Electrical Potentials and Na, K, and Cl Concentrations in the Vacuole and Cytoplasm ofNitella translucens

Abstract

The potential differences across the tonoplast and plasmalemma membranes have been measured in the single cells of Nitella translucens, the cells being immersed in an artificial pond water (composition: NaCl _1.0 mM., KC1 _0.1 mM., CaCl₂, _0.1 mM.). The potential of the cytoplasm is –138 m V with respect to the bathing medium and –18 mV with respect to the vacuole. The concentrations of Na, K, and Cl have been measured in the two cell fractions. The concentrations in the flowing cytoplasm are: Na 14 mM., K 119 mM., and Cl 65 mM.; the vacuolar concentrations are: Na 65 mM., K 75 mM.,and Cl 160 mM. The observed potential differences across the two membranes are compared with the Nernst potentials for all three ions. This analysis shows that all three ions are actively transported at the plasmalemma: Na is pumped outwards while K and Cl are pumped inwards. At the tonoplast Na is pumped into the vacuole while K and Cl are close to electrochemical equilibrium. The inhibitor, ouabain, has no effect on the cell resting potential.