32 THE CHEMIHTRY AM) I'llY^WS OF THE CELL 



ing a delicate 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 

 t^'pes of substances is merely relative and not absolute. AVithin the 

 cell, also, the colloids probably exist as a more or less well-developed 

 emulsion, so that we have here a practically limitless amount of 

 surface formation all through the protoplasm; such a structure could 

 permit the endless number of reactions of a living cell to go on 

 side by side in the same cell. Recent studies of G. L. Kite seem 

 to show that all the protoplasm has much the same relation to solu- 

 tions as does the external layer or cell membrane, for he found 

 that if drops of solutions which can penetrate a cell from outside 

 be injected directly into a cell they diffuse through it, but sub- 

 stances which camiot penetrate from outside are also unable to 

 diffuse through the cell after they have been injected into it. 



Since osmotic pressure, exactly like gas pressure, is presumably 

 produced by the bombarding of the walls of the container by parti- 

 cles 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 mole- 

 cules, and so a solution of sugar or urea will produce an osmotic 

 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 relativelj^ high osmotic pressure as compared with an 

 equivalent solution of a non-electrolyte, since each molecule yields 

 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 

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

 interesting to consider also that colloids under ordinary conditions 

 do not greatly modify the diffusion of crystalloids through a solution 

 containing 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 nuMiibranes. But as all the eleav- 

 age products of proteins after tliey liave passed the peptone stage are 

 crystalloids, by decomposition of the intracelhdar 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, 



10 Undor cxpciiinontal conditions it is found that tlie nature of the membrane 

 preatly modifies tlic osmotic ])reasure; for if a jjfiven colloid is soluble in a cer- 

 tain membrane and a certain crystalloid is not, the colloid will ditVuse tln-oujrh 

 tlie membrane wliile the crystalloid is held back. (Kahlenberff, Jour. Physical 

 Chem., 1 !)()() (10), 141.) 



