Bilayers, vesicles, and membranes

Some micelles at concentrations well above the CMC form extended parallel sheets two molecules thick, called planar bilayers. The individual molecules lie perpendicular to the sheets, with hydrophilic groups on the outside in aqueous solution and on the inside in nonpolar media. When segments of planar bilayers fold back on themselves, unilamellar vesicles may form where the spherical hydrophobic bilayer shell separates an inner aqueous compartment from the external aqueous environment. 

Bilayers show a close resemblance to biological membranes and are often a useful model on which to base investigations of biological structure. However, actual membranes are highly sophisticated structures, in which phospholipid molecules form layers instead of micelles because the hydrocarbon chains are too bulky to allow packing into nearly spherical clusters. 

There are two views of the motion of integral proteins in the bilayer. In the fluid mosaic model shown in Fig. 11.53, the proteins are mobile, but their diffusion coefficients are much smaller than those of the lipids. In the lipid raft model, a number of lipid and cholesterol molecules form ordered structures, or rafts, that envelop proteins and help carry them to specific parts of the cell. 

The mobihty of the bilayer enables it to flow around a molecule close to the outer surface, to engulf it, and to incorporate it into the cell by the process of endocytosis. Alternatively, material from the cell interior wrapped in cell membrane may coalesce with the cell membrane itself, which then withdraws and ejects the material in the process of exocytosis. An important function of the proteins embedded in the bilayer, however, is to act as devices for transporting matter into and out of the cell in a more subtle manner, as discussed in Section 8.6. 

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