22 



'HYSIOLOGICAL CHEMISTRY 



the gas laws. This pressure of a dissolved substance is called 

 osmotic pressure. Obviously it will be impossible to measure 

 this pressure by observing the pressure of the solution on its 

 container. It will be necessary to devise a separating wall or 

 membrane through which the solvent, but not the dissolved sub- 

 stance can pass. Such a membrane is called a semipermeable 

 membrane. A sheet of parchment or a thin film of collodion 

 serves such a purpose. Since the solvent can pass through the 

 membrane, it will tend to pass through the membrane freely. 

 If an apparatus is set up with such a membrane, on one side 

 of which is a solution, on the other pure solvent, the solvent will 

 pass into the solution faster than the reverse process, and tend 

 to dilute the solution. It is possible to place the solution in 

 connection with a manometer, and thus measure the osmotic 

 pressure. Osmotic pressure is directly proportional to the 

 amount of dissolved substance, and to the absolute temperature, 

 at least for dilute solutions. Aside from its usefulness in deter- 

 mining molecular weights and other theoretically important 

 data, it is at once apparent that osmotic pressure will play an 

 enormously important role in the body. The body and cell 

 fluids are solutions, and the cell membranes are semipermeable 

 membranes. The maintenance of a proper distribution of the 

 liquids in the body depends on a delicate balance of osmotic 

 equilibrium, as do also such processes as secretion, absorption, 

 etc. 



A biological method of measuring osmotic pressure consists 

 in immersing cells in a liquid of known osmotic pressure. If 

 the pressure in the surrounding liquid is higher than that in the 

 cell, water passes out of the cell, and its contents can be seen 

 to shrink away from the cell wall. If the reverse is the case, 

 water passes into the cell, even to the point of rupture. The 

 passage of water through the membrane is called osmosis. The 

 above described method of determining it is called plasmolysis. 



If the pores of the membrane are large enough to allow the 

 passage not only of water molecules, but also of simple chemical 

 substances such as salts, but still too small to allow the passage 



