A particularly important example in animal cells is cholesterol, which helps strengthen the bilayer and decrease its permeability. The structure of biological membranes typically includes several types of molecules in addition to the phospholipids comprising the bilayer. Vesicles made by model bilayers have also been used clinically to deliver drugs. Many of these properties have been studied with the use of artificial 'model' bilayers produced in a lab.

The packing of lipids within the bilayer also affects its mechanical properties, including its resistance to stretching and bending. 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. Just like the heads, the tails of lipids can also affect membrane properties, for instance by determining the phase of the bilayer. 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. 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.īiological bilayers are usually composed of amphiphilic phospholipids that have a hydrophilic phosphate head and a hydrophobic tail consisting of two fatty acid chains. Lipid bilayers are ideally suited to this role, even though they are only a few nanometers in width, because they are impermeable to most water-soluble ( hydrophilic) molecules. 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. The cell membranes of almost all organisms and many viruses are made of a lipid bilayer, as are the nuclear membrane surrounding the cell nucleus, and membranes of the membrane-bound organelles in the cell. These membranes are flat sheets that form a continuous barrier around all cells. The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules.

The three main structures phospholipids form in solution the liposome (a closed bilayer), the micelle and the bilayer.

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