Scanning Electron Microscopy


A number of scientists in the early 20th century conceived cell water and ions as existing in unique "colloidal" or "bound" states. Work in the 1930's, however, established the assumption that most of cell water and of K exist in a free state like that of a dilute aqueous solution, so that the osmotic activity of cell K serves to balance that of the high concentration of extracellular Na. This assumption underlies theoretical concepts of membrane pumps, Donnan equilibria, and cell potentials, and led to the now classical membrane-osmotic-pump-leak concept of cell ionic distributions and volume maintenance. Nevertheless, the results of a number of physiological studies, accumulated beginning in the 1950's, are incompatible with the classical membrane-osmotic pump-leak concepts and led to radically different concepts of the physical state of cell water and ions. Of these the most coherent theory is that of Ling. It is now realized that the interpretation of many of the physical studies of cell water, such as dielectric and NMR relaxations, is model-dependent, and that the relaxations are dominated by small fractions of "bound" water. That the bulk of cell water exists in an ordered state is now firmly established by diffusion measurements using NMR and quasielastic neutron scattering. The properties of bulk water in this state are well-defined by recent studies in polymeric model systems, and include relative solute exclusion, reduced rotational and translational diffusion, a distribution of correlation times on the order of 10-11 sec., high osmotic pressures, and particular patterns of freezing and thawing. The specific adsorbed state of most of cell K+ is now established by electron microscopic, and near-edge x-ray absorption studies.

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