Phospholipids contain a polar head group and two long-chain fatty acyl moieties, one of which is generally unsaturated. The head group is a glycerol or serine phosphate attached to a polar group such as choline. These molecules are a major constituent of cellular membranes, where their diverse structures and asymmetric distributions play major roles in determining membrane properties (Dowhan 1997). The four major classes of phospholipids in human plasma membranes are phosphatidylethanolamine, phosphatidylserine, phosphatidylcholine, and sphingomyelin. The first three are derivatives of glycerol while sphingomyelin is a derivative of serine.
Here, pathways for the metabolism of glycerophospholipids, phosphphatidylinositol (PI), and sphingolipids are annotated.