Explain why lipids are used as waterproofing agents on the surfaces of leaves and in the waxy cuticle of insects, and outline the elements that make up a lipid molecule.

AQA A-Level Biology (7402) — 3.1.3 Lipids · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Plants and insects both produce lipid-based waxy coatings on their outer surfaces. These coatings reduce water loss and provide protection.

Model answer (5 marks)

Lipids are insoluble in water because they are hydrophobic.

Because of this hydrophobicity, water cannot penetrate the waxy layer, so evaporation from the leaf surface or insect cuticle is reduced.

This prevents dehydration, helping the plant or insect conserve water.

A lipid molecule is made up of carbon (C) and hydrogen (H) atoms, and usually contains oxygen (O) atoms as well.

Examiner tips

  • Use the word ‘hydrophobic’ to show understanding of insolubility.
  • Explain the benefit (prevention of dehydration) to link to the function.
  • Mention C, H and O to satisfy the structure part.
  • Keep the answer concise – 5 marks only.

Common mistakes

  • Forgetting to state that lipids are hydrophobic.
  • Not linking the waterproofing to dehydration prevention.
  • Omitting the oxygen atom in the structure description.

Mark scheme (5 marks)

  1. Lipids are insoluble in water (hydrophobic / do not dissolve in water)
  2. Because lipids are insoluble/hydrophobic, water cannot pass through the waxy layer, reducing water loss by evaporation
  3. This is an advantage for the plant / insect because it helps prevent dehydration (conserves water)
  4. Lipids contain carbon (C) and hydrogen (H)
  5. Lipids also contain oxygen (O) — accept: carbon, hydrogen, and oxygen

Key terms in this question

lipid · waxy cuticle · elements

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