280 PROTOPLASM 



suggestion of the mechanism of cell permeability and the proto- 

 plasmic membrane is still applicable. Overton centered his 

 attention on fat solvents such as alcohol, ether, chloroform, 

 xylene, methane, etc., and found that they all enter the living cell 

 very rapidly. Salts, on the other hand, enter very slowly. He 

 therefore believed that the plasma membrane is chiefly lipoid. 

 Lipoid is a word freely used by biological chemists but not of 

 very precise meaning. It includes those substances which are 

 not true fats or oils but resemble them in being "oily" in nature 

 (page 463).' Lecithin and, less accurately so, cholesterin (or 

 cholesterol) are among the best known lipoids. Lecithin is 

 better classified as a phosphatide, and cholesterin as a sterol. 

 Solvents (chloroform, etc.) of these fatlike substances quickly 

 dissolve their way into the cell, while water-soluble salts enter 

 very slowly. Overton neglected the salts, but his own hypothesis 

 permits an interpretation of the entrance of salts into the cell 

 on the basis of the hydrophilic nature of lipoids. Lecithin 

 becomes milky when shaken in water ; this means that it has taken 

 up water and is permeated by it without going into true solution. 

 After years of doubt and criticism, biologists have returned to a 

 modified interpretation of Overton's hypothesis. It is now 

 generally recognized that the protoplasmic membrane is fatty 

 in nature at its outer surface but only like fat, for the true fats 

 and oils, such as olive oil and petroleum (Nujol) oil, do not enter 

 the cell at all. 



It has long been known (since 1846, when first shown by 

 DuBois Reymond) that the electric resistance of tissues is very 

 high. This quality may rest upon the selective permeability of 

 the cell membrane (for, if cations can pass and anions not, then 

 no current can penetrate, and the resistance will be high), 

 or the fatty nature of the membrane may be responsible. Oster- 

 hout takes this latter view and assumes that high electric resist- 

 ance of the protoplasmic surface of the alga Nitella can be 

 accounted for only if the membrane is of nondissociable, non- 

 aqueous substances, i.e., of fatty material. 



The high protein content of protoplasm, which suggests the 

 presence of protein at the surface, stands in contrast to the above 

 evidence in support of a fatty nature of the cell membrane. Fur- 

 thermore, the elastic properties of the membrane require it. Nath- 

 anson thought the cell membrane to be a mosaic of fat and protein. 



