Chemical, physical and ultra-structural observations of the plasma membrane suggest that it is a tripartite structure, i.e., it is made up of three layers in which internal and external electron dense layers of about 20A to 40A thickness are separated by a central layer of low density of about 35A thickness.
The layered structure reflects the molecular organisation. The outer dense layer seems to be composed chiefly of protein molecules although carbohydrates (oligosaccharides) are also present.
The middle less dense layer is actually a double layer of lipid molecules (Schmitt and Palmer, 1940; Mitchison, 1953).
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The lipid molecules occur in chain and two molecular chains of lipids remain parallel to each other forming a bimolecular or double-layered structure.
The inner dense layer is also made up of protein molecules. There is a considerable evidence that some of the protein of both the outer and inner layers, protrudes into the middle layer.
The lipid layers of the plasma membrane are linked with each other by the inner ends of. lipid molecules which are non-polar and hydrophobic (Gr., hydra-water, phobe-hate) in nature.
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Both these lipid layers are held together due to Vander Waal’s forces at these non-polar ends.
The lipid molecules remain linked with the molecules of protein layers by their outer, polar and hydrophilic (Gr., hydra- water, phil-loving) ends. In the hydrogen bonds, ionic linkages or electrostatic forces bind the molecules of lipids and proteins together.
The tripartite structure of the plasma membrane was proposed by Da nielli and Davson in 1935 and later Picken (1960) and Robertson (1962, 1964) confirmed the Danielli and Davson’s observations about the plasma membrane.
Robertson in 1960 called the plasma membrane as the unit membrane but Korn (1966, 1969) criticized the unit merfibrane concept.
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Recently Green and Perdue proposed that biological membranes are built of lipoprotein macromolecular repeating units. Frey- Wyssling (1957); Sjostrand (1963) and others have reported the presence of globular particles as repeating units in a variety of animal and plant cells.