OSMOSIS 161 



centrated solution to dilute the stronger one, whilst, on the 

 other hand, salt will pass from the more concentrated to the 

 weaker solution, until, as a result of both these processes, 

 the two solutions on opposite sides of the membrane become 

 of one uniform strength, i.e. each contains the same number 

 of molecules in the same volume. 



Not only does this occur when the solutions contain the 

 same salt, but two solutions of different salts will also tend 

 to diffuse so that there are the same number of molecules of 

 each on either side of the membrane. The rate at which 

 osmosis takes place is proportional to the difference in mole- 

 cular concentration of the two solutions. 



Consider the effect of a membrane allowing the passage of 

 water but holding back the molecules of the dissolved 

 substance. Any membrane will do this for substances 

 known as colloids, such as albumin, gum, or starch, and when 

 tested with these it is said to be semi-permeable. There are, 

 moreover, membranes which prevent the passage of, or are 

 semi-permeable for, salts, and in fact the cell walls and cell 

 membranes in plants and animals often possess this property. 

 A cell thus surrounded and containing some small amount 

 of salt in solution in its contents, will, if immersed in a very 

 weak salt solution or in pure water, take up water to dilute 

 the salt solution inside it and become tense and turgid. 

 A plant, for example, when immersed in water or a very 

 dilute salt solution takes up water, its stems stiffen and 

 become firm because of the osmotic pressure in its cells 

 drawing the water in. If, on the other hand, the cell be 

 immersed in a concentrated solution of a salt, water now 

 leaves the cell to dilute the salt solution round about it, and 

 the cell shrinks. If the solution outside has the same 

 molecular concentration as the cell contents then no ap- 

 parent diffusion of water either in or out will take place. 

 Such solutions in equilibrium (because equimolecular) are 

 called isotonic, the more concentrated of two is called 

 hypertonic, and the less concentrated hypotonic. As used in 

 pharmacology the word isotonic signifies a solution of equal 

 osmotic pressure to that of the blood. Thus, ' normal saline 

 solution ' is a solution of sodium chloride isotonic with the 

 blood. When red blood corpuscles are placed in such a 



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