Background The membranes covering cells and intracellular organelles are composed of lipid bilayer membranes. Membranes are active components of biological systems. They regulate entry and exit of specific solutes and act as energy stores. They transfer information about the extracellular environment to the internal regions of a cell.
Membranes are exceedingly thin, on the order of a few nanometers.
Each lipid in a bilayer has a hydrophilic region with charged or hydrogen-bonding groups, plus a hydrophobic region rich in hydrocarbons. These molecules are termed amphiphilic because they can interact favorably with both polar and nonpolar solvents. Phosphatidylcholine and other phospholipids that are the major components of many biomembranes carry two long fatty acyl chains esterified to the one and two positions of glycerol. The three position of the glycerol contains a hydrophilic phosphate diester anion. Phospholipids spontaneously form lipid bilayers in an aqueous medium.