Supercooled cell water

To survive freezing, a cell must be cooled in such a way that it contains little or no freezable water by the time it reaches the temperature at which internal ice formation becomes possible. Above that temperature, the plasma membrane is a barrier to the movement of ice crystals into the cytoplasm. The critical factor is the cooling rate. Even in the presence of external ice, most cells remain unfrozen, and hence, supercooled, 10 to 30 degrees below their actual freezing point (-0.5 °C in mammalian cells). Supercooled cell water has a higher chemical potential than that of the water and ice in the external medium, and as a consequence, it tends to flow out of the cells osmotically and freeze externally (Figure 1). 

The cell in Fig. 4.5 represents a fixed value of chemical drive just as the original meter and the original kilogram in Paris represent fixed length and mass values. This example shows the solidification of supercooled heavy water (freezing point 276.97 K),... 

There are aspects of cell membranes other than their permeability to water and solutes that also play a critical role in the responses of cells to freezing. The structure of the plasma membrane allows cells to supercool and probably determines their ice-nucleation temperature. The nucleation temperature along with the permeability of membranes to water are the chief determinants of whether cells cooled at... 

K. Striking is the broad distribution of jump times of water in cell walls coextending from times of liquid water to ice. We can compare water in cell walls with supercooled water with a broad scale of mobilities. The reduction of the apparent T may be induced by the interaction water/mucopolysaccharid groups. Water in charcoal with mean pore radius of 13 A shows a broader distribution of t but with a... 

In general, intracellular freezing induced with extracellular ice crystal initiates around -5°C and most freezable water freezes by the time the cells reach -20°C. Thus, freezing injury of the cells should be concentrated in this temperature region. On the other hand, water molecules cannot endure in a supercooled state under —40°C even if there is no seeding of ice crystals. This suggests that reduction of cell viability is restricted to temperatures above -40°C. The results shown in Figure 9, also support this conjecture. 

As was demonstrated previously when cells are cooled without external ice, a supercooling of the cellular water increases, and then if finally ice crystals are formed in the cells spontaneously, their size would be too small to cause injury to the cells. Another factor in maintaining cell viability would be to stabilize the cell membrane itself. It is well known that the structural stabilities of the cell membrane are very important to the function of the cells, and when the cell membrane is injured by external or internal ice, the inevitable consequence will be cell damage. 

The same apparatus was used to measure the kinetics of emulsion crystallization under shear. McClements and co-workers (20) showed that supercooled liquid n-hexadecane droplets crystallize more rapidly when a population of solid n-hexa-decane droplets are present. They hypothesized that a collision between a solid and liquid droplet could be sufficient to act as a nucleation event in the liquid. The frequency of collisions increases with the intensity of applied shear field, and hence shearing should increase the crystallization rate. A 50 50 mixture of solid and liquid n-hexadecane emulsion droplets was stored at 6 -0.01 °C in a water bath (i.e., between the melting points and freezing points of emulsified n-hexadecane). A constant shear rate (0-200 s ) was applied to the emulsion in the shear cell, and ultrasonic velocities were determined as a function of time. The change in speed of sound was used to calculate the percentage solids in the system (Fig. 7). Surprisingly, there was no clear effect of increased shear rate. This could either be because increase in collision rate was relatively modest for the small particles used (in the order of 30% at the fastest rate) or because the time the interacting droplets remain in proximity is not affected by the applied shear. 

Note There are three allotropes of carbon graphite, diamond, and buckminsterfullerene (Cgo) the latter, discovered in 1985, is composed of soccer-ball-shaped molecules. The thermodynamic stability of buckminsterfullerene has not yet been determined. The validity of its inclusion on the C phase diagram is, therefore, uncertain. (Metastable phases, such as supercooled water, do not appear on phase diagrams.) The crystal structure is face-centered cubic with Ceo molecules at the corners and faces of a cubic unit cell. The unit cell is shown below. 

If freezing of tissues occurs slowly, ice forms in extracellular areas as water flows out of the cell by exosmosis. As a result, the cell dehydrates and does not freeze intracellularly. However, if the cell is cooled rapidly, it cannot lose water fast enough to maintain equilibrium with its environment, and it therefore becomes increasingly supercooled and eventually freezes intracellularly (27). Mazur (27,28) suggested that injury from intracellular ice and its subsequent growth by recrystallization is a direct... 

Much work has been carried out on pressure-dependent studies of properties (including diffusion measurements and spin-lattice relaxation times) in water and aqueous solutions. There has been particular interest in supercooled metastable states. Heterogeneous nucleation is dependent on the sample volume. Consequently, a small volume capillary cell is required for high-pressure NMR studies on aqueous solutions under these conditions. This was discussed in Section 5.1.2.3. In order to stabilize aqueous solutions under... 

Model II supposes that the cell membrane becomes ruptured, whereupon the supercooled water freezes immediately, since it is now in direct contact with ice. However, several lines of evidence indicate that such rupture does not occur in cells that survive low-temperature exposure and sometimes not even in cells that are killed One piece of evidence, which comes from microscopy, shows that cell membranes prevent seeding of the supercooled water within, at least at temperatures above — 10°C P]. 

Alternatively, equilibrium could also be established if the supercooled water were to flow out of the cell and freeze externally instead of internally. The result of this process... 

The MTD simulations were initialized in the supercooled liquid state. The models contained 512 and 576 molecules. TIP4P force fields [35] were used to model the interactions between water molecules. The authors used different supercells, including orthorhombic and hexagonal unit cells commensurate with the 4 symmetry and cubic cells commensurate with 4. However, regardless of the choice of the unit cell, only the nucleation of 4 ice was observed. The authors concluded that this form of ice is more stable than the hexagonal one, at least within the limits of the force field. 

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