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Saccharomyces cerevisiae does not accumulate ethanol against a concentration gradient

It has been reported that yeast cells accumulate ethanol against a concentration gradient. We initiated a study of the mechanism involved in this phenomenon. However, we found that this accumulation does not occur and that ethanol permeates the yeast cell plasma membrane by simple diffusion. The following evidence supports this conclusion. (i) Uptake and outflow of ethanol in yeast cells followed first-order kinetics and were insensitive to the presence of structural analogs of ethanol, to drastic pH changes, and to the action of reagents of amino and thiol groups. These results strongly suggest that ethanol permeates the yeast cell plasma membrane without involvement of any carrier. (ii) The outflow rate of ethanol seems greater than the ability of this organism to produce ethanol, indicating that intracellular accumulation of ethanol is not possible. (iii) The intracellular concentration of ethanol found was similar to the concentration in culture media in all tested conditions. With the available information, it is difficult to ascertain the reasons for the discrepancy between our results and those previously reported by other authors. However, the inadequacy of the cell-sampling procedure and of the chromatographic conditions used by those authors suggests that the discrepancy may be due to artifacts in the measurements of ethanol.