Cell membrane Danielli model

On the basis of electron microscopic studies, Robertson (1967) has modified the Danielli-Davson model, presenting a new model—the unit membrane hypothesis—for all cell membranes. This model was similar to that of Danielli-Davson but with a few significant changes. The number of bimolecular phospholipid layers in Robertson s model was restricted to one, and the membrane was considered to be asymmetrical, with mucopolysaccharide or mucoprotein on the outside and unconjugated protein on the inner part of the membrane (Fig. 2). The... 

Sodium and potassium ions are vital to the normal functioning of the nerve cell. The ions are separated by the cell membrane with sodium on the outside and potassium on the inside of the resting cell. A model of the basic membrane, which in principle was built as a bilayer according to the well known Davson-Danielli-Robertson scheme but which... 

Figure 1 The Danielli and Davson model for cell membrane Until the late 1960 s bilayer models were often perceived to be static structures but studies with a series of biophysical techniques involving X-ray diffraction, nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spectroscopy, fluorescence microscopy, electron microscopy and others have revealed the dynamic nature of the cell membranes which have been summarized in the Singer and Nicolson s fluid mosaic model proposed in 1972 (see Figure 2).

Cell Membranes The sequence of models of membrane structure Danielli-Davson Robertson S inger-N icholson The fluid mosaic model of cell membrane structure The interplay between data and models in the development of models of cell structure... 

The time line is provided in the published resourees. It shows that evidence about the movement of lipids within cell membranes did not appear until after the development of the Danielli-Davson and Robertson models. Claire also refers back to the fluid mosaic model at two further points in the lesson during whole class discussions. 

Treatment of HeLa cells with trypsin results in the liberation from the surface membrane of carbohydrate and protein. This does not affect the viability of the cells (Weiss, 1958) although as much as 22% reduction of dry mass was observed in experiments where only 20-60% of the total sugars were released (Shen and Ginsburg, 1968). The trypsin-accessible glycoproteins of the HeLa cell membrane do not, therefore, play a major structural role and would be among su-perstructural protein units described by the Danielli model. 

Protein first came into the model as a result of considerations of interfacial tension. Danielli and Davson (1934/5) pointed out that measured interfacial tension at lipid-water interfaces was much higher than that evident at cell surfaces. Therefore they put forward a new model in which the lipid bilayer was encased in a sandwich of protein, thereby avoiding this problem and accommodating the protein known to be present in membranes. Thirty years later, the basic premise upon which protein was added to the model was shown to be incorrect (Haydon and Taylor, 1963) phospholipids have interfacial... 

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