Describe the unit membrane hypothesis ?

The unit membrane hypothesis of J. D. Robertson was based on electron micrographs taken of red blood cells and the outer myelin sheath that usually surrounds the nerve fibers of all vertebrates. This sheath is derived from the winding growth of Schwann cells around the axon. These micrographs suggested that the membrane possessed a trilaminar (three-layer) structure, in which two electron-dense layers surrounded a broader, less dense layer, much as two slices of bread enclose an inner layer of cheese in a sandwich. The dense inner and outer ‘‘bread’’ layers were postulated to be protein, while the lighter interior layer was described as a double layer of lipid. This lipid layer was believed to be largely phospholipid with some cholesterol.

Robertson’s model of the cell membrane tended to support an earlier hypothesis of membrane structure developed by J. F. Danielli of London in 1940. Danielli reasoned that the permeability characteristics of red blood cells and other types of cells were consistent with a membrane containing a double layer of lipid and surrounded by an inner and an outer protein coat. Danielli further suggested that the double layer of lipid was oriented so that the outermost, polar ends of each phospholipid layer faced the inner or outer protein surfaces while the innermost, nonpolar ends of each phospholipid layer lay next to one another. The membrane was also traversed by nonlipid channels scattered randomly throughout its structure. In a number of ways, the electron micrograph interpretations of Robertson were consistent with the theoretical model developed by Danielli; this led to the popularization of the unit membrane hypothesis as the basis for all membrane structure within the cell.

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