Freezing

FREEZING. In their spread across the earth, living organisms also occupy areas where they are naturally subjected to cooling significantly below 0°. In this case, all plant organisms, and among animals, poikilothermic ones (i.e., those unable to regulate their body temperature independently of the environmental temperature), are subject to freezing. The process of freezing in living organisms is closely connected with the phenomenon of water freezing out of complex solutions that constitute their mass, where the freezing of water amounts to its crystallization from aqueous solutions in the form of ice. Thus, setting aside the physiological effect of cooling as a condition that reduces the rate of all biochemical and biophysical reactions occurring in the organism, the effect of the freezing process itself consists of two factors: on the one hand, the removal of water from the aqueous solutions that make up the protoplasm, which is equivalent to its drying out, and on the other hand, the destruction of normal protoplasmic structures by ice crystals precipitating within them. Corresponding in its physical significance to the process of water freezing out of any aqueous solutions, its freezing out of the substance of living organisms depends to a large extent on the osmotic concentration of the cooled solutions. However, due to the unique structural features of living organisms, in particular in connection with the influence of capillary forces acting in cellular and tissue spaces, the process of water freezing out of the organism's fluids is complicated by the phenomenon of their supercooling. As a result, in some cases, freezing occurs only after the organism has been supercooled to a temperature significantly below 0° [corresponding figures are: for frog muscle -0.9°; for insects-Vanessa (butterfly) -1.7°, Aporia (caterpillar of a butterfly) -10.5°; Pirola (plant) -32.1°]. The dependence of the onset of freezing on the characteristics of the living organism is evident, for example, from the fact that in living leaves of Pirola, ice formation occurs only at -31.65° and even at -32.1°, whereas in killed leaves of this plant, ice crystallizes out already at -3.1° (and not below -3.5°). The very moment of freezing, i.e., the crystallization of water from such supercooled solutions, is characterized by the so-called 'temperature jump,' i.e., a rise in temperature almost to 0° (see Anabiosis). Supercooling and the associated freezing are in themselves not yet the cause of death of organisms; for example, frog muscle completely freezes at -4.06°, but at this temperature the process is still reversible; for the final death of the muscle, cooling to -4.1° is required. The cause of death of organisms during freezing lies in the violation of the semipermeable properties of protoplasm due to the sharp removal of water and the precipitation of ice crystals, since both these processes disrupt the normal ratios of constituent substances and basic cellular structures. An example of cell permeability during freezing is the observation that when thawing, frozen beet colors the water with the pigment that has come out of its cells. Freezing as a reversible process is widely distributed in the organic world and is one of the essential factors in the adaptation of organisms to the environment (for example, of polar plants to temperatures of -40° and below). Since freezing is associated with the drying out of protoplasm, it is easily tolerated by all organisms that can also withstand drying at ordinary or high temperatures. For example, fungal spores (Mucor) withstand freezing at -110°, bacteria at -190° (temperature of liquid air), and some arachnids (Macrobiotus) at -253° (temperature of liquid hydrogen) and even at -271.8° (temperature of liquid helium). After preliminary removal of water by simple drying, subsequent freezing is tolerated by organisms much more easily, which explains the exceptional resistance to the action of freezing of dry seeds, bacterial spores, eggs of rotifers, tardigrades (Macrobiotus), etc. With careful and gradual thawing, frozen organisms can return to a state of active life (see Anabiosis). The phenomenon of freezing is sometimes advantageously used as a method for isolating and purifying various unstable substances, such as enzymes, toxins, vitamins, etc. The advantage of this method is that, thanks to it, it is possible to avoid using high temperatures for this purpose, which adversely affect the activity of the mentioned substances.