Explain how the sequence of amino acids in a polypeptide determines the three-dimensional shape of a globular protein.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The primary structure of a protein is the linear sequence of amino acids, which is fixed by the codon sequence in the gene/mRNA. This sequence determines the pattern of side‑chains (R groups) along the polypeptide. The R groups interact through hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions, causing the chain to fold spontaneously. Hydrophobic side‑chains are driven into the interior of the protein, away from water, stabilising the tertiary structure. Because the pattern of R groups is fixed, a specific and unique three‑dimensional conformation is produced, which in turn determines the protein’s function.
Examiner tips
- Use the term ‘primary structure’ for the amino‑acid sequence; mention codons/gene/mRNA. Explain the key non‑covalent interactions and the role of hydrophobic collapse. Show that the fixed sequence leads to a unique 3‑D shape and link this to function. Keep each point brief and use the exact terminology from the mark scheme.
Common mistakes
- Confusing primary structure with secondary structure; forgetting to mention codons/gene. Omitting the role of hydrophobic interactions or the unique shape–function relationship. Using vague terms like ‘folding’ without specifying the interactions involved.
Mark scheme (4 marks)
- The primary structure (sequence of amino acids) is determined by the sequence of codons in the gene / mRNA.
- Interactions between R groups of amino acids (e.g. hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions) cause the polypeptide to fold spontaneously.
- Hydrophobic R groups are orientated towards the interior of the protein, away from the aqueous environment, stabilising the tertiary structure.
- Because the sequence of R groups is fixed by the primary structure, a specific and unique three-dimensional conformation results, which determines the protein's function.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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