. Biochemistry of plants and animals, an introduction. Biochemistry. 32 GENERAL BIOCHEMISTRY I/) Q- Critical point. Temperature >- FIGURE 2-4. The phase diagram of water. The figure is drawn out of scale to reveal clearly the trends of the three individual curves. can be predicted that water will evaporate, sublime, freeze, melt, or remain unchanged, depending upon the conditions altered. Biological Effects of Freezing and Thawing Since water is the principal component of living cells, the latter freeze at or just below the freezing point of water itself. Profound biological changes result
. Biochemistry of plants and animals, an introduction. Biochemistry. 32 GENERAL BIOCHEMISTRY I/) Q- Critical point. Temperature >- FIGURE 2-4. The phase diagram of water. The figure is drawn out of scale to reveal clearly the trends of the three individual curves. can be predicted that water will evaporate, sublime, freeze, melt, or remain unchanged, depending upon the conditions altered. Biological Effects of Freezing and Thawing Since water is the principal component of living cells, the latter freeze at or just below the freezing point of water itself. Profound biological changes result from the change in state, but the reason for these changes is not always clear. Some may arise in part from the reduced rate of chemical reactions when temperature is lowered. Reactions in cells are often blocked by freezing because leactants must diffuse to a particular site before they can combine. Change from a liquid to a solid state prevents this necessary diffusion and the ensuing reactions. However, when the cells are warmed to their original tem- peratures, their processes should resume at normal rates if the above factors are the only ones involved. Quite often cells do not recover but are killed by the freezing and thawing. Hence other changes must be taking place. Since water expands on freezing, it has been stiggested that the increase in volume ruptures cell membranes and allows essential components to leak out when the cell thaws. Killing may occur this way but often there is no sign of cell rupture. To account for death of apparently intact cells, it was then proposed that growing ice crystals might develop between essential cell structures and force them permanently apart. But in this case also stipporting evidence is usually not available, for large ice crystals can seldom be found; yet the cells are Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of
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