Physical Principles of Membrane Damage due to Dehydration and Freezing

Abstract

Water is the solvent for the ions, the organic solutes and many of the biochemicals necessary for active life, and the substrate for some of its reactions including photosynthesis. Water also allows relatively rapid diffusion of the reagents and products of vital reactions. Water is necessary, and it must be liquid water: ice is a much poorer solvent, and the diffusion in ice of solutes other than the hydroxyl ion is very slow. Therefore there is almost no active life in organisms or tissues from which most of the water is removed, or in which most of the water is frozen. Any remaining unfrozen water forms highly concentrated solutions with very high viscosities, or is bound tightly to other molecules. The rates of diffusion and of biochemical reactions are very slow or zero. As a result, the nearly dry or frozen state is, for some tissues and organisms, a state of suspended animation 1 : the processes of biochemistry and physiology slow almost to a halt, and recommence only when liquid water reappears. Seeds, for instance, survive with only a small fraction of water (5-15% by weight); so do the spores of bacteria. The tissues of resurrection plants are capable of rehydration to active life. The animals and microorganisms of the soil such as nematodes, rotifers and tardigrades survive drying and rehydration. Similarly, some organisms, tissues and cells survive temperatures which freeze the environmental water (eg. woody tissues of plants, some insects, suspensions of some cells).