CHAPTER 2 



Water 



INTRODUCTION 



ATER IS AN ESSENTIAL Constituent of ajl living things; it is 

 the universal biological solvent, the diffuse phase in 

 which most of the cellular reactions of metabolism occur, 

 and the most necessary to life of all environmental constituents. Life undoubt- 

 edly began in a watery medium. Numerous exits from water to land have 

 been made in the course of evolution, but only a few groups of animals have 

 been successful in maintaining themselves out of water. Each group which 

 has made the exit from water has used its own set of adaptations to life in air, 

 some being more successful than others. Only the insects have made the exit 

 complete, and some of them return to water for at least part of their life cycle. 

 All other animals, including birds and mammals, return to a watery medium at 

 least for embryonic life. 



One problem of animal life is to maintain inside the organism just the 

 proper amount of water— not too much, not too little. Terrestrial animals must 

 retain and use what water is available; fresh-water animals must exclude water 

 to prevent self-dilution; some marine and parasitic forms are in osmotic 

 equilibrium with their medium, whereas others are more dilute and have the 

 problem of taking in enough water while living in a plenitude of it. 



Nearly all fresh-water and terrestrial plants, by virtue of their cellulose walls 

 and active plasma membranes, maintain their cellular constituents, particularly 

 their vacuolar sap, at concentrations higher than those of the fluids which 

 bathe their tissues. The cells are continually more concentrated than the 

 tissue fluids and hence turgid. In animals, however, cellulose walls are absent, 

 and the effective intracellular concentration equals, or slightly exceeds, the 

 concentration of body fluids. Regulation of osmotic concentration in animals 

 takes place, then, not in single cells, as in plants, but in the organism as a 

 whole. 



Physical Factors. If the body fluids of an animal are more concentrated 

 than the outside medium, the organism is said to be hypertonic; if they are 

 more dilute the organism is hypotonic to the medium; if the concentrations 

 inside and out are equal, the organism and the medium are isotonic. If a cell 

 is hypertonic to the medium and is surrounded by a semipermeable mem- 

 brane, that is, one which will pass water but not solute, water will tend to 

 enter from the dilute medium. The pressure necessary to prevent such en- 

 trance of water is equal to the force by which water tends to enter and is 

 termed osmotic pressure. Strictly semipermeable membranes rarely if ever 

 exist in living organisms, otherwise there would be no exchange of solutes, 

 but for the present discussion we can consider all animal cell membranes as 

 essentially semipermeable. The osmotic pressure of a solution is proportional 



6 



