Cholesterol is found embedded within the cell membrane, specifically between the phospholipids in the bilayer. Its rigid ring structure interacts with the hydrophobic fatty acid tails of phospholipids, being located primarily in the hydrophobic core of the membrane. Cholesterol is distributed asymmetrically between the two monolayers (leaflets) of the plasma membrane, with a higher concentration in the outer leaflet, often associated with sphingomyelin.
Location in the Membrane
- Cholesterol molecules insert into the membrane with their hydrophobic rings aligned alongside the fatty acid tails of the phospholipids in the inner part of the bilayer.
- The small polar hydroxyl group of cholesterol orients toward the membrane surface, near the polar head groups of the phospholipids.
- It resides predominantly in the hydrophobic interior of the membrane bilayer, stabilizing the lipid structure by filling spaces between phospholipid molecules.
Distribution
- Cholesterol composition can comprise a large portion (up to 50%) of the plasma membrane lipids.
- The outer leaflet of the plasma membrane tends to have more cholesterol (about 63% of total cholesterol) than the inner leaflet, due to interactions with sphingomyelin and membrane lipid asymmetry.
- Cholesterol-rich areas, called lipid rafts, occur mainly in the outer leaflet, where cholesterol clusters with sphingolipids to form more ordered microdomains that regulate membrane fluidity and protein localization.
Functional Implications
- Cholesterol modulates membrane fluidity, making the membrane less permeable and more stable.
- It prevents phospholipids from packing too closely, maintaining membrane flexibility while providing structural support.
In summary, cholesterol is embedded within the hydrophobic core of the cell membrane bilayer, primarily concentrated in the outer leaflet, interacting closely with phospholipids to maintain membrane stability and fluidity.