These tend to close on themselves to form sealed compartments, thus, eliminating free edges where the hydrophobic tails would be in contact with water. For the same reason compartments formed by lipid bilayers tend to reseal when they are torn. The lipid bilayer is a two-dimensional fluid. Flip-flop, or the translocation from one leaflet of the membrane to the other, rarely occurs spontaneously although, in eucaryotic cells specific phospholipid translocator enzymes mediate it. Animal cells in particular have large amounts of cholesterol.
Cholesterol decreases the permeability of the bilayer to small water-soluble molecules.
Although, cholesterol tends to make bilayers less fluid, it also prevents the fatty acid chain from coming together and crystallizing. Only phosphatidylserine carries a net negative charge while the other three are neutral at physiological pH. Inositol phospholipids are present in much smaller quantities and they have very important functional roles in cell signaling. Glycolipids have been proposed as serving many functions among which their being receptors for yet unknown physiological ligands may be the most interesting. The ganglioside Gm1 acts as receptor for cholera toxin. The asymmetry derives from the cytosolic side of the ER membrane being the site of synthesis of phospholipids.