention that water is removed in synthesis and added in hydrolysis. Using vague terms like "join together" without specifying peptide bonds or dehydration synthesis. Not recognising that digestion is a hydrolysis reaction.

Mark scheme (5 marks)

  1. Amino acids are the monomers that join together to form a protein polymer
  2. Many amino acids join together (by chemical bonds) to form the polymer / large molecule
  3. The process of joining monomers together requires a chemical reaction
  4. The protein polymer can be broken down by digestion / hydrolysis
  5. Water is used / added to break the bonds during hydrolysis / digestion

Key terms in this question

monomer · polymer · amino acid

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