SODIUM, POTASSIUM, AND CHLORIDE TRANSPORT AND MEMBRANE POTENTIALS IN VALONIA VENTRICOSA

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Abstract
The marine alga V. ventricosa regulates its ionic content by means of several ion pumps. An active extrusion of Na+ and probably an active uptake of K+ occur at the plasmalemma. At the tonoplast there is an inward transport of Na+ and especially K+. The evidence for these ion pumps is the large electrochemical gradient for Na+ across the plasmalemma and for both Na+ and K+ across the tonoplast under steady-state conditions. Ion distribution and membrane potentials across plasmalemma and tonoplast were measured. The ion concentrations were: Na+ 508, K+ 12.1, and Cl- 596 mM/liter sea water; Na+ 40, K+ 434, Cl- 138 mM/liter protoplasm water; Na 44, K+ 625, Cl- 643 mM/liter sap. The resting potential across the plasmalemma was about 71 mv, cytoplasm negative to sea water. The potential across the tonoplast was about 88 mv, cytoplasm negative to sap. The large potential across the tonoplast is a feature not found in other vacuolated plant cells. Ion fluxes between vacuole and sea water were measured in growing cells of Valonia. The fluxes were: Na+ 3.3-3.6, K+ 86-89, Cl- 11-18 [mu][mu]M/cm2 sec. The ratio of influx: efflux for Na+ and K+ was close to unity, whereas the flux ratio for Cl- was 1.70. The latter value is close to that predicted by the Ussing-Teorell equation for a passively moving ion, which suggests that Valonia, unlike other vacuolated plant cells, does not actively transport Cl- into the vacuole.