According to Robertson, the unit membrane consisted of a bimolecular lipid leaflet sandwiched between outer and inner layers of protein organized in the pleated sheet configuration.
Such an arrangement was presumed to be basically the same in all cell membranes.
In the late 1950s, electron microscopy provided additional information about the structure of the plasma membrane.
J. D. Robertson was a pioneer in this area, showing that membranes fixed with osmium tetroxide revealed a characteristic tri-laminar appearance consisting of two parallel outer dark (osmiophilic) layers and a central light (osmiophobic) layer (Fig. 15-6).
The osmiophilic layers typically measured 20-25 Å (2.0-2.5nm) in thickness and the osmiophobic layers measured 25-35 Å (2.5-3.5 nm), yielding a total thickness of 65-85 Å (6.5-8.5 nm). This value compared favorably with the thickness predicted on the basis of chemical studies.
Robertson and others demonstrated that the tri-laminar pattern was characteristic of many other cellular membranes, including the endoplasmic reticulum. In view of the underlying unity in the appearance of the cell membranes studied, Robertson proposed his now famous unit membrane model. According to Robertson, the unit membrane consisted of a bimolecular lipid leaflet sandwiched between outer and inner layers of protein organized in the pleated sheet configuration. Such an arrangement was presumed to be basically the same in all cell membranes.
Though Robertson acknowledged specific chemical differences between membranes (i.e., the particular molecular species that make up each membrane differ), he proposed that the pattern of molecular organization was fundamentally the same. Although there can be no doubt about the similar electron-microscopic appearance of nearly all membranes, so strict a chemical interpretation to account for the uniformity is no longer supportable.
Robertson extended his unit membrane model to include the notion that continuity exists between the membranes of the nuclear envelope and the plasma membrane via the endoplasmic reticulum. The occurrence of such continuities has been confirmed in electron-microscopic studies of many different cells and tissues.
Furthermore, Robertson suggested that vesicular organelles might arise from this continuous membrane system and that they are subsequently pinched off to form separate structures. There is some evidence in support of this notion in the case of lysosomes and micro-bodies.