Explain collagen fibres and tensile strength
Collagen’s triple-helical molecules align and bundle into fibres that resist tension in connective tissues.
Section 2.3 · Proteins
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain syllabus concepts; self-checked practice does not measure practical competence or qualify as teacher-reviewed assessment.
Before you begin
- Recognise that biological molecules have structures that relate to their roles in cells and organisms.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many polypeptide chains form a collagen triple helix?
What you will learn
- Describe collagen molecules and their arrangement into fibres
- Relate collagen molecule and fibre structure to tensile strength
Three chains form a collagen molecule
A collagen molecule consists of three polypeptide chains twisted together into a triple helix. The chains have a repeated amino-acid pattern that allows close packing, with glycine at regular positions and other amino acids contributing to the structure.
Several collagen molecules are arranged in parallel, staggered arrays. Bonds and interactions between neighbouring molecules help assemble them into collagen fibrils and fibres. Do not confuse one triple-helical molecule with the larger fibre made from many molecules.
Fibre organisation resists pulling
Collagen fibres are strong and have high tensile strength: they resist being pulled apart. Their bundled, cross-linked arrangement supports tissues such as tendons, ligaments, skin and the organic matrix of bone.
A structure-function explanation should move from triple helix to aligned bundles and then to resistance to tension. The fibre is not a single giant polypeptide; it is an assembly of many collagen molecules.
Pause and explain
What mechanical property is associated with collagen fibres?
Worked example
A tissue must resist strong pulling forces. Explain how collagen molecule and fibre structure help.
Show the worked solution
- Three chains make one triple-helical molecule.
- Many molecules align and bundle.
- The fibre resists tension.
Answer Three polypeptide chains form each collagen triple helix. Many collagen molecules align in staggered bundles and assemble into fibres; interactions between molecules help the fibres resist tensile forces in the tissue.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe how collagen molecules are arranged to form fibres.
Give me a hint
Move from chains to molecule to bundles.
Compare my reasoning
- Three polypeptide chains form a triple helix.
- Many molecules align and assemble into fibrils and fibres.
Three polypeptide chains twist together to form a collagen triple helix. Many collagen molecules arrange in parallel, staggered arrays and assemble into fibrils and then fibres.
Look for these in your work
- Triple helix described
- Molecular assembly explained
Relate collagen fibre structure to function in a tendon.
Give me a hint
Use resistance to pulling.
Compare my reasoning
- Aligned molecules form strong bundles.
- The fibres have tensile strength.
Collagen molecules align and bundle into fibres with high tensile strength. These fibres resist pulling forces, helping a tendon transmit force between muscle and bone.
Look for these in your work
- Structure linked to tensile strength
- Tissue role described
A diagram shows one collagen molecule labelled as a fibre. Correct the scale of organisation.
Give me a hint
Distinguish the triple helix from a bundle.
Compare my reasoning
- One collagen molecule is a triple helix of three chains.
- Many molecules assemble into a fibre.
One collagen molecule consists of three polypeptide chains twisted into a triple helix. Many collagen molecules align and assemble into fibrils and fibres; the fibre is not a single molecule.
Look for these in your work
- Molecule structure
- Fibre assembly
Common mistakes
- Calling a collagen fibre a single polypeptide
- Describing collagen as a globular protein
- Explaining strength without connecting molecular arrangement to tension
What can you explain now?
How many polypeptide chains form a collagen triple helix?
Compare with the explanation
Three
Collagen molecules contain three intertwined chains.
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Make it stick
How do collagen chains, molecules and fibres build tensile strength?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.