94 PHYSICAL CHEMISTRY 



The plasma membrane is probably a separate phase, and if so, 

 in considering the permeability of a cell we must consider the 

 partition of the solute between the medium and the plasma mem- 

 brane, as well as between the plasma membrane and the proto- 

 plasm. In many cells the .protoplasm consists of more than one 

 phase. In the frog's egg, four phases in the cytoplasm and two in 

 the nucleus may be separated by means of the centrifuge (Mc- 

 Clendon, 1910 a). The partition of a solute between these phases 

 is therefore to be considered. Some solutes may be in a different 

 molecular condition within the cell, or they may be precipitated 

 or adsorbed to phase boundaries. All of these factors make the 

 subject of cell permeability a very complex one, no general rules 

 without exceptions having been found. AH we can do at present 

 is to collect data on the permeability of cells to various sub- 

 stances. The most exact method is the determination of the 

 partition of a substance between water, or some aqueous solu- 

 tion, and the cell, but unfortunately very little of this data has 

 been collected. When a substance is precipitated within a cell, 

 a partition equilibrium may never be reached. If the substance 

 in question is absolutely insoluble in the plasma membrane no 

 diffusion can take place. If the plasma membrane changes with 

 the physiological condition of the cell, substances may become 

 imprisoned in a cell that becomes impermeable to them later. 

 Furthermore, substances may be formed in cells by chemical 

 reactions which will not penetrate cells from the exterior.- 



Traube attempted to show the ease with which a substance 

 penetrates a cell to be inversely proportional to its molecular 

 attraction to water. Therefore, the more a substance lowers 

 the surface tension of water the more easily it penetrates cells. 

 As was stated above, this is only one of the factors involved. 

 It has been found that substances which very greatly lower the 

 surface tension of water do penetrate cells, but there is not 

 always a constant ratio between the surface tension lowering and 

 the penetration. This same idea championed by Batelli was at- 

 tacked by Fluzin, who claims that the attraction of the mem- 

 brane for the solvent is also important. The idea that degree 

 of hydration on the two sides of the membrane influences osmose 

 is also held by H. N. Morse. 



Overton supposed plasma membranes to be composed of what 



