The ionic state of single internodal cells of a fresh water characean, Nitella translucens, has been studied. In mature cells the vacuolar concentrations were 78 mM K, 60 mM Na, and 151 mM Cl, compared with concentrations of 0.1 mM K, 1.0 mM Na, and 1.3 mM Cl in the bathing medium. The results suggest an active influx of potassium and an active efflux of sodium at the plasmalemma, and an active influx of chloride, probably at the tonoplast. The cation transport is inhibited by ouabain, and is more efficient in young cells; the chloride transport is insensitive to ouabain, and unaffected by age. Thus the two systems appear to be independent. It is suggested that the active fluxes are 0.5 to 0.6 µµmoles K/cm2 sec. inwards, and 0.45 µµmoles Na/cm2 sec. outwards. The passive influxes, 0.3 µµmoles K/cm2 sec. and 0.55 µµmoles Na/cm2sec., give a value for the relative permeabilities of the plasmalemma, PNa/PK, of 0.18. The absolute magnitudes of the permeabilities, compared with those derived from resistance measurements, suggest that potassium ions interact strongly in the membrane. The cation fluxes at the tonoplast are much higher than those at the plasmalemma. The active influx of chloride is 0.85 µµmoles/cm2 sec. in light, but only 0.052 µµmoles/cm2 sec. in the dark. The potassium influx is also reduced in the dark. Thus the energy for both active transport processes is closely geared to light-dependent metabolism, rather than to respiration.

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