Explain how the primary structure of a protein determines its three-dimensional shape and, therefore, its function.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. The primary structure is the linear sequence of amino acids in a polypeptide chain.
2. The R groups of the amino acids interact through hydrogen bonds, ionic bonds, disulfide bonds and hydrophobic interactions, producing secondary and tertiary folding.
3. Because the sequence of amino acids is fixed, it dictates the final three‑dimensional (tertiary) shape of the protein.
4. The specific 3‑D shape creates a unique active or binding site, so the protein can only interact with complementary molecules and perform its particular function.
2. The R groups of the amino acids interact through hydrogen bonds, ionic bonds, disulfide bonds and hydrophobic interactions, producing secondary and tertiary folding.
3. Because the sequence of amino acids is fixed, it dictates the final three‑dimensional (tertiary) shape of the protein.
4. The specific 3‑D shape creates a unique active or binding site, so the protein can only interact with complementary molecules and perform its particular function.
Examiner tips
- Use the exact terminology: primary, secondary, tertiary, R groups, active site.
- Show the causal chain: sequence → interactions → folding → function.
- Keep each point short and to the point to hit the 4 marks.
Common mistakes
- Confusing primary structure with secondary structure.
- Failing to mention the role of R‑group interactions in folding.
- Giving a generic statement about function without linking it to the 3‑D shape.
Mark scheme (4 marks)
- Primary structure is the specific sequence of amino acids in a polypeptide chain
- The R groups of amino acids interact with each other (e.g. hydrogen bonds, ionic bonds, disulfide bonds, hydrophobic interactions) to produce secondary and tertiary folding
- The sequence of amino acids (primary structure) therefore dictates the final three-dimensional (tertiary) shape of the protein
- The specific three-dimensional shape produces a specific active site / binding site, so the protein can only interact with complementary molecules and carry out its particular function
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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