A student is investigating a protein found in human hair. The protein has a specific three-dimensional shape that is essential for its function. Describe the structure of an amino acid and explain how a sequence of amino acids can fold into a three-dimensional protein shape, including why this shape is important for the protein's function.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Human hair contains a protein called keratin. Keratin's three-dimensional shape gives it structural properties that make hair strong and flexible.
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). The R group gives each amino acid its unique chemical properties.
When amino acids are linked by peptide bonds, the resulting polypeptide chain can interact through the R groups. Hydrogen bonds, ionic bonds, van der Waals forces and disulphide bridges between side chains drive the chain to fold into a specific three‑dimensional shape.
The folded shape determines the protein’s function; for example, keratin’s structure gives hair strength and flexibility. If the shape is altered, the protein cannot perform its role.
When amino acids are linked by peptide bonds, the resulting polypeptide chain can interact through the R groups. Hydrogen bonds, ionic bonds, van der Waals forces and disulphide bridges between side chains drive the chain to fold into a specific three‑dimensional shape.
The folded shape determines the protein’s function; for example, keratin’s structure gives hair strength and flexibility. If the shape is altered, the protein cannot perform its role.
Examiner tips
- Use the exact terms – amino group, carboxyl group, R group, peptide bond, folding interactions, functional shape.
- Show the chain of reasoning: structure of amino acid → peptide chain → side‑chain interactions → 3D shape → function.
- Keep it concise – 5 marks, no extra words.
Common mistakes
- Confusing the amino acid backbone with the side chain; forgetting the α‑carbon.
- Lacking the link between 3D shape and function; saying only that shape is ‘important’ without explaining why.
Mark scheme (5 marks)
- An amino acid contains an amino group (-NH2)
- An amino acid contains a carboxylic acid group (-COOH)
- Different amino acids have different R groups / side chains, giving each amino acid different properties
- Interactions between R groups / side chains in the chain of amino acids cause the protein to fold into a specific three-dimensional shape
- The specific three-dimensional shape is essential for the protein's function, so if the shape changes the protein can no longer perform its function
Key terms in this question
amino acid · three-dimensional shape · protein
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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