Derived from either glycerol, in which case they are known as phosphoglycerides, or sphingosine (an 18-carbon amino alcohol with an unsaturated hydrocarbon chain, one step past MCAT level), phospholipids are prominent in all biological membranes. In addition to carbon, hydrogen, and oxygen, phospholipids also contain nitrogen and phosphorus. Like soaps, phospholipids are amphipathic. The phosphatidicholine end is hydrophilic while the hydrocarbon chains are lipophilic, but because the two fatty acid chains are too bulky to fit on the inside of a micelle, unlike soaps which form micelles, phospholipids form bimolecular sheets in aqueous media. The formation of lipid bilayers occurs by self-assembly, thermodynamically favored because the state in which the fatty acid tails are sequestered is the state in which the water molecules of the medium enjoy the maximum degree of intermolecular access to each other. Any other state would involve the input of energy to separate water from itself to make room for the nonpolar hydrocarbon tails.