THE CHEMISTRY AND PHYSICS OF THE CELL 39 



(plasmorrhexis, plasmoptysis) does not seem to be a normal occur- 

 rence in animal tissues. We shall be most nearly correct, 

 probably, if we look upon the animal cell as possessing a deli- 

 cate diffusion membrane at its surface, through which water 

 passes more readily than do most crystalloids, and through 

 which colloids pass almost not at all, but the exclusion of each 

 of these types of substances is merely relative and not abso- 

 lute. 



Since osmotic pressure, exactly like gas pressure, is produced 

 by the bombarding of the walls of the container by particles in 

 the solution, the amount of pressure will vary in proportion to 

 the number of particles present. With such substances as 

 sugar and urea, the non-electrolytes, the moving particles seem 

 to be molecules, and so a solution of sugar or urea will produce 

 an osmotic pressure directly proportional to the number of 

 molecules it contains. 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 yields two or more ions. Col- 

 loids, 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 

 membrane, and in preventing the diffusion of one another. It 

 is interesting to consider also that colloids under ordinary con- 

 ditions do not greatly modify the diffusion of crystalloids 

 through a solution containing both classes of matter. The fact 

 that a cell is full of dissolved colloids does not seriously affect 

 the osmotic properties of the intracellular crystalloids, provided 

 it is not condensed in such a way as to form diffusion mem- 

 branes. But as all the cleavage products of proteids after they 

 have passed the peptone stage are crystalloids (e. g., leucin, 

 tyrosin, glycocoll, etc.), by decomposition of the intracellular 

 proteids the osmotic 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 proteids that have 

 no osmotic pressure crystalloidal substances that do have 

 osmotic pressure, cause intracellular osmotic conditions to be 

 continually varying. As a result, streams of diffusing parti- 

 cles 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 



