io PHYSICAL CHEMISTRY 



this process the membrane seems to change from a colloid struc- 

 ture to a mass of microscopic crystals. 



We do not know the nature of the plasma membrane, but the 

 normal cell is impermeable, or very poorly permeable, to certain 

 salts and some other substances. Some fish eggs are impermeable 

 to salts and to water. The permeability of cells is increased on 

 stimulation, and goes back to normal during rest. All cells are 

 permeable to a host of substances that reduce the surface tension 

 of water to a marked degree, and to many substances that dis- 

 solve in certain non-aqueous media, although some substances 

 that do not reduce the surface tension of water or dissolve in 

 these media penetrate cells. 



If any solution is separated from pure water or a less concen- 

 trated solution by a membrane to which the dissolved substance 

 (solute) is impermeable, the solute exerts pressure on the mem- 

 brane, and the water passes through the membrane into the solu- 

 tion. This pressure is called osmotic pressure, and depends not 

 on the percentage of solute, but on the number of molecules and 

 ions per unit volume. The standard concentration is the molecular 

 weight of a substance in grams dissolved in a liter of water. This 

 exerts an osmotic pressure of 22.4 atmospheres and is called a 

 mol. If the molecules partly dissociate into ions the osmotic 

 pressure is increased. The osmotic pressure is difficult to meas- 

 ure directly because it is difficult to obtain membranes that can 

 stand the pressure. Since it has been found that the osmotic 

 pressure is proportional to the lowering of the freezing point of 

 pure water due to adding the dissolved substance, the freezing 

 point is usually determined and the osmotic pressure calculated. 

 When the osmotic pressure is greater on the inside than on the 

 outside of a cell, the cell expands unless the cell wall is strong 

 enough to stand the full pressure without stretching. In green 

 plants the osmotic pressure is greater on the inside than in the 

 water bathing the roots, because the dissolved substances are 

 mostly manufactured on the inside. Plants use osmotic pressure 

 as the force for growth, and can force asunder great rocks when 

 they grow in a crack between them. The osmotic pressure of 

 the blood of mammals is almost as constant as the body tempera- 

 ture, but we do not thoroughly understand its role in physiology. 

 When an organ works, large molecules are broken down into 



