SUMMARY: The passive permeation and facilitated diffusion of glycerol into K1060, an unsaturated fatty acid auxotroph, were studied as a function of temperature and membrane lipid fatty acid composition using a stopped-flow spectrophotometric assay of glycerol permeation. The relative rates of glycerol passive and mediated entry were both significantly influenced by the fluidity of the membrane lipids, increasing as the gel to liquid-crystalline phase transition midpoint temperature of the membrane lipids decreased. The rate of passive glycerol permeation, but not the rate of glycerol facilitated diffusion, decreased as the membrane lipids were converted to the gel state. The apparent activation energies for passive and facilitated diffusion of glycerol, measured in cells whose membrane lipids were in the liquid-crystalline state, were 15–16 and 10–11 kcal mol, respectively, and neither value was significantly influenced by the fatty acid composition or fluidity of the membrane lipids. The mechanistic implications of these observations for the function of the glycerol facilitated diffusion system of are discussed.


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