Hint: This scientist observed structurally similar to the plasma membrane and the membranes of all cell organelles. Although light microscopy doesn't overcome the resemblance, it is seen in electron micrographs.
- In 1950, J. D. Robertson proposed the unit membrane hypothesis which mainly focused on electron micrographs taken from red blood cells and the outer myelin sheath which normally covers all vertebrate nerve fibers.
- This sheath is produced from the winding growth of cells around the axon in Schwann cells. These micrographs indicated that the membrane had a trilaminar (three-layer) structure in which two thin layers of an electron surrounded a larger, less dense layer, just like two slices of bread enclosed an inner layer of cheese in a sandwich.
- The lighter inner layer was described as a double layer of lipid, while the dense inner and outer "bread" layers were postulated as protein. It was assumed that this lipid layer was mostly phospholipid, with some cholesterol.
Robertson's cell membrane model appeared to back up an earlier membrane structure hypothesis proposed by J. F. Danielli of London (1940). He argued that the features of permeability of red blood cells and other cell types were consistent with a membrane containing a double layer of lipid and surrounded by an inner and outer protein layer.
Danielli further proposed that the double layer of lipid be arranged such that the outermost, polar ends of each phospholipid layer faced the inner or outer protein surfaces, while the innermost, non-polar ends of each phospholipid layer lay next to each other.
Nonlipid channels, distributed randomly across its structure, often traversed the membrane. Robertson's observations of electron micrographs in a variety of ways were consistent with Danielli's theoretical model; this led to the popularisation of the unit membrane theory as the basis for all membrane structures inside the cell.
Note: The tightly packed bimolecular lipid layer easily accounted for the presence in the membrane of 40% lipid by weight. The unit membrane exists in the plasma membrane, endoplasmic reticulum, golgi complex, and lysosomes as a single trilaminar structure but in mitochondria, plastids, and nuclei the boundaries have an outer and an inner membrane, each with a unit membrane structural level.