Proteins are large molecules that are essential for life. Describe the structure of a protein, starting from its amino acid building blocks.

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

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

Model answer (5 marks)

Proteins are made up of many amino acids.

Amino acids are joined together by peptide bonds.

The joining of amino acids is a condensation reaction, so water is released.

Two amino acids joined together form a dipeptide; a small number of amino acids joined together form a peptide.

A long chain of many amino acids forms a polypeptide, which is a protein.

Examiner tips

  • Use the exact terms: amino acids, peptide bond, condensation reaction, dipeptide, peptide, polypeptide, protein.
  • Show the sequence of ideas – building blocks, bond type, reaction, small chain, long chain.
  • Keep each point short and to the point to fit the 5 marks.

Common mistakes

  • Using ‘linkage’ instead of ‘peptide bond’
  • Omitting the fact that water is released
  • Confusing ‘dipeptide’ with ‘polypeptide’

Mark scheme (5 marks)

  1. Proteins are made up of (many) amino acids
  2. Amino acids are joined together by peptide bonds
  3. A condensation reaction occurs when amino acids join / water is released when amino acids join
  4. Two amino acids joined together form a dipeptide / a small number of amino acids joined together form a peptide
  5. A long chain of many amino acids forms a polypeptide / protein

Key terms in this question

amino acid · protein

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