THE PERMEABILITY OF CELLS 511 



occur in cells (inorganic salts, sugars, etc.). Further, the en- 

 trance of KNO3 can be demonstrated by the diphenylamine 

 reaction, while the cell remains plasmolysed. Consequently 

 Overton and Hoeber assumed that cells have a physical and a 

 physiological permeability. The lipoid soluble substances enter 

 in a purely physical manner, by simple diffusion, involving solu- 

 tion in the lipoids of which the plasma membrane is assumed to 

 be composed. The lipoid-insoluble substances enter in some 

 other way, but not by diffusion. As evidence of this Overton 

 calls attention to the fact that the neutral salts, sugars, etc., are 

 just the substances which are known to pass into cells or through 

 cells from regions of lower to regions of higher concentration 

 without chemical change. Some other factor than diffusion must 

 be involved.^ Hoeber ('09) now admits that the lipoid theory 

 does not hold for dyes and that it must undergo more or less re- 

 modelling (p. 78). 



On the other hand, Overton's opponents draw no distinction 

 between lipoid-soluble and lipoid-insoluble substances, but re- 

 gard the entrance of any solute as a process of simple difTusion, 

 (without considering how the substance passes the membrane), 

 the only difference lying in the relative rates of diffusion. 



The presence of salts in cells in different proportions from the 

 medium is explained as due to combination \^ ith proteids in 

 the cell. Indeed, Moore and Roaf ('07) have gcae so far as to 

 deny any importance to the existence of a surface membrane in 

 regulating the entrance of salts into red blood corpuscles, but 

 regard the difference in salt content between cell and medium as 

 entirely due to formation of salt proteid compounds. 



A great deal of confusion has arisen from the fact that a cell 

 may change in permeability from time to time. Differences are 



1 It would seem that we must draw the distinction between the accumulation 

 of salts in solution in vacuoles and the existence of salts (as determined by chem- 

 ical analysis) in the protoplasm of cells without such structures. The former is 

 a phenomenon comparable to the passage of NaCl through loops of intestine from 

 dilute to more concentrated solution. Two bounding surfaces are involved. 

 The latter is possibly explainable by the formation of ion-proteid compounds 

 within the cell as developed by Moore and Roaf (Biochemical Journal, Vol. 3, 

 p. 55, 1907) to account for the high K content of the red blood corpuscle. 



