<P> The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules . These membranes are flat sheets that form a continuous barrier around all cells . The cell membranes of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus and other sub-cellular structures . The lipid bilayer is the barrier that keeps ions, proteins and other molecules where they are needed and prevents them from diffusing into areas where they should not be . Lipid bilayers are ideally suited to this role because, even though they are only a few nanometers in width, they are impermeable to most water - soluble (hydrophilic) molecules . Bilayers are particularly impermeable to ions, which allows cells to regulate salt concentrations and pH by transporting ions across their membranes using proteins called ion pumps . </P> <P> Biological bilayers are usually composed of amphiphilic phospholipids that have a hydrophilic phosphate head and a hydrophobic tail consisting of two fatty acid chains . Phospholipids with certain head groups can alter the surface chemistry of a bilayer and can, for example, serve as signals as well as "anchors" for other molecules in the membranes of cells . Just like the heads, the tails of lipids can also affect membrane properties, for instance by determining the phase of the bilayer . The bilayer can adopt a solid gel phase state at lower temperatures but undergo phase transition to a fluid state at higher temperatures, and the chemical properties of the lipids' tails influence at which temperature this happens . The packing of lipids within the bilayer also affects its mechanical properties, including its resistance to stretching and bending . Many of these properties have been studied with the use of artificial "model" bilayers produced in a lab . Vesicles made by model bilayers have also been used clinically to deliver drugs . </P>

Where can the phosphate portion of the phospholipids in a membrane be found