Ca Fluxes and Membrane Potentials inNitella translucens

Abstract

The concentrations of Ca have been measured in the flowing cytoplasm and the vacuole of the single cells of Nitella translucens, the cells being immersed in an artificial pond Water (composition: NaCl, 1.0 mM; KCl, 0.1 mM; CaCl₂, 0. mM). In the flowing cytoplasm the total concentration is 8 mM and in the vacuole 12 mM. Measurements of the electrical potential differences across the plasmalemma and tonoplast membranes show that the cytoplasm is at a potential of —134 mV with respect to the bathing medium and —24 mV with respect to the vacuole. An attempt has been made to measure the tracer fluxes of Ca and it is shown that the cells are not in flux equilibrium. The influx is 0.046 μμ moles cm⁻² sec⁻¹; the efflux was too small to measure with any degree of accuracy. The observed potential differences across both membranes are compared with the Nernst potentials for Ca. This analysis shows that Ca is not in electrochemical equilibrium across either membrane and that the driving forces on Ca are directed from the bathing medium and the vacuole into the cytoplasm. It is suggested that there is no necessity for a metabolically driven Ca pump at the plasmalemma because the low cytoplasmic Ca content could be due to the low permeability of the plasmalemma; the Goldman flux equation gives a value of P_Ca = 4.3×10⁻⁸ cm sec⁻¹. A Ca pump at the tonoplast appears to be necessary to explain the steep electrochemical potential gradient from the vacuole to the cytoplasm. The efflux of Ca from the isolated cell wall has been measured. From these measurements it was possible to estimate the concentration of indiffusible anions in the Donnan Free Space; the value obtained was 0.74 equiv. 1.⁻¹.