INTRACELLULAR POTASSIUM COMPARTMENTS IN NITELLA AXILLARIS

Three intracellular compartments for potassium exchange have been observed in intact cells of the giant-celled alga, Nitella axillaris. These compartments have been compared with the exchange properties of isolated subcellular structures. The smallest and fastest compartment (apparent half-time, 23 seconds) appears to involve passive absorption on the cell wall. The next largest (apparent half-time, 5 hours) may represent exchange with the cytoplasmic layer through the plasma membrane, the chloroplasts being in rapid equilibrium with the surrounding cytoplasm. The largest and slowest compartment (apparent half-time, 40 days) has been identified with the central vacuole. The vacuolar membrane and the plasma membrane have similar properties with respect to K permeability. Thus, the experimental data from the whole cell can be accounted for by a structural model of the compartments. Cyanide in concentrations up to 10(-3) M causes no net loss of K. The fastest compartment in Nitella and in higher plants is compared, and the ecological significance of the slow rate of potassium transport in Nitella is discussed.