A biologist's view of the relevance of thermodynamics and physical chemistry to cryobiology

Abstract

Thermodynamics and physical chemistry have played powerful roles the past 45years in interpreting cryobiological problems and in predicting cryobiological outcomes. The author has been guided by a few core principles in using these concepts and tools and this paper discusses these core principles. They are (1) the importance of chemical potentials and of the difference between the chemical potentials of water and solutes inside the cell and outside in determining the direction and rate of fluxes of water and solutes. (2) The influence of the curvature of an ice crystal on its chemical potential and on the ability of ice to pass through pores in cell membranes, on the nucleation temperature of supercooled water, and on the recrystallization of ice. (3) The use of Le Chatalier's Principle in qualitatively predicting the direction of a reaction in response to variables like pressure. (4) The fact that the energy differences between State A and State B are independent of the path taken to go from A to B. (5) The importance of being aware of the assumptions underlying thermodynamic models of cryobiological events. And (6), the difficulties in obtaining experimental verification of thermodynamic and physical-chemical models.

Fig. 1

Schematic of changes in the osmolalities of water and solutes and in the volume of water in a cell abruptly transferred from isotonic (0.3 osmolal) PBS to a solution of 1 osmolal ethylene glycol (EG) in hypotonic salt (0.15 osmolal). The cell is assumed to be permeable to the former but impermeable to the later. The superscripts i and e refer to inside and outside the cell, M_s and M_T are the osmolalities of salt and the total osmolality (salt + EG). V_w is the volume of water in the cell relative to its volume in an isotonic cell. The time scale is arbitrary except to indicate that the swelling process in C–G is much slower than the shrinking process in A to B. The circles represent the variations in cell size but are not drawn quantitatively to scale.