Separation between polar groups lipid bilayers

From the data presented here several conclusions may be reached regarding the effect of cholesterol on lipid bilayers. It is shown that, even if the presence of cholesterol in bilayers serves to moderate temperature-induced changes, its ability to affect the location of solubilized molecules is highly temperature dependent We have also shown, in accord with previous work (11), that the presence of cholesterol in the gel phase results in a larger separation between the lipid polar groups and this in turn allows water to penetrate into the lipid hydrophobic core. 

The cell membrane, also called the plasma membrane, surrounds the cytoplasm and physically separates the intracellular components from the extracellular environment. It has a thickness around 7.5-10 nm and controls the flux of molecules between the cytosol and the extracellular environment. The plasma membrane is composed of a lipid bilayer, in which the polar groups are exposed to the internal and external cell aqueous medium, protecting the embedded hydrophobic tails. Various proteins with distinct functions can be found partially or totally immersed in this bilayer. 

Lipids. After 15 years of systematic measurement, interactions between amphiphilic assemblies, particularly between phospholipid bilayer membranes, are as well elaborated as for any class of materials (for a recent review, see reference 6). By their very nature, amphiphilic molecules self-assemble in water into structures that separate polar group-water compartments from hydrophobic compartments. Aqueous spaces in these assemblies swell and shrink under the combined influence of all the forces that stabilize the molecular assemblies as well as those that determine their association. Because the subject has been so thoroughly reviewed, we need mention only general points here. 

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