Explain how the structure of chitin makes it suitable for its role as a structural polysaccharide in the exoskeletons of arthropods and the cell walls of fungi.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Chitin is a polysaccharide made of repeating N‑acetylglucosamine (NAG) units linked by β‑1,4‑glycosidic bonds. The β‑1,4 bonds force adjacent monomers to alternate in orientation, so the chains lie flat and form straight, unbranched fibres. Adjacent chains hydrogen‑bond through their amino and acetyl groups, bundling into microfibrils that give the material high tensile strength. Because the chains are tightly packed and the β‑1,4 bonds are resistant to hydrolysis, chitin is insoluble and chemically stable, so it resists enzymatic degradation and maintains the structural integrity of arthropod exoskeletons and fungal cell walls.
Examiner tips
- Use the exact terms (β‑1,4‑glycosidic bonds, N‑acetylglucosamine, microfibrils).
- Show the chain‑flatness → straight fibres → tensile strength link.
- Mention insolubility and resistance to degradation.
Common mistakes
- Confusing α‑1,4 bonds with β‑1,4; describing chitin as branched; omitting the hydrogen‑bonding step.
Mark scheme (4 marks)
- Chitin is a polysaccharide composed of repeating N-acetylglucosamine (NAG) monomers linked by β-1,4-glycosidic bonds.
- The β-1,4-glycosidic bonds cause adjacent monomers to alternate orientation, allowing chitin chains to lie flat and form straight, unbranched fibres.
- Adjacent chitin chains form hydrogen bonds between their amino/acetyl groups, bundling into microfibrils that confer high tensile strength.
- Chitin is insoluble and chemically stable, meaning it resists enzymatic degradation by most organisms, maintaining structural integrity of the exoskeleton or cell wall.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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