72 - The Cell 



viclually separated and distributed throughout the 



water. 



The solvent capacity of water is important 

 because the many compounds in protoplasm 

 tie dissolved in a common medium, and this 

 intimate mixing of the different molecules 

 greatly enhances their chemical reactivity. 

 Dissolved substances interact together much 

 more rapidly than mixtures in a dry state. 

 Therefore, much of the complex metabolism 

 of the cell would not be possible if the proto- 

 plasmic dispersion medium cotdd not en- 

 compass such a rich variety of chemical 

 components. Moreover, the protein compo- 

 nents of the protoplasm require an aqueous 

 environment if each is to maintain its molec- 

 ular configuration and metabolic activity 

 (p. 87). 



Thermal Properties of Water. The freezing 

 (0° C) and boiling (100° C) points— between 

 which water remains in liquid state — repre- 

 sent the approximate extremes that are tol- 

 erable to protoplasm. 5 If temperatures in 

 this range had not prevailed on the earth for 

 many years, life as we know it cotdd not have 

 been evolved. Likewise, the exceptionally 

 high heat capacity'' 1 of water plays an impor- 

 tant role in the Hie of organisms generally. 

 When the temperature of the environment 

 changes rapidly, that of the living body 

 changes much more slowly, owing to its large 

 content of water. Accordingly, organisms 

 have time to take measures of self-preserva- 

 tion when drastic changes occur in the tem- 

 perature of the environment. A frog, for 

 example, may sit on a rock in the broiling 

 sun, while the temperature of the rock — 



5 In dormant .slate, a number of cells are able to 

 endure lengthy exposure to subzero (<° C) tempera- 

 tures, but few if any can continue to function during 

 the exposure. 



6 The heat capacity (specilic heat) of a substance 

 specifies the quantity of heat (number of calories) re- 

 quired to raise the temperature of 1 gram of the sub- 

 stance 1 degree Centigrade. For water, the heat capac- 

 ity is one (cal), an exceptionally high value. Or. to 

 use a different unit, one large calorie (1 Cal), which 

 is designated bv the capital C. is required to produce 

 a 1° C elevation in the temperature of a liter (= 1000 

 g) of water. 



which has a relatively low heat capacity — 

 rises with considerable rapidity. But the tem- 

 perature of the frog rises much more slowly, 

 and this allows ample time for the frog to 

 seek a nearby pond. 



Another important thermal property of 

 water is its exceptionally high heat of vapor- 

 ization," which provides a cooling factor in 

 organisms generally. In the case of man and 

 other higher animals, perspiration is vapor- 

 ized from the surface of the body, and this 

 process dissipates large amounts of heat, 

 which otherwise would elevate the body 

 temperature. In higher plants, the process of 

 transpiration likewise keeps the temperature 

 of the leaves below the lethal point; and at 

 the same time, transpiration generates a force 

 that draws more water up from the soil 

 (Chap. 13). 



Chemical Activity of Water. Water partici- 

 pates in many metabolic reactions in all 

 cells. Hydrogen and oxvgen are constituents 

 of virtually all organic compounds, and con- 

 sequently water frequently appears among 

 the initial or end products, when such com- 

 pounds are metabolized in the protoplasm. 



Water in Relation to Dissociation. In 

 solution many molecules tend to dissociate, 

 or ionize, forming electrically charged frag- 

 ments, called ions. Not all substances can 

 dissociate appreciably, but the tendency, if 

 present, always reaches a maximum when a 

 substance is dissolved in water. 



The dissociation ot table salt (NaCl) is 

 given in the following equation: 



which shows that each molecule of NaCl, 

 when it dissociates, liberates one sodium ion 

 (Na + ) and one chloride ion (Cl~). 



7 The heat of vaporization (expressed in Calories) 

 specifies the quantity of heat required to vaporize one 

 gram of a liquid, and the value for water (0.540 Cal) 

 is unusualh high. 



