1. When yeast oxidizes propan-2-ol in the presence of KCl no uptake of K+occurs. 2. When propionate is added to suspensions containing propan-2-ol, or if the suspensions are bubbled with CO2, a considerable uptake of K+occurs. 3. Maximum K+uptake occurs at a propionate concentration of 2mm. 4. The addition of 20mm-propionate to the suspension lowers the intracellular pH of the yeast from a resting value in the region of 6.2 to approx. 5.6. 5. When K+uptake is measured in the presence of 20mm-propionate, progressive changes in the rate of K+uptake and intracellular pH occur. The optimum rate of K+uptake occurs at an intracellular pH of 5.70. 6. The effect of both intra- and extra-cellular pH on K+–K+exchange was studied and an optimum rate was found at an extracellular pH of 5.35, the corresponding intracellular pH being 6.44. 7. When a Na+-loaded yeast oxidizes propan-2-ol in the presence of KCl, a steady efflux of Na+and influx of K+occurs. The addition of 10mm-propionate to the suspension markedly inhibited the Na+efflux but only slightly decreased the K+influx. 8. The effect of both extra- and intra-cellular pH on Na+efflux was studied with propan-2-ol and with glucose. The results can be best interpreted in terms of intracellular pH changes, and an optimum was obtained at approx. pH6.40.

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