A marathon runner's body uses lipids as a fuel source during a long-distance race. Explain how the structure of a lipid molecule relates to its function as an energy source, and state one other function of lipids in the human body.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Lipids are biological molecules that provide more than twice the energy per gram compared to carbohydrates. They are stored in adipose tissue and mobilised during prolonged exercise.
Model answer (5 marks)
Lipids are tri‑glycerols composed of a glycerol backbone and three fatty‑acid chains. The fatty acids are long chains of carbon and hydrogen atoms, giving the molecule a very high proportion of C–H bonds. When these bonds are oxidised in the mitochondria, a large amount of energy is released, so lipids store more than twice the energy per gram than carbohydrates.
Because the long hydrocarbon chains are non‑polar, lipids are insoluble in water. This property means they do not disturb the water potential of cells and can be stored safely in adipose tissue.
Other functions of lipids include forming the phospholipid bilayer of cell membranes, providing insulation and waterproofing, and protecting organs.
Because the long hydrocarbon chains are non‑polar, lipids are insoluble in water. This property means they do not disturb the water potential of cells and can be stored safely in adipose tissue.
Other functions of lipids include forming the phospholipid bilayer of cell membranes, providing insulation and waterproofing, and protecting organs.
Examiner tips
- Mention glycerol + fatty acids, C–H bonds, high energy release, insolubility, and give one extra function such as membrane structure or insulation
Common mistakes
- Confusing lipids with proteins or carbohydrates, or failing to state the high C–H bond content; not recognising the insolubility of lipids; giving a function that is not a lipid function (e.g. saying lipids are the main source of dietary fibre).
Mark scheme (5 marks)
- Lipids are made from / contain glycerol and fatty acids
- Lipids contain a high proportion of carbon–hydrogen bonds / are rich in hydrogen, which releases large amounts of energy when oxidised / broken down
- This means lipids store more energy per gram than carbohydrates
- Lipids are insoluble in water / do not dissolve, so they do not affect the water potential of cells / do not interfere with cell chemistry
- One other function of lipids in the human body — e.g. insulation (thermal), waterproofing the skin, forming cell membranes / phospholipid bilayer, protection of organs
Key terms in this question
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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