Phospholipids Spontaneously Form Ordered Structures in Water

Phospholipids Spontaneously Form Ordered Structures in Water 

Structures formed by (a) detergents and (b) phospholipids in aqueous solution. Each molecule is depicted schematically as a polar head-group ( ) attached to one or two long, nonpolar chains. Most detergents have one nonpolar chain phospholipids have two. At very low concentrations, detergents or phospholipids form monolayers at the air-water interface. At higher concentrations, when this interface is saturated, further molecules form micelles or bilayer vesicles (liposomes). 

Multilayered vesicles (liposomes) formed from sonically dispersed phosphatidylcholine in the presence of 10% diacetylphosphate and 2% potassium phosphotungstate. Each vesicle has a trilaminar structure consisting of two dark layers separated by a light layer. The dark layers contain the electron-dense phosphotungstate ion the light layer corresponds to the hydrophobic interior of the bilayer. 

In an early inaccurate model for the structure of biological membranes, a phospholipid bilayer was coated on both sides by protein in an unfolded or jS-pleated sheet conformation. This model reflected the prevailing view of membrane structure from about 1940 until the early 1970s. 

The idea of a lipid bilayer was first proposed by E. Gorter and F. Grendel, who showed in 1925 that the phospholipid content of the erythrocyte plasma membrane is approximately the amount needed to enclose the cell with a bilayer. Subsequent x-ray diffraction measurements confirmed this picture (fig. 17.8). 

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Due to their amphipathic nature, phospholipids spontaneously form ordered structures in water. When phospholipids are agitated in the presence of excess water, they tend to aggregate spontaneously to form bilayers, which strongly resemble the types of structures they form in biological membranes. 

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