Explain why lipids are not soluble in water and describe one consequence of this property for living organisms.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Lipids are a group of biological molecules that include fats and oils. Unlike most biological molecules, lipids do not dissolve in water.
Model answer (5 marks)
Lipids are mainly composed of carbon, hydrogen and oxygen in a ratio that gives them very few polar bonds; they are therefore non‑polar and hydrophobic.
Water is a polar molecule and, according to the principle of "like dissolves like", it only dissolves other polar or charged substances.
Because lipids are non‑polar they cannot be surrounded by water molecules and so they are insoluble.
A consequence of this insolubility is that lipids can assemble into cell membranes.
The phospholipid bilayer forms a stable, selectively permeable barrier that prevents water‑soluble substances from passing freely through the membrane, thereby controlling the internal environment of the cell.
Water is a polar molecule and, according to the principle of "like dissolves like", it only dissolves other polar or charged substances.
Because lipids are non‑polar they cannot be surrounded by water molecules and so they are insoluble.
A consequence of this insolubility is that lipids can assemble into cell membranes.
The phospholipid bilayer forms a stable, selectively permeable barrier that prevents water‑soluble substances from passing freely through the membrane, thereby controlling the internal environment of the cell.
Examiner tips
- Use the "like dissolves like" principle to link polarity to solubility. Show the lipid composition (C,H,O) and the high C/H to O ratio. Mention the phospholipid bilayer as the key consequence. Explain how the barrier controls permeability.
Common mistakes
- Confusing lipids with carbohydrates or proteins. Failing to mention the C/H to O ratio. Giving a consequence that is not a membrane (e.g. energy storage) without linking it to solubility.
Mark scheme (5 marks)
- Lipids are made of carbon, hydrogen and oxygen (accept: non-polar molecules / hydrophobic)
- The ratio of hydrogen and carbon to oxygen is much higher than in carbohydrates / lipids have very few polar bonds
- Water is a polar molecule and only dissolves other polar / charged substances (like-dissolves-like principle)
- Named consequence — lipids can form cell membranes / phospholipid bilayer (accept: membranes are selectively permeable / control what enters and leaves the cell)
- 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 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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