WATER 



against a strictly mechanistic point of view concerning the 

 entrance of substances into cells. 



The normal living cell carries out certain reactions in 

 which water plays a part; it, therefore, needs either to take 

 in water or to get rid of it. Also in cyclical changes, as 

 those of division, in which the nucleus breaks down 

 and reforms, water moves back and forth between nucleus 

 and cytoplasm as well as between yolk and ground-sub- 

 stance. Whilst the cell-contents may vary from moment 

 to moment with respect to water present, the water within 

 a cell tends to maintain a certain level characteristic 

 of that cell. This level varies with different cells. Thus, 

 the water content of human cells differs: the enamel 

 of the teeth, spermatozoa, and bone cells are poor in 

 water while cells of the liver, kidney, intestine, etc., 

 are rich in water. The level also varies in a given cell 

 with its activity or stage in life-history. Thus, gland cells 

 when actively secreting and when at rest show different 

 levels. An egg-cell will show different levels at different 

 stages of development. Finally, the level varies with 

 changes in the surrounding medium. Every cell tends with 

 respect to water to come into equilibrium with its surround- 

 ings. The level at which this equilibrium establishes itself 

 depends upon the cell's specific composition, stage in its 

 life-history and its activity at a given time. 



Since the water level within the cell tends to remain con- 

 stant under the changes brought about by reactions, water 

 moves in or out in order to maintain this level. In a way 

 the movement of water between cell and environment is 

 comparable to the movement of oxygen; it is a diffusion- 

 phenomenon for water moves to the region of most concen- 

 trated material, i.e., from the region of more to that of less 

 water. 



The air which human beings breathe is composed of 79 

 per cent, nitrogen, 20.96 per cent, oxygen, less than one per 



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