Lipids are hydrophobic because most of their structure consists of non-polar hydrocarbon regions, which cannot form favourable interactions such as hydrogen bonds with polar water molecules. Consequently, lipids do not dissolve readily in water.
This is assessed at both SL and HL in IB Biology subtopic B1.1, Carbohydrates and lipids.
The Molecular Explanation
Water is polar because oxygen attracts the shared electrons more strongly than hydrogen. This gives oxygen a partial negative charge and the hydrogen atoms partial positive charges, allowing water molecules to form hydrogen bonds with each other and with other polar or charged substances.
Most lipids contain long hydrocarbon chains made mainly of carbontocarbon and carbontohydrogen bonds. These bonds have little polarity, so the hydrocarbon regions cannot form hydrogen bonds with water. Water molecules therefore remain hydrogen-bonded to one another while excluding and surrounding the lipid molecules.
Lipids tend to group together, reducing the hydrocarbon surface exposed to water. This behaviour is called the hydrophobic effect. It helps explain both fat droplets and the spontaneous formation of phospholipid bilayers.
| Lipid type or region | Interaction with water |
|---|---|
| Fatty acid hydrocarbon chain | Non-polar and hydrophobic |
| Triglyceride | Mostly non-polar and insoluble in water |
| Phospholipid phosphate head | Polar and hydrophilic |
| Phospholipid fatty acid tails | Non-polar and hydrophobic |
Phospholipids are therefore amphipathic: they have a hydrophilic phosphate-containing head and hydrophobic fatty acid tails. In water, their heads face the aqueous environment while their tails cluster away from it, producing a bilayer.
A common misconception is that water actively “repels” lipids. More precisely, non-polar lipids cannot replace the favourable hydrogen bonds between water molecules, so they separate from water.
Exam Technique
For an explain question, link molecular structure to behaviour: non-polar hydrocarbon regions → no hydrogen bonding with polar water → low solubility. Do not simply state that lipids are hydrophobic; explain why using polarity and intermolecular interactions.
