Amino acids can join together to form peptides and proteins. Describe how amino acids are structured and explain how they join together to form a dipeptide, including the type of bond formed and the small molecule released.

Eduqas A-Level Chemistry — 4.7 Amino acids, peptides and proteins · Describe and Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

An amino acid has a central ‘α’ carbon bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom and a variable side chain (R group). When two amino acids join, the amino group of one reacts with the carboxyl group of the other, forming a peptide bond. This is a condensation reaction, so a molecule of water (H₂O) is released.

Examiner tips

  • Use the exact terms – amino group, carboxyl group, peptide bond, condensation reaction, water released.
  • Show the reaction as –NH₂ + –COOH → –NH–CO– + H₂O.
  • Mention the central α‑carbon and the R side chain to demonstrate full structure.
  • Keep the answer concise – 5 marks only.

Common mistakes

  • Writing the bond as an ionic bond instead of a covalent peptide bond.
  • Forgetting to mention that water is released (or incorrectly stating it is produced).
  • Omitting the central α‑carbon or the R side chain in the description of the amino acid.

Mark scheme (5 marks)

  1. An amino acid contains an amino group (–NH₂)
  2. An amino acid contains a carboxyl (carboxylic acid) group (–COOH)
  3. The amino group of one amino acid reacts with the carboxyl group of another amino acid
  4. A peptide bond is formed between the two amino acids
  5. Water is released / produced during the reaction (condensation reaction)

Key terms in this question

amino acid · dipeptide

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