Some molecules in cells are hydrophobic because they are non-polar and cannot form favorable interactions, especially hydrogen bonds, with polar water molecules. Instead of dissolving, they tend to associate with one another and minimize contact with water.
The Reasoning
Water is a polar molecule because oxygen attracts the shared electrons more strongly than hydrogen. This produces partial charges, allowing water molecules to form hydrogen bonds with one another and with ions or other polar molecules.
For a substance to dissolve, water must form sufficiently favorable attractions with its particles. Polar and charged substances are therefore generally hydrophilic: water molecules surround them in hydration shells and keep them dispersed.
Non-polar molecules, such as many lipids, contain mainly carbon/carbon and carbon/hydrogen bonds with little charge separation. They cannot form hydrogen bonds with water. Water molecules remain hydrogen-bonded to each other and organize around the non-polar molecules, causing the non-polar molecules to cluster together. This is the hydrophobic effect.
A common misconception is that water chemically “repels” hydrophobic molecules. It does not. Their aggregation occurs because water forms more favorable interactions with itself than with the non-polar molecules.
| Type of substance | Behaviour in water | Biological example |
|---|---|---|
| Polar or charged | Usually dissolves because it interacts with water | Glucose or mineral ions |
| Non-polar | Usually remains insoluble and clusters away from water | Triglycerides or steroids |
| Amphipathic | Hydrophilic region contacts water; hydrophobic region avoids it | Phospholipids |
This property is essential in cells. Amphipathic phospholipids spontaneously form bilayers: their hydrophilic phosphate heads face aqueous cytoplasm or tissue fluid, while their hydrophobic fatty-acid tails face inward. The resulting membrane separates aqueous compartments and provides a barrier to many ions and polar molecules.