Explain how the structural features of waxes make them suitable for their role in reducing water loss from the surfaces of leaves and insect cuticles.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Waxes are esters of long‑chain fatty acids and long‑chain alcohols, not glycerol, giving a large non‑polar, hydrophobic molecule.
The long, saturated hydrocarbon chains allow wax molecules to pack tightly together, forming a solid or semi‑solid layer at biological temperatures.
Because of this hydrophobicity, water molecules cannot form hydrogen bonds with the wax and are therefore repelled or cannot pass through the layer.
The solid, continuous wax layer acts as a barrier that greatly reduces evaporation or transpiration of water from the surface, limiting water loss.
The long, saturated hydrocarbon chains allow wax molecules to pack tightly together, forming a solid or semi‑solid layer at biological temperatures.
Because of this hydrophobicity, water molecules cannot form hydrogen bonds with the wax and are therefore repelled or cannot pass through the layer.
The solid, continuous wax layer acts as a barrier that greatly reduces evaporation or transpiration of water from the surface, limiting water loss.
Examiner tips
- Use the word ‘hydrophobic’ to link structure to function.
- Mention the tight packing of saturated chains.
- Show that the wax layer is solid at body temperature.
- Explain that water cannot penetrate the layer, reducing evaporation.
Common mistakes
- Confusing waxes with other lipids such as triglycerides.
- Failing to mention the long saturated chains or the solid nature of the layer.
- Using vague terms like ‘water‑repellent’ without explaining the molecular basis.
Mark scheme (4 marks)
- Waxes are esters formed from long-chain fatty acids and long-chain alcohols (rather than glycerol), giving a large non-polar / hydrophobic molecule.
- The long, saturated hydrocarbon chains allow wax molecules to pack tightly together, forming a solid / semi-solid layer at biological temperatures.
- The hydrophobic nature of wax means water molecules cannot form hydrogen bonds with it and are therefore repelled / cannot pass through the wax layer.
- The solid, continuous wax layer therefore acts as a barrier that greatly reduces evaporation / transpiration of water from the surface, limiting water loss.
Key terms in this question
wax · cuticle
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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