There are three major classes of lipids in membranes: (1) glycolipids, (2) cholesterol, and (3) phospholipids. Glycolipids have a role in the cell surface-associated antigens as well as cell-surface receptors, whereas cholesterol serves to regulate fluidity. Phospholipids have fatty acids attached at carbons 1 and 2. It is usual to find a saturated fatty acid attached at carbon 1 and an unsaturated fatty acid at carbon 2. In addition, phosphatidylethanolamine and phosphatidylserine usually have fatty acids that are more unsaturated than PI and phosphatidylcholine. Less than 10% of the membrane phospholipid is PI. Plasma membranes have no cardiolipin, and the mitochondrial membranes have very little phosphatidylserine. Several of these phospholipids have important roles in the signal transduction processes that mediate the action of a variety of hormones. Phosphatidylinositol and its role in the PI cycle is one of the most important. Phosphatidylcholine and phosphatidyl¬ ethanolamine also play a role in these systems. The PIP cycle functions in moving the calcium ion from its intracellular store to the inner aspect of the cell membrane where it stimulates protein kinase C. Phosphatidylinositol also serves to anchor glycoproteins to the membrane. Glycoproteins are tethered to the external aspect of the plasma membrane and play a role in the cell recognition process. Antigens, pathogens, and foreign proteins are recognized by these structures.