Cholesterol is both hydrophobic and hydrophilic, making it amphipathic (or amphiphilic).
Cholesterol's structure dictates its dual nature:
- Hydrophilic Portion: The hydroxyl (-OH) group on one end of the cholesterol molecule is polar and therefore attracted to water, making it hydrophilic. This group can interact favorably with the polar heads of phospholipids in cell membranes.
- Hydrophobic Portion: The majority of the cholesterol molecule consists of fused carbon rings and a hydrocarbon tail. These are nonpolar and repel water, making this part of the molecule hydrophobic. This bulky, rigid structure allows it to embed within the hydrophobic core of cell membranes, interacting with the fatty acid tails of phospholipids.
Because it contains both hydrophobic and hydrophilic regions, cholesterol can insert itself into cell membranes, with its hydrophilic hydroxyl group oriented towards the aqueous environment and its hydrophobic ring system buried within the lipid bilayer. This amphipathic property is crucial for cholesterol's role in modulating membrane fluidity and permeability.
