CHAPTER XXI 

 THE ROLE OF WATER 



Water, as the solvent or dispersion medium of all other con- 

 stituents of protoplasm, becomes one of the most significant 

 of the components of living matter. It is the solvent, the carrier 

 of the reacting substances. It serves not only as the medium 

 in which reactions take place but as the vehicle by means of 

 which needed substances are brought to the cell and waste 

 products carried away. 



The role of water in living processes is so great that a meager 

 chapter such as the present one cannot possibly do it justice. 

 The ascent of sap in trees, the part that water plays in that most 

 fundamental of all organic reactions photosynthesis, its part in 

 the metabolism of the human body are all problems that cannot 

 be adequately dealt with here. Brief mention of some general 

 problems in water metabolism, with special reference to the role 

 of colloidal water, is the limited scope of this chapter. The high 

 surface tension of water, its tendency to form hydrates, its high 

 dielectric constant, its high specific and latent heats, but, above 

 all, its great power as a solvent make it the sine qua non of 

 biological processes. Water lubricates and protects the organs 

 of the body; the tendon sheaths are protected by water; the 

 synovial and cerebrospinal fluids are lubricants. Absorption, 

 secretion, diffusion, and excretion could not take place without 

 water. Water exists not only as a pure solvent but as water of 

 hydration, combined with carbohydrate and protein. In addi- 

 tion to water taken into the body, there is water formed by the 

 polymerization or synthesis (the reverse of hydrolysis) of com- 

 pounds and by the oxidation of hydrogen-containing substances. 

 Such water is known as metabolic water. In the case of the com- 

 bustion of a fat, slightly more than its own weight in water is 

 produced. From 10 to 14 grams of metabolic water are formed 

 per 100 cal. in the ordinary diet. 



The amount of water in organisms is great (about 75 per cent), 

 but greater still is the amount being constantly lost and added. 



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