Membrane Structure: The Unit Membrane Model, Unit Membrane Structure, Cell Surfaces of Eukaryotes and Prokaryotes, Cell Junctions, New Techniques

Abstract

The unit membrane concept that says that all membranes have an underlying bilayer composed of phospholipids was originally proposed by Danielli and Davson in 1935. This concept extends to a wide variety of membranes from different organisms, each of which has a specific function. Although each of these membranes may subserve different functions, they all share two common properties: they exhibit both a selective permeability to lipid solvents as well as a high electrical resistance. These two properties suggest the presence of an underlying unit consisting of a lipid bilayer. The bilayer concept of membrane structure is consistent with the trilamellar image of membranes that arose from electron microscopic observations as well as X-ray diffraction analysis of myelin. The unit membrane concept has proven useful for describing the underlying phospholipid bilayer of membranes; however, when it became necessary to determine the localization of different proteins within membranes, a better representation of membrane ultrastructure was needed. The techniques such as freeze-fracture electron microscopy, thin-section electron microscopy with improved fixation, nuclear magnetic resonance, and immunological labeling of membranes with specific probes made it clear that integral as well as peripheral proteins can be localized within membranes.