The following points highlight the top three theories of plasma membrane. The theories are: 1. Lamellar Theory 2. Unit Membrane Theory 3. Micellar Theory.
1. Lamellar Theory:
“Sandwitch model” proposed by Danielli and Davson (1935), the plasma membrane exhibit trilaminar structure i.e., a lipid layer is bounded by protein layer on both the sides. In this model, proteins are usually represented in globular form. Subsequently electron microscopic studies favour the idea of trilaminar structure of plasma membrane.
2. Unit Membrane Theory:
The universal occurrence of trilaminar pattern of membranes of cells led Robertson to propose in 1959, the concept of a ‘unit membrane’ with a trilaminar appearance. The unit membrane forms not only plasma membrane but also contains other membranes in most of the cell organelles.
All the unit membranes consist of a bimolecular layer of phospholipids covered by a layer of proteins. Phospholipid is the main part of lipid. For example, lecithin, cephalin etc.
Besides it, other lipids like cholesterol, cerebroside and ganglioside as well as some polysaccharides; it is likely that these molecules provide stability to plasma lemma. Phospholipids are oriented with their fatty acid groups towards each other and their water soluble ends (polar groups) facing outwards.
3. Micellar Theory:
Hilleir and Hoffman (1963) proposed this theory. According to them the plasma membrane includes a mosaic of globular subunits (micelles) which are closely packed together having a central core of lipid molecules with hydrophilic (water loving) polar end. Each lipid micelle measures about 40-70 A in diameter.
A single layer of globular protein is present on either sides of the lipid micelle. The spaces between the globular micelles represent water filled pores which measures about 4 A in diameter. The pores are bounded partly by the polar groups of protein molecules.
Electron microscope support both models of plasma membrane:
(i) Robertson (1959) and
(ii) Hilleir and Hoffman (1963).