Amino acids can join together to form peptides and proteins. Explain how amino acids are linked together to form a dipeptide, and describe how the structure of a protein is related to its function.

WJEC A-Level Chemistry (Wales) — 4.7 Amino acids, peptides and proteins · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Two amino acids join together to form a dipeptide. A peptide bond is formed between the carboxyl group of one amino acid and the amino group of the other, releasing a molecule of water (a condensation reaction). Many amino acids join together in a chain to form a polypeptide or protein. The sequence or order of amino acids determines the protein’s shape, and this specific shape allows the protein to carry out its function.

Examiner tips

  • Use the term "peptide bond" and mention water is released.
  • Explain that the sequence determines the 3‑D structure and hence function.
  • Show the condensation reaction explicitly.
  • Keep the answer concise and to the point.

Common mistakes

  • Forgetting to mention that water is released.
  • Using the wrong term (e.g., "amide bond" instead of "peptide bond").
  • Failing to link the sequence to the protein’s shape and function.

Mark scheme (5 marks)

  1. Two amino acids join together to form a dipeptide
  2. A peptide bond is formed between the two amino acids
  3. Water is released / eliminated during the formation of the peptide bond (condensation reaction)
  4. Many amino acids join together in a chain to form a polypeptide / protein
  5. The sequence / order of amino acids determines the shape of the protein, and the specific shape allows the protein to carry out its function

Key terms in this question

amino acid · dipeptide · protein

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