Once freezing and thawing the yeast leads to incomplete destruction of the cell membrane and to too low values of the cell pH determined in the cell suspension with the glass electrode. The cells are rendered completely permeable through boiling, treatment with acetone, or freezing and thawing the cell repeatedly. Cell pH's obtained in this ways are definitely higher. The differences in cell pH's observed with completely broken cells and partly broken cells can be accounted for quantitatively to the presence of intact cells in the only once frozen and thawed cell suspension.

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The effect of Cd2+ poisoning of Saccharomyces cerevisiae on ' W a , Io9Cd and [ I4C]tetraphenylphosphonium (TPP) uptake and cell pH was examined. At Cd2+ concentrations that produced substantial K+ efflux the rates of uptake of 45Ca, lo9Cd and [I4C]TPP increased progressively during incubation of the cells with Cd?+, and the cell pH was lowered concomitantly. The initial rates of uptake of the divalent cations and of TPP were increased in cells pre-loaded with Cd2+, which shows that stimulation of the ion fluxes was exerted by the Cd2+ that accumulated in the cells. The distribution ratio of TPP between cells and medium, however, was decreased by Cd2+. Although hyperpolarization of the cell membrane by Cd2+ cannot be excluded, it is argued that Cd2+ primarily stimulated divalent cation uptake by increasing the cation permeability of the cell membrane allowing the cations to enter the cells more easily.

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