What lipids are found in the plasma membrane?

There are three major classes of lipids found in the plasma membrane – the phosphoglycerides, sphingolipids and sterols.

The phosphoglycerides and sphingolipids can be combined as one class, the phospholipids. Phospholipids are the most abundant lipids found in the plasma membrane. They are made up of a polar head group and two hydrophobic fatty acid tails. The fatty acid tails differ in length and may contain anywhere between 14 and 24 carbon atoms. One tail is unsaturated and has one or more cis-double bonds that create a small kink in the tail. The second tail is saturated and straight as it does not have any cis-double bonds. The differences in the length and saturation of the fatty acid tails impact membrane fluidity by affecting the ability of phospholipid molecules to pack against each other.  

Sterols have a single polar hydroxyl head group attached to a rigid steroid ring structure and a short non-polar hydrocarbon tail. Cholesterol is the major sterol component in cell membranes of animals. Other eukaryotic cell membranes may have different sterols. Sterols are smaller than phospholipids. They insert into the lipid bilayer with their hydroxyl head groups oriented with the phospholipid polar groups. This arrangement aligns the sterol’s rigid ring structure with the hydrocarbon tail of phospholipid, which reduces phospholipid mobility. This stiffening effect does not affect membrane fluidity but it reduces the water-soluble permeability of the bilayer.