A Levels Biology (9700)•9700/12/O/N/19

Explanation
Bonds in Fibrous Protein Chains
Steps:
- Fibrous proteins consist of linear amino acid chains linked by peptide bonds forming the backbone.
- Peptide bonds (Y) covalently connect the carboxyl group of one amino acid to the amino group of the next.
- Hydrophobic interactions (X) occur between non-polar side chains, stabilizing the protein's elongated structure.
- Analyze the diagram: Y links sequential residues, while X aligns side chains in a non-covalent manner.
Why B is correct:
- Peptide bonds define the primary structure by amide linkages (–CO–NH–) between amino acids, while hydrophobic interactions drive folding of non-polar residues away from water, as per protein folding principles.
Why the others are wrong:
- A: Disulfide bonds are covalent S–S links between cysteines, not hydrophobic; hydrophobic interactions are non-covalent.
- C: Hydrogen bonds stabilize secondary structures like alpha helices, but ionic bonds involve charged side chains, not matching chain linkages.
- D: Ionic bonds form between oppositely charged residues, but Y is specifically the covalent peptide bond, not ionic.
Final answer: B
Topic: Proteins
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