Describe the structure of a lipid molecule and explain why lipids are important in the human body.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
A lipid is a glycerol‑based molecule.
1. It contains one glycerol backbone.
2. Three fatty acid chains are esterified to the glycerol.
3. The long hydrocarbon chains make the molecule hydrophobic.
4. Lipids store energy – when oxidised they release more energy than carbohydrates.
5. They provide thermal insulation and form the phospholipid bilayer of cell membranes.
1. It contains one glycerol backbone.
2. Three fatty acid chains are esterified to the glycerol.
3. The long hydrocarbon chains make the molecule hydrophobic.
4. Lipids store energy – when oxidised they release more energy than carbohydrates.
5. They provide thermal insulation and form the phospholipid bilayer of cell membranes.
Examiner tips
- Use the exact terms ‘glycerol’, ‘fatty acid’, ‘esterified’, ‘hydrophobic’, ‘energy storage’, ‘insulation’, ‘cell membranes’.
- Show the structure in words: glycerol + three fatty acids = triglyceride.
- Explain the function before listing the examples.
Mark scheme (5 marks)
- A lipid is made of one glycerol molecule
- A lipid is made of three fatty acid molecules joined to the glycerol
- Lipids are used as a store of energy / provide energy when respired
- Lipids are used for insulation (to keep the body warm) / thermal insulation
- Lipids are used to make cell membranes
Key terms in this question
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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