The Membrane Potential ofNitella translucens

Abstract

The effects of changing the external concentrations of Na, K, Ca, and Cl on the potentials of the cytoplasm and the vacuole with respect to the bathing medium of the internodal cells of Nitella translucens have been investigated. The potential difference between the vacuole and the cytoplasm is practically unaffected by the concentration changes. The observed changes of potential difference are therefore attributed to the boundary separating the cytoplasm from the medium; this boundary is possibly a plasmalemma–cell wall complex. The difference of potential between the cell wall and the medium has also been measured and, in the presence of Ca, shown to be markedly sensitive only to the external Ca concentration. The results are divided into two sections: (a) for cells pretreated in 5 mM NaCl, the subsequent experiments being carried out in Ca-free media, and (b) for cells initially immersed in a standard artificial pond water containing the chlorides of Na, K, Ca. With the pretreated cells the external Na/K ratio was varied with the total NaCl+KCl concentration kept constant at 1.1 mM. The results suggest that over a limited range of concentrations the cytoplasm-medium potential difference can be described by an equation similar in form to a Goldman equation but containing only terms for Na and K, the average value of the permeability ratio α(= P_Na/P_K) being 0.27. In the presence of Ca the effects of Na and K on the cytoplasm-medium potential difference are greatly reduced, while the effect of Ca is relatively large. The results cannot be fitted to any form of Goldman equation containing terms for the major ions. The possibility of a contribution to the plasmalemma potential from electrogenic pumps is briefly discussed. Measurements of the Na and K content of the cytoplasm and the vacuole have been made for the pretreated cells. The Na concentration in the cytoplasm is 37 mM and in the vacuole 73 mM; the K concentration is 93 mM in the cytoplasm and 67 mM in the vacuole. The Nernst potentials for both ions are compared with the cytoplasm-medium and cytoplasm-vacuole potential differences. This analysis shows that Na is actively transported from the cytoplasm into the medium as well as into the Vacuole; K is pumped into the cytoplasm from the medium but appears to be close to electrochemical equilibrium across the tonoplast. This Confirms previously published work.