what are the models of Plasma …

http://www.biologydiscussion.com/cell/plasma-membrane/top-4-historical-models-of-plasma-membrane-cell-biology/83448

This is also known as unit membrane model. This model was proposed by Davson Daniell and Robertson. When surface tension measurements made on the membranes, it suggests the presence of proteins. After the existence of proteins the initial lipid bilayer model proposed by Gorter and Grendel was modified. It was suggested that surface tension of cells is much lower than what one would expect if only lipids were involved. It may also be observed that if protein is added to model lipid water system, surface tension is lowered. This suggested indirectly the presence of proteins. On this basis Davson and Danielli proposed that plasma membrane contained a lipid bilayer with protein on both surfaces. Initially they supposed that proteins existed as covalently bonded globular structures bound to the polar ends of lipids. Subsequently they developed the model in which the protein appears to be smeared over the hydrophilic ends of the lipid bilayer. This model makes its popularity for a long time. With the availability of electron microscope later, fine structure of plasma membrane could be studied. Definite plasma membrane of 6 nm to 10 nm (10nm = 100 Å; 1 nm = 10_6mm) thickness was observed on surface of all cells, and plasma membranes of two adjacent cells were found to be separated by a space, 1-15nm wide.

2. Unit Membrane Model (Protein-Lipid Bilayer-Protein):

The following points highlight the top four historical models of Plasma Membrane. The models are: 1. Lipid and Lipid Bilayer Models 2. Unit Membrane Model (Protein-Lipid Bilayer-Protein) 3. Fluid Mosaic Model 4. Dannelli Model. 1. Lipid and Lipid Bilayer Model: This model to explain the structure of plasma membrane was given by Overton, Gorion and Grendel. Previously only indirect information was available to explain the structure of plasma membrane. In 1902, Overton observed that substances soluble in lipid could selectively pass through the membranes. On this basis he stated that plasma membrane is composed of a thin layer of lipid. Subsequently, Gorter and Grendel in 1926 observed that the extracted from erythrocyte membranes was twice the amount expected if a single layer was present throughout the surface area of these cells. On this basis they stated that plasma membrane is made up of double layer of lipid molecules. These models of Gorter and Grendel could not explain the proper structure of plasma membrane but they put the foundation of future models of membrane structure.