Die pH-Abhängigkeit der Aufnahme von H2PO 4 - , SO 4 = , Na+ und K+ und ihre gegenseitige Beeinflussung bei Ankistrodesmus braunii

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Abstract
Ion uptake was studied using 32P, 35S, 22Na and 42K as tracers in synchronized cells of Ankistrodesmus, which were slightly starved with respect to the ions to be investigated. In the light and in the dark, phosphate uptake is maximal between pH 5.5 and 6.5. Whereas Na+ in comparison to K+ enhances phosphate uptake in the light (8 to 9-fold) and in the dark, Ca++ exerts only a slightly stimulatory effect. The stimulation of phosphate binding by Na+ occurs rapidly, even after less than 5 sec of incubation, and also in the presence of an equimolar concentration of K+. The pH-dependence of Na+-uptake in the light and in the dark is comparable to a dissociation curve: Na+-uptake increases with decreasing extracellular H+-concentration and is inversely proportional to phosphate uptake in the absence of Na+. The light:dark ratio of Na+-uptake at pH 8 amounts to 7:1. Mere adsorption of Na+ is similarly dependent on the pH. K+ strongly competes with Na+-uptake, even at pH 8. K+-uptake proceeds in a quite different manner from Na+-uptake and has an optimum at pH 7. Sulfate is taken up linearly in a biphasic process as a function of time; the pH-optimum lies between pH 7.5 and 8. K+ but not Na+ slightly enhances sulfate uptake. The Na+-enhancement of phosphate uptake can be related neither to a sodium-potassium exchange pump nor to a photosynthesis-dependent ion-exchange reaction. The results suggest that the uptake of phosphate, Na+ and K+, and the influence of alkali cations on phosphate uptake, but not sulfate uptake, are strongly dependent on fixed charges of the plasmalemma or even of the cell wall. These fixed charges may even prevent an active ion uptake.