Glycerol is the backbone of the fundamental phospholipids used as the self-assembling units of lipid membranes. It is interesting that both enantiomeric glycerol configurations appear in nature. The metabolic intermediate for all glycerophospholipids is phosphatidic acid (1,2-diacylglycerol-3-phosphate). Lysophosphatidic acid, which lacks the acyl chain in the 2-position, is an important cellular messenger. Phosphatidylcholine (lecithin) is the most abundant phospholipid in animal and plant tissues. Phosphatidylethanolamine (cephalin) is an abundant phospholipid in microbial, plant, and animal cells. Phosphatidylglycerol has important functions in lung surfactants, chloroplasts, and bacterial membranes and is a metabolic intermediate in the biosynthesis of cardiolipin. Phosphatidylserine is involved in biological processes including apoptosis, blood coagulation, and activation of protein kinase C. It can be found in plasma membranes and other membranes of animals, plants, and microorganisms. A class of phospholipids with a high rate of metabolism is the phosphatidylinositols which have varying degrees of phosphorylation in the polar head group myo-inositol. The metabolic conversion of phosphatidylinositols to diacylglycerols and inositol phosphates is important in the regulation of vital cellular functions such as differentiation, proliferation, and apoptosis, and in anchoring proteins via a glycosyl-bridge to the plasma membrane.