# 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.

> AQA A-Level Biology (7402) — 3.1.3 Lipids · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. Lipids are made from / contain glycerol and fatty acids
2. Lipids contain a high proportion of carbon–hydrogen bonds / are rich in hydrogen, which releases large amounts of energy when oxidised / broken down
3. This means lipids store more energy per gram than carbohydrates
4. Lipids are insoluble in water / do not dissolve, so they do not affect the water potential of cells / do not interfere with cell chemistry
5. 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

- [lipid](https://www.gradenine.co.uk/glossary/lipid)

## Related

- [Revision notes for AQA A-Level Biology (7402)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-marathon-runner-s-body-uses-lipids-3496ac90) · Published by Druglandscape Ltd.