Unit Membrane Concept

The molecular structure of cell membrane is still not clearly known. Several models have been proposed from time to time to explain its structure based on its physico-chemical properties.

In the year 1935 Danielli and Dawson proposed that the cell membrane is made up of a lipid layer sandwiched between two layers of proteins. This idea came to be known as lamellar or sandwich model. The model assumed the cell membrane to be a stable structure with very little variability and functional specificity.

In the year 1959 David Robertson based on the electron microscopic studies, proposed the idea of unit membrane. He noticed a three-layered (dark-light-dark) arrangement for all the membranes that he studied. This idea holds that a membrane consists of a phospho lipid bilayer sandwiched between two protein monolayers and that various membranes in a cell are unit membranes. Now it is accepted that the idea of unit membrane is applicable with some exceptions. Cell membrane, membranes of endoplasmic reticulum, golgi complex and lysosomes are unit membranes while nuclear membrane, membranes of mitochondria and plastids are double unit membranes.