30 GENERAL BIOCHEMISTRY 



themselves may be of different kinds. Thus pohn molecules inteiact 

 and polar substances tend to be most soluble in polar liquids. It so 

 happens that a great many molecules of biological importance are 

 polar and thus are soluble in water. Indeed water is the solvent 

 par excellence in biology, thus fulfilling yet another vital role. 



Water shows unusual aspects in almost all its physical properties, 

 and all these features are biologically important. For instance, water 

 has an exceptionally high specific heat, thus exerting a moderating 

 effect on temperature changes. Climatological stabilization by the 

 proximity of large bodies of water may be cited as a well-known illus- 

 tration. 



Everyone knows that ice floats in water. Yet a density in the solid 

 state below that for the liquid is rather a rarity. Conceive of the con- 

 sequences if water were an ordinary substance in this respect. Ex- 

 posure of water to cold air would produce ice at the surface, but this 

 solid would sink and accumulate until the liquid was completely 

 frozen. Even oceans would freeze solid when deprived of the insulating 

 effect of a layer of floating ice. Melting of the surface would occur in 

 warm seasons, but huge quantities of ice would persist in deep water. 

 Thus life could hardly become particularly abundant in the oceans. 

 Moreover, the land might become much more arid than now without 

 open water throughout part of the year to recharge the atmosphere 

 with moisture. 



In addition to these and other important physical roles played by 

 water, this substance is of particular chemical consequence as well. 

 As will be illustrated later, water is often either consumed as a re- 

 actant or appears as a product in the metabolic reactions of living 

 things. Thus water is a critical material in both photosynthesis and 

 respiration. 



The dipolar character of water molecules leads to their association 

 with other charged particles, a process called hydration. In this way 

 some of the water of cells is restricted in its movement, at least to a 

 limited and temporary extent. Some investigators have felt that water 

 so involved is tightly and rather permanently attached and have called 

 it "bound water." Moreover, they have attempted to show experi- 

 mentally that much of the water in cells may be in this state. However, 

 other types of experiments yield results in disagreement with such an 

 interpretation. The basis and application of these latter findings seem 

 sound, and rather serious objections have been made to the earlier 

 work. Hence, most current opinion holds that water of hydration is 

 in rapid equilibrium with the ordinary liquid water of cells. Further- 

 more, it is believed that water of hydration either is a small fractiou 



