# Explain why lipids are not soluble in water and describe one consequence of this property for living organisms.

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

> Lipids are a group of biological molecules that include fats and oils. Unlike most biological molecules, lipids do not dissolve in water.

## Mark scheme (5 marks)

1. Lipids are made of carbon, hydrogen and oxygen (accept: non-polar molecules / hydrophobic)
2. The ratio of hydrogen and carbon to oxygen is much higher than in carbohydrates / lipids have very few polar bonds
3. Water is a polar molecule and only dissolves other polar / charged substances (like-dissolves-like principle)
4. Named consequence — lipids can form cell membranes / phospholipid bilayer (accept: membranes are selectively permeable / control what enters and leaves the cell)
5. Explanation of consequence — because lipids do not dissolve, they form a stable barrier / prevent water-soluble substances from passing freely through the membrane

## Key terms

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

## 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/explain-why-lipids-are-not-soluble-da02486b) · Published by Druglandscape Ltd.