Uptake and Loss of Na+, Rb+, and Cs+ in Relation to an Active Mechanism for Extrusion of Na+ in Scenedesmus

The mechanism for extrusion of Na(+) from Scenedesmus cells is characterized physiologically. It is stimulated by phosphate but oxygen is not necessary. Rb(+) and Cs(+) may also be extruded, but in the presence of Na(+) they cannot compete for the sites on the inside of the transport system. When Na(+) is extruded, Rb(+) and, by inference, K(+) seems to be transported as counter ion from the outside, and sodium ions compete only weakly for this external site. The parallelism between these findings and the Na(+)-K(+)-activated adenosine triphosphatases known from animal tissues is pointed out. With low additions of phosphate, the extrusion mechanism can keep the cells practically free from Na(+). Increasing the concentrations of external phosphate stimulates uptake more than extrusion, and a net uptake occurs. As for Rb(+) and Cs(+), they are taken up in the absence of external phosphate, but additions of P will greatly enhance the amounts absorbed. Two different ways of uptake are indicated.