A-Level Biology May/June 2024 Q2(a): Describe the structure of a collagen molecule and the structure of a collagen fibre.

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  Structure of a Collagen Molecule

  A collagen molecule is a fibrous protein with a quaternary structure. It consists of three helical polypeptide chains that are tightly wound around each other to form a stable triple helix.

  These three chains are held together by numerous hydrogen bonds that form between them.

  The primary structure of each polypeptide chain is notable for its repeating amino acid sequence, where every third amino acid is glycine. Glycine is the smallest amino acid, which allows the three chains to pack very closely together within the triple helix.

  Structure of a Collagen Fibre

  Many individual collagen molecules assemble to form larger structures called fibrils, which in turn bundle together to form a collagen fibre.

  Within a fibril, the collagen molecules are arranged in parallel to one another. However, they are also staggered, meaning the ends of adjacent molecules do not align. This staggered arrangement eliminates any weak points along the fibril.

  To provide great tensile strength, strong covalent cross-links form between adjacent collagen molecules in the fibril. These cross-links hold the molecules firmly together, making the resulting collagen fibre very strong and inelastic.

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  How the marks are awarded

  • B1 — Describing the molecule as having three helical polypeptide chains, forming a triple helix (quaternary structure).
  • B1 — Stating that hydrogen bonds form between the polypeptide chains to hold the triple helix together.
  • B1 — Correctly identifying that every third amino acid in the polypeptide chain is glycine.
  • B1 — Describing the arrangement of molecules within a fibre as being staggered relative to one another.
  • B1 — Stating that covalent cross-links form between adjacent collagen molecules to hold the fibre together.

  Common mistakes

  • Confusing the bonding: stating that covalent bonds hold the triple helix together (it's hydrogen bonds) or that hydrogen bonds hold the fibre together (it's covalent cross-links).
  • Only describing the collagen molecule and failing to describe the structure of the collagen fibre, or vice versa, thereby limiting the maximum marks achievable.
  • Giving vague descriptions, such as 'strong bonds' without specifying the type (hydrogen or covalent) and location (between chains or between molecules).
  • Confusing the structure of collagen with haemoglobin, for example by mentioning alpha and beta subunits or the presence of a haem group.

  Examiner tip: For structural proteins, always learn the details at each level of organisation: from the primary sequence to the quaternary molecule and finally to the macromolecular fibre.

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