The Control of Na Uptake intoNitella translucens

Abstract

Movement of Na into cells of Nitella translucens is a ‘downhill’ process; the ions move across the plasmalemma down an electrochemical potential gradient. Nevertheless, measurements of Na influxes under a wide range of experimental conditions have shown that there must be links between Na uptake and processes controlled by metabolism. When Ca ions are present in the bathing solution, Na influxes are greatly increased by light under conditions where photosynthesis can proceed (i.e. when both photosystems are active). In the presence of Ca, the influx of Na increases only slightly when the external Na concentration is raised above 1 mM, and the light-promoted Na influx is considerably inhibited when Cl is removed from the bathing solution. When the Cl concentration is kept constant, the Na influx in light is determined by the concentrations of other cations present in solution (K, Ca, or NH₄). In the absence of Ca from the cell wall and solution, the influx is stil enhanced by light, but does not saturate when the external Na concentration is raised above 1 mM. It is suggested that the Na influx in light is partly linked to the inward Cl pump, but there is also a separate (Cl-independent) effect of light on the permeability of the plasmalemma to Na. Links between Na and Cl uptake could be maintained by effects of Cl on electrochemical driving forces controlling Na entry; alternatively, chemical coupling between the two processes may be involved.