The intracellular reference phase (RP) method and ultra-low temperature micro-dissection were used for isothermal and isotopic phase distribution studies of Na+, K+, and water in amphibian oocyte cytoplasm. One-third of the cytoplasmic water is available as solvent for [3H]sucrose. This fraction, designated c1, quantitatively coincides with the water volume in which Na+ and K+ are freely diffusible. Two-thirds of the cytoplasmic water is inaccessible to sucrose and is designated c2. The Na+ and K+ associated with c2 are extremely slowly exchanging (bound) and at different concentrations than in c1. The cations in c1 are in mass-action equilibria with those in c2, each described by an equation of the for
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