THE idea that active ion transport is linked directly to an electron-transferring respiratory system was first expressed clearly by Stiehler and Flexner1 and has since been elaborated by other investigators2,3. On the basis of this hypothesis, the maximum value of the ratio (ion pairs transported/oxygen consumed) = 4 4,5, since four electrons are transferred in the reduction of one molecule of oxygen to water. In support of the theory, Robertson and Wilkins5 showed experimentally with slices of carrot tissue that the ratio approached but did not exceed 4 as the concentration of salt in the medium was increased.