Describe the main difference between a polypeptide chain and a protein.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A polypeptide chain is a linear sequence of amino acids linked by peptide bonds.
A protein is a polypeptide that has folded into a specific three‑dimensional structure and performs a biological function.
The main difference is that a protein has a defined tertiary (and often quaternary) structure, whereas a polypeptide chain is just the amino‑acid sequence.
This structure determines the protein’s function, allowing it to act as an enzyme, structural component, signal molecule, etc.
A protein is a polypeptide that has folded into a specific three‑dimensional structure and performs a biological function.
The main difference is that a protein has a defined tertiary (and often quaternary) structure, whereas a polypeptide chain is just the amino‑acid sequence.
This structure determines the protein’s function, allowing it to act as an enzyme, structural component, signal molecule, etc.
Examiner tips
- Use the words ‘polypeptide chain’ and ‘protein’ exactly as defined; mention folding and function; keep answer to 4 short points.
- Show the link between structure and function to secure the functional significance mark.
Common mistakes
- Confusing a polypeptide with a protein and vice versa; not mentioning folding; ignoring the functional aspect of the difference.
Mark scheme (4 marks)
- Understanding of the definition of a polypeptide chain
- Understanding of the definition of a protein
- Explanation of the main difference between a polypeptide chain and a protein
- Understanding of the functional significance of the difference