30 THE CHEMISTRY AND PHYSICS OF THE CELL 



pressure directly proportional to the number of molecules it con- 

 tains. In the case of the electrolytes, however, the ions produce 

 pressure as well as the molecules, and hence an electrolyte in solution 

 will produce a relatively high osmotic pressure as compared with an 

 equivalent solution of a non-electrolyte, since each molecule may jaeld 

 two or more ions. Colloids, however, exert so slight an osmotic 

 pressure that it is difficult of detection; this probably depends on 

 the great size and slight motility of their molecules. In the many 

 and important osmotic processes of the animal organism, therefore, 

 the colloids take no part except in helping to form the diffusion mem- 

 brane, and in preventing the diffusion of one another. ^^ It is interest- 

 ing to consider also that colloids under ordinary conditions do not 

 greatly modify the diffusion of crystalloids through a solution con- 

 taining both classes of matter. The fact that a cell is full of dis- 

 solved colloids does not seriously affect the osmotic properties of the 

 intracellular crystalloids, provided the colloids are not condensed 

 in such a way as to form diffusion membranes. But as all the cleav- 

 age products of proteins after they have passed the peptone stage are 

 crystalloids, by decomposition of the intracellular proteins the os- 

 motic pressure may be greatly raised. As long as the cell is living 

 there can be no constancy in composition, for metabolic processes, 

 by producing from proteins that have no osmotic pressure crystal- 

 loidal substances that do have osmotic pressure, cause intracellular 

 osmotic conditions to be continually varying. As a result, streams 

 of diffusing particles are moving about in every direction, setting up 

 new chemical reactions and consequent new osmotic currents. The 

 greater the difference in osmotic pressure between a cell and its 

 environment, and between the different parts of the same cell, the more 

 powerful the osmotic effects, and as a result the greater the capacity 

 for accomplishing work. 



Indeed, we may look upon cell life as a constant attempt at the 

 establishment of equilibrium, both chemical and osmotic, ichich is nether 

 achieved because the move towards one sort of equilibrium is always 

 against the other. All the food-stuffs — -fats, carbohjairates and pro- 

 teins — are characterized by being colloids when intact and crystalloids 

 when disintegrated, thus: 



colloidal proteins «=^ crystjilloidal aiiiino acids 

 colloidal f!;lycoG;en <=i crystalloidal siijiar 

 nondilTiisil)Ie fats ^ dilTusihlo soaps and {glycerol. 



In consequence of this, if the crystalloids difTuse from the blood into a 

 cell there is at once an excess of this end of the equation, and, hastened 

 by the intraccillular enzymes, partial syntliesis to the colloid soon 



'" Under experimental conditions it is found that the nature of the nienibrane 

 greatly modifies the osmotic jiressure; for if a ^iven colloid is soluV^le in a cer- 

 tain mend)rane and a certain crystalloid is not, the colloid will diffuse through 

 the membrane wliile the crystalloid is held back. (Kaldenberj!;, ,Iour. Physical 

 Chem., 1900 (10), 111.) 



