32 



GENERAL BIOCHEMISTRY 



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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 killed. 



