<P> According to the fluid mosaic model of S.J. Singer and G.L. Nicolson (1972), which replaced the earlier model of Davson and Danielli, biological membranes can be considered as a two - dimensional liquid in which lipid and protein molecules diffuse more or less easily . Although the lipid bilayers that form the basis of the membranes do indeed form two - dimensional liquids by themselves, the plasma membrane also contains a large quantity of proteins, which provide more structure . Examples of such structures are protein - protein complexes, pickets and fences formed by the actin - based cytoskeleton, and potentially lipid rafts . </P> <P> Lipid bilayers form through the process of self - assembly . The cell membrane consists primarily of a thin layer of amphipathic phospholipids that spontaneously arrange so that the hydrophobic "tail" regions are isolated from the surrounding water while the hydrophilic "head" regions interact with the intracellular (cytosolic) and extracellular faces of the resulting bilayer . This forms a continuous, spherical lipid bilayer . Hydrophobic interactions (also known as the hydrophobic effect) are the major driving forces in the formation of lipid bilayers . An increase in interactions between hydrophobic molecules (causing clustering of hydrophobic regions) allows water molecules to bond more freely with each other, increasing the entropy of the system . This complex interaction can include noncovalent interactions such as van der Waals, electrostatic and hydrogen bonds . </P> <P> Lipid bilayers are generally impermeable to ions and polar molecules . The arrangement of hydrophilic heads and hydrophobic tails of the lipid bilayer prevent polar solutes (ex. amino acids, nucleic acids, carbohydrates, proteins, and ions) from diffusing across the membrane, but generally allows for the passive diffusion of hydrophobic molecules . This affords the cell the ability to control the movement of these substances via transmembrane protein complexes such as pores, channels and gates . Flippases and scramblases concentrate phosphatidyl serine, which carries a negative charge, on the inner membrane . Along with NANA, this creates an extra barrier to charged moieties moving through the membrane . </P> <P> Membranes serve diverse functions in eukaryotic and prokaryotic cells . One important role is to regulate the movement of materials into and out of cells . The phospholipid bilayer structure (fluid mosaic model) with specific membrane proteins accounts for the selective permeability of the membrane and passive and active transport mechanisms . In addition, membranes in prokaryotes and in the mitochondria and chloroplasts of eukaryotes facilitate the synthesis of ATP through chemiosmosis . </P>

The cell membrane around a cell forms a barrier that protects and regulates the cell