Membrane models
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 bilayer.
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.
Evert Gorter and François Grendel (Dutch physiologists) approached the discovery of our present model of the plasma membrane structure as a lipid bi-layer.
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.
In 1935, Davson and Danielli proposed that biological membranes are made up of lipid bi-layers that are coated on both sides with thin sheets of protein and they simplified their model into the "pauci-molecular" theory.
The lipid bi-layer suggestion agrees with previous models but views proteins as globular entities embedded in the layer instead of thin sheets on the surface.
Henderson and Unwin have studied the purple membrane by electron microscopy, using a method for determining the projected structures of unstained crystalline specimens.
By applying the method to tilted specimens, and using the principles put forward by DeRosier and Klug for the combination of such two-dimensional views, they obtained a 3-dimensional map of the membrane at 7 Å resolution.
[6] High-resolution micrographs of crystalline arrays of membrane proteins, taken at a low dose of electrons to minimize radiation damage, have been exploited to determine the three-dimensional structure by a Fourier transform.
Recent studies on negatively stained rat hepatocyte Gap™ junctions subjected to 3-dimensional Fourier reconstructions (of low-dose electron micrographs) indicate that the six protein sub-units are arranged in a cylinder slightly tilted tangentially, enclosing a channel 2 nm wide at the extracellular region.
[7] Further details of the molecular organization should emerge as more methods of preparation become available, so that high-resolution 3-dimensional images comparable to the purple membranes are obtained.
[8] The proteolipid code relies on the concept of a zone, which is a functional region of membrane that is assembled and stabilized with both protein and lipid dependency.
