Osmotic response of individual cells during freezing. II. Membrane permeability analysis.

An analytical model is presented to simulate the freezing of individual yeast cells. In addition the model is solved numerically on a digital computer to obtain values for cell volume as a function of temperature, based on the thermal protocol during freezing, and the transport parameters of the cell membrane. The numerical procedure was modified to enable values for the membrane hydraulic permeability reference coefficient, Lpg, and activation energy, ELp, to be deduced by nonlinear analysis of complementary experimental data (10). It was observed that the apparent values of both Lpg and ELp increase with cooling rate, from Lpg = 0.0116 micrometer 3 micrometers-2 atm-1 min-1 and ELp = 19.4 kJ mol-1 for 9 degrees K/min to Lpg = 2.11 micrometers 3 micrometer-2 atm-1 min-1 and ELp = 101 kJ mol-1 for 35 degrees K/min. The deduced permeabilities fall within the range of values determined in a prior study by Levin (6). Analysis with the model also indicates that the turgor pressure exerts a negligible effect on yeast exposed to freezing stress.