Abstract
Abstract Radioisotope equilibration techniques have been used to determine the intracellular concentration of K+, Na+ and Cl−, together with the unidirectional ion fluxes across the plasmalemma of Porphyra purpurea. Influx and efflux of 42K+, 24Na+ and 36C1− are biphasic, the rapid, initial uptake and loss of tracer from individual thalli being attributable to desorption from extracellular regions. Cellular fluxes are slower and monophasic, cells discriminating in favour of K+ and Cl− and against Na+. A comparison between the equilibrium potential of individual ion species and the measured membrane potential demonstrates that there is an active component of K+ and Cl− influx and Na+ efflux. ‘Active’ uptake and ‘passive’ loss of K+ and Cl− are reduced when plants are kept in darkness, suggesting that a fraction of the transport of K+ and Cl− may be due to ‘exchange diffusion’ (K+/K+ and Cl−/Cl−antiport).
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