Models proposed for the plasma membrane
Overton was the pioneer to postulate that plasma membrane is a thin layer of lipid. He proposed this because he found that lipid soluble substances are easily transported across the membrane.
The amount of lipids present in the erythrocyte membrane was nearly twice that of its total surface area. This made Gorten and Grendel to propose that lipids in the membrane exist as bilayers.
This model was proposed by Davson and Danielli and was shaped by Robertson. Experiments showed that the surface tension of the biological membranes are lower than that of the pure lipid bilayers, suggesting the presence of proteins in them. Based upon this, Davson and Danielli proposed that proteins are smeared over the lipid bilayer.
When electron microscope was invented, plasma membrane appeared as three layers. With this observation, Robertson formulated a unit membrane model, which states that the proteins are present on either side of the lipid bilayer. According to this model, the membrane will be like a lipid layer sandwiched between two protein layers.
This is the universally accepted model for plasma membrane. On the basis of several experiments, S.J. Singer and G.L. Nicolson in 1972 proposed this model.
The essential features of the Fluid mosaic model are
· Lipids and proteins are present in a mosaic arrangement within the bilayer.
· Phospholipids act as a fluid matrix, in which some proteins are integral and others are associated with the surface of the membrane.
· Lipids and proteins are mobile in the membrane.
· They can move laterally, rotate but not from one monolayer to the other.
· The membrane is asymmetric in nature, the outer and inner leaflets of the bilayer differ in composition.