Membrane models

Before the emergence of electron microscopy in the 1950s, scientists did not know the structure of a cell membrane or what its components were; biologists and other researchers used indirect evidence to identify membranes before they could actually be visualized. Specifically, it was through the models of Overton, Langmuir, Gorter and Grendel, and Davson and Danielli, that it was deduced that membranes have lipids, proteins, and a bi-layer. The advent of the electron microscope, the findings of J. David Robertson, the proposal of Singer and Nicolson, and additional work of Unwin and Henderson all contributed to the development of the modern membrane model. However, understanding of past membrane models elucidates present-day perception of membrane characteristics. Following intense experimental research, the membrane models of the preceding century gave way to the fluid mosaic model that is accepted today.

Evert Gorter and François Grendel (Dutch physiologists) approached the discovery of our present model of the plasma membrane structure as a lipid bi-layer. They simply hypothesized that if the plasma membrane is a bi-layer, then the surface area of the mono-layer of lipids measured would be double the surface area of the plasma membrane. To examine their hypothesis, they performed an experiment in which they extracted lipids from a known number of red blood cells (erythrocytes) of different mammalian sources, such as humans, goats, sheep, etc. and then spreading the lipids as a mono-layer in a Langmuir-Blodgett trough. They measured the total surface area of the plasma membrane of red blood cells, and using Langmuir's method, they measured the area of the mono-layer of lipids. In comparing the two, they calculated an estimated ratio of 2:1 _{Mono-layer of lipids:Plasma membrane}. This supported their hypothesis, which led to the conclusion that cell membranes are composed of two apposing molecular layers. The two scientists proposed a structure for this bi-layer, with the polar hydrophilic heads facing outwards towards the aqueous environment and the hydrophobic tails facing inwards away from the aqueous surroundings on both sides of the membrane. Although they arrived at the right conclusions, some of the experimental data were incorrect such as the miscalculation of the area and pressure of the lipid mono-layer and the incompleteness of lipid extraction. They also failed to describe membrane function, and had false assumptions such as that of plasma membranes consisting of mostly lipids. However, on the whole, this envisioning of the lipid bi-layer structure became the basic underlying assumption for each successive refinement in modern understanding of membrane function.

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