Membrane lipids

The three major classes of membrane lipids are phospholipids, glycolipids, and cholesterol.

Phospholipids
Phospholipids and glycolipids consist of two long, nonpolar (hydrophobic) hydrocarbon chains linked to a hydrophilic head group.

In the phospholipids the head consist of phosphorylated either:
 * Glycerol (and hence the name phosphoglycerides given to this group of lipids).
 * Sphingosine (with only one member - sphingomyelin).

Glycolipids
In the glycolipids the head contains of sphingosine with one or several sugar units attached to it. The hydrophobic chains belong either to:
 * two fatty acids - in the case of the phosphoglycerides.
 * one FA and the hydrocarbon tail of sphingosine - in the case of sphingomyelin and the glycolipids.

Fatty acids
The fatty acids in phospho- and glycolipids usually contain an even number of carbon atoms, typically between 14 and 24. The 16- and 18-carbon FAs are the most common ones. FAs may be saturated or unsaturated, with the configuration of the double bonds nearly always cis. The length and the degree of unsaturation of FAs chains have a profound effect on membranes' fluidity.

Phosphoglycerides
In phosphoglycerides, the hydroxyl groups at C-1 and C-2 of glycerol are esterified to the carboxyl groups of the FAs. The C-3 hydroxyl group is esterified to phosphoric acid. The resulting compound, called phosphatidate, is the simplest phosphoglycerate. Only small amounts of phosphatidate are present in membranes. However, it is a key intermediate in the biosynthesis of the other phosphoglycerides.

Sphingosine
Sphingosine is an amino alcohol that contains a long, unsaturated hydrocarbon chain. In sphingomyelin and glycolipids, the amino group of sphingosine is linked to FAs by an amide bond. In sphingomyelin the primary hydroxyl group of sphingosine is esterified to phosphoryl choline. In glycolipids, the sugar component is attached to this group. The simplest glycolipid is cerebroside, in which there is only one sugar residue, either Glc or Gal. More complex glycolipids, such as gangliosides, contain a branched chain of as many as seven sugar residues.