Net and Steady-state Cation Fluxes inChlorella pyrenoidosa

Abstract

The addition of K⁺ to Chlorella cells grown so as to be abnormally rich in Na⁺ induces a net Na⁺ efflux and a concomitant uptake of K⁺. The net Na⁺ extrusion shows first-order kinetics with time constants of about 10 min for illuminated cells, and occurs at rates in the region of 10 to 15 pmol cm1² s. The corresponding time course for the net K⁺ influx also approximates to first-order kinetics but is more complicated because it not only involves a K⁺/Na⁺ component but also a K⁺/H⁺ exchange. The H⁺ extrusion usually represents less than 20 per cent of the net cation movement and may account both in magnitude and in rate for the difference between K⁺ and Na⁺ movements. The magnitudes of the net K⁺ and Na⁺ fluxes differed from steady-state flux rates in normal high K⁺-containing cells being as much as 20 times greater for K⁺ and over 100 times greater for Na⁺. There is some indication that K⁺ competes for Na⁺ entry into Na⁺-rich cells, suggesting that both the Na⁺/Na⁺ and K⁺/Na⁺ exchanges may share the same entry site. The K⁺/Na⁺ exchange rates saturate at low external K⁺ concentrations; the half-maximum rate was at about 0.2 mM K⁺. The Na⁺/K⁺ exchange is sensitive to temperature and between 0 and 25 °C an activation energy of about 25 k cal/mole was calculated from the Arrhenius equation.