By the 1930s, chemical analyses of vertebrate tissues—chiefly muscle—and of isolated cells—chiefly red blood cells—had demonstrated a peculiar and puzzling asymmetry. Tissues and cells generally contained high concentrations of potassium ions (K+) relative to sodium ions (Na+); by contrast, the environments of these tissues and cells, such as the blood plasma, contained the opposite concentration ratio: high Na+/low K+. Meanwhile, studies on the changeable contents of tissues and cells suggested that a membrane, although not visible by the microscopy of that time, surrounded each cell. Such a membrane could then have different permeabilities to solutes such as Na+ and K+. And investigations of excitable tissues, such as nerve and muscle, also focused on Na+ and K+ contents, with the electrical activity explained by some in terms of varying permeabilities to ions.
KeywordsElectrochemical Gradient Flavin Adenine Dinucleotide Matrix Space Active Transport System Secondary Active Transport
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