Ionic relationships in the giant ameba Chaos chaos were studied by analyzing bulk preparations of ground cytoplasm for K, Na, and Cl. Ion levels under normal conditions were compared with the levels in cells exposed to varying concentrations of different ions, for varying times and at different temperatures. By standard intracellular electrode techniques, the bioelectric potential, electrical resistance, and rectifying properties of the plasmalemma were studied on intact cells in media of different composition. The results obtained, when related to evidence from other studies on ion fluxes and osmotic relationships, suggest the following concept of ionic regulation in Chaos chaos. In the absence of active membrane uptake, the plasmalemma is essentially impermeable to anions but permeable to both K and Na, which enter passively. In the cold the cell does not discriminate between K and Na, the cytoplasmic level of K + Na is determined by a Donnan distribution, and osmotic imbalance leads to slow swelling. At normal temperatures active processes are added: Na and water are pumped out by the contractile vacuole system; Cl is accumulated, along with the colloid components of the cytoplasm, only during feeding and growth, which depend upon membrane uptake and intracellular membrane transformations. There is no evidence for active transport of any ion species directly across the plasmalemma.

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