Carrier-mediated Potassium Efflux Across the Cell Membrane of Red Beet

The flux ratio (influx/efflux) of K⁺ across the plasmalemma of beet cells at an external potassium concentration of 0.6 mm does not respond to changes of membrane potential in the manner expected for the free diffusion of ions. The K⁺ efflux is affected by the presence of adsorbed Ca²⁺, but is apparently unrelated to the electrical potential or to the net uptake of potassium. The K⁺ efflux is greater than the efflux of the sulfate and organic anions which are accumulated with potassium, and is partially dependent on the presence of external potassium. Thus the loss of ⁴²K from the cell does not appear to be a leakage of freely diffusing K⁺ ions, nor a leakage of ion pairs, but a carrier-mediated transport or exchange of potassium across the cell membrane.