SUMMARY: Anaerobically grown yeast, suspended in glucose solution, leaked cell contents non-selectively. The rate of leakage increased with temperature but reached a maximum rate close to the maximum temperature of growth ( ) of the yeast. The total leakage induced by glucose was much greater above than below it, because above the yeast lost the ability to take up released material. The rate of uptake of C-labelled amino acids was also substantially slowed above -Yeast heat-stressed in the presence of glucose lost the ability to establish and maintain a concentration gradient of sorbose, and simultaneously ATPase activity could be measured, all characteristics of yeast with a ruptured cytoplasmic membrane. These effects were uniquely caused by utilizable sugars, were essentially independent of sugar concentration and could be partially inhibited by Ca or inhibitors that prevent sugar utilization. Yeast heated above in water suspension was essentially undamaged as determined by the tests used, but if glucose was subsequently added below an effect of heat damage could be demonstrated. We conclude that one factor that determines the of a yeast is the temperature sensitivity of the cytoplasmic membrane in the presence of the utilizable sugar of the growth medium.


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