![]() Sjostrand (1963) to be different from the plasma membrane. (i) Unit membrane concept required the uniformity in the feature of membrane but mitichondrial, endoplasmic reticular and plastid membranes have been shown by F.S. Some objections to the unit membrane model were advanced during 1960s which are as follows: High permeability of natural membranes to non-polar molecules could be explained by their solubility in the nonpolar lipid phase and at the same time accounted for relative impermeability to small ions which do not dissolve readily in this medium. Hydrocarbons are poor electrical conductors, so the continuous hydrocarbon phase of the natural membrane would explain the known high electrical resistance of membranes.ĥ. Phospholipids spontaneously form a bimolecular system in vitro when added to an aqueous environment and there is no requirement for work input to maintain the minimum energy conformation of the synthetic membrane.Ĥ. The model accounted for the three layered staining pattern of fixed membranes as observed in electron micrographs.ģ. On the outer surface it is mucoprotein while on the inner surface it is non-mucoid protein.Ģ. In unit membrane model the protein layers are assymetrical. ![]() The plasma membrane surrounding the cell is thicker at the free surfaces of the cell than where it is in contact with other cells. In other words, the unit membrane is 75 Å thick with a 35 Å thick phospholipid layer between two 20 Å thick protein layers. The clear Osmiophilic zone 35 A in thickness is a bimolecular layer of lipids without the polar groups. Each dense osmiophilic band is made up of protein (20 Å) and the polar groups of phospholipids (5 A) and is thus 25 Å thick. The unit membrane concept implies a trilaminar appearance with a bimolecular lipid layer between two protein layers (Fig. The physical appearance of this trilaminar model has led to the term unit membrane. The unit membrane model visualises cell membrane as a trilaminar and indicates structure consisting of two dark osmiophilic layers separated by a light osmiophilic layer. This conclusion led Robertson in 1953 to propose unit membrane hypothesis according to which all biological membranes show generalised unit membrane construction. In still other variations the proteins are thought to be in coiled or globular form on both sides of lipid layers or they are thought to be asymmetrical, with a folded P-chain on one side and globular proteins on the other. Membrane models are usually postulated to contain protein lined polar pores of about 7 Å diameter which probably permit the passage of small ions and water molecules across the membrane. Perhaps, these units form micelles of membranes indicated in recent electron micrographs. Again Davson and Danielli (1943) and Danielli (1954) considered proteins to be in the form of a folded P-chain. Danielli (1938) suggested the presence of two types of proteins tangentially arranged in contact with the lipid and globular proteins on the outer surface. This basic model has been modified from time to time. The plasma membrane would thus consist of a double layer of phospholipid molecules sandwiched between two essentially continuous layers of protein (Fig. The hypothesis also suggested that the polar ends of lipid molecules are associated with monomolecular layer of globular proteins. Danielli and Davson model was the first attempt to describe membrane structure in terms of molecules and to relate the structure to biological and chemical properties.Īccording to bimolecular model of Danielli and Davson, plasma membrane consists of two layers of phospholipid molecules (a bimolecular leaflet) in which phospholipid molecules are arranged in such a way that hydrophilic heads of the phospholipid molecules face outside and hydrophobic non-polar lipid chains are associated in the inner region of leaflet. This conclusion led James Danielli and Hugh Davson in 1935 to suggest bimolecular leaflet model of cell membrane. Harvey and Coley (1931) and Danielli and Harvey (1935) studied surface tension of cell membrane and on the basis of their observation they pointed out the existence of protein molecules adsorbed on the surface of lipid droplets which reduce the surface tension of droplets.
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