Membrane Surfactants or Lipids

The surface-active components of biological membranes are generally referred to as lipids, with the majority consisting of double-chained phospholipids or glycolipids. The hydrophobic tails normally contain chains of 16-18 carbons. 

The size, structure, and fluidity of membrane lipids are also important because those aspects of the molecules make it possible for them to pack efficiently into a variety of convoluted bilayer membrane structures with various degrees of curvature and flexibihty. Tliat flexibility also makes possible the inclusion of the various other important components of the cell wall, including proteins, carbohydrates, and cholesterol. In terms of the geometric concepts discussed previously (see Fig. 15.15), one can visualize where one class of lipid will have a critical packing factor. Pc (= v/adc) 1, which will produce a truncated cone shape, while another will have F i for an inverted truncated cone. Combinations of the two can then accommodate the inclusion of, for example, proteins and cholesterol, while maintaining an overall planar structure (or a given degree of curvature), or increase curvature to produce a smaller associated unit. 

FIGURE 15.15. Using the molecular geometry of one or several surfactant molecules it becomes possible to visualize how a surfactant or mixture can produce the wide variety of aggregate structures encountered in nature. 

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