Ion Transport in Hyclrodictyon africanum

The concentrations of K, Na, and CI in the cytoplasm and vacuole, the tracer fluxes of these ions into and out of the cenocyte, and the electrical potential difference between bathing solution and vacuole and cytoplasm, have been measured in Hydrodictyon africanum. If the ions were acted on solely by passive electrochemical forces, a net efflux of K and CI and a net influx of Na would be expected. Tracer fluxes indicate a net influx of K and CI and efflux of Na in the light; these net fluxes are consequently active, with an obligate link to metabolism. The effects of darkness and low temperature indicate that most of the tracer K and CI influx and Na efflux are linked to metabolism, while the corresponding tracer fluxes in the direction of the free energy gradient are not. Ouabain specifically inhibits the metabolically linked portions of tracer K influx and Na efftux. Alterations in the external K concentration have similar effects on metabolically mediated K influx and Na efflux. It would appear that K influx and Na efflux are linked, at least in the light. I N T R O D U C T I O N Much of the recent work on the ionic relations of algae has been directed towards discovering which ions are actively transported into or out of the cells. By active is meant transport against an electrochemical potential gradient. If flux equilibrium can be shown, the measured potential difference (PD) between the vacuole and the external medium may be compared with the equilibrium potential calculated from the internal and external concentration of a given ion, using the Nernst equation (Briggs, Hope, and Robertson, 1961; Dainty, 1962). Discrepancies between the observed potential and the equilibrium potential calculated from the Nernst equation for any given ion may be at tr ibuted to active transport. If isotopic measurements of influx and efflux indicate that the ceils are not in flux equilibrium for a given ion, then a more complicated expression must be used, involving the flux ratio expected from the observed electrochemical gradient. Measurements necessary to specify active transport between two phases are the ion concentrations in the two phases, the PD between the two phases, and the ion flux between the two phases. Work in which these measurements have been made on algal cells includes that of MacRobbie and Dainty (1958 Reprinted from THE JOUaNAL OF GENERAL PHYSIOLOGY, VoL 5o, No. 6 (Part I), pp. 16o7-1625, Printed in U.S.A. 1607 The Journal of General Physiology on A ril 2, 2017 D ow nladed fom Published July 1, 1967