Thin sections of biological membranes examined by electron microscopy appear as two dark lines separated by a lighter space (1). The dark lines have been equated to proteins and other polar groups, the intervening lighter space to lipid fatty acids (2). However, other interpretations are possible (3, 4), and the electron microscope observations do not validate any one molecular model of the biological membrane (5). More direct information regarding the molecular configurations within biological membranes are provided by physical probes such as differential scanning calorimetry (6), X-ray diffraction (7, 8), nuclear magnetic resonance (9), and electron paramagnetic resonance (10, 11). All of these probes show that a major portion of the lipids in a variety of membrane systems are in a bimolecular layer; protein must cover large portions of this bilayer, but the other proteins may penetrate through its hydrophobic center.
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