The uptake of pyruvate by yeast was studied under a variety of conditions of temperature, extracellular concentration, O2 pressure, and pH. It was shown that physical diffusion adequately explains the permeation of undissociated pyruvic acid into the outer and inner regions of the cell. The entrance of pyruvate ion into the outer region appears to take place by the same process. The passage of pyruvate ion across the membrane separating the outer from the inner region was found to occur only under conditions under which pyruvate is metabolized. The mechanism of the active process involved in the uptake of pyruvate is not known.

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