Saccharomyces cerevisiae does not accumulate ethanol against a concentration gradient

Area of Science:

• Biochemistry
• Cell Biology
• Microbiology

Background:

• Previous studies suggested yeast cells accumulate ethanol against a concentration gradient.
• The mechanism of this reported ethanol accumulation was not well understood.

Purpose of the Study:

• To investigate the mechanism of ethanol accumulation in yeast cells.
• To determine if ethanol transport across the yeast cell plasma membrane involves active transport or simple diffusion.

Main Methods:

• Studied ethanol uptake and outflow kinetics in yeast cells.
• Assessed the effect of ethanol structural analogs, pH changes, and chemical reagents on ethanol transport.
• Measured intracellular ethanol concentrations under various conditions.

Main Results:

• Ethanol uptake and outflow followed first-order kinetics.
• Transport was insensitive to structural analogs, pH changes, and thiol/amino group reagents, indicating no carrier involvement.
• Ethanol outflow rates exceeded production rates, precluding intracellular accumulation.
• Intracellular ethanol concentrations mirrored external media concentrations.

Conclusions:

• Ethanol permeates the yeast cell plasma membrane via simple diffusion.
• The previously reported ethanol accumulation in yeast is likely due to experimental artifacts, such as inadequate cell-sampling and chromatography.
• Yeast cells do not actively transport or accumulate ethanol against a concentration gradient.