Water permeability of yeast cells at sub-zero temperatures

Abstract

A combined cryomicroscopic-multiple nonlinear regression analysis technique has been used to determine the water permeability of the yeast cell Saccharomyces cerevisiae during freezing. The time rate of change in volume of "supercooled" yeast cells was photographically monitored using a "cryomicroscope" which is capable of controlling in a programmable manner both the temperature and the time rate of change in temperature of the cell suspension being studied. Multiple nonlinear regression analysis together with a thermodynamic model of cell water transport during freezing was then used to statistically deduce the subzero temperature dependence of the cell water permeability. The water permeability process for S. cerevisiae being cooled at subzero temperatures was found to be rate-limited by the passage of water through either the plasmalemma, the cell wall, or a combination of these two permeability barriers. The hydraulic water permeability coefficient for yeast at 20 degrees C is approximately 1--2 x 10(-13) cm3/dyne sec, if extrapolation from subzero temperatures to room temperature is permissible, while the apparent activation energy governing the permeability process at subzero temperatures is approximately 45--68 kJ/mol (11--16 kcal/mol).