The Permeability of Cells for Acids. 155 



pigment diffuses away. We have no evidence of fat-combinations with 

 proteins or other substances which would behave like fat solvents 

 in the absorption of acids. There are without doubt fat or oil combina- 

 tions with proteins in cells in which the physical characteristics of the 

 fat are marked. The elaioplasts of the leaves of the vanilla orchid 

 are the best examples. We have no visible indication that they 

 contain fat until the cells are treated with acid when the fat appears as 

 definite globules. Living cells behave very much as if they were drop- 

 lets of oil or fat solvents. 1 This is especially well seen in the alkalies 

 and in the narcotizing concentrations of series of alcohols and esters. 

 The alcohol distributes itself between cell and medium as between oil 

 and water and this is true even for plant cells which contain a mini- 

 mum of lipoid material. It is quite possible that cells are largely made 

 up of fat-protein combinations whose solubility relations are much like 

 those of pure fat or fat solvents. At present we know nothing of such 

 substances. 



Any theory of permeability must explain the behavior of cells toward 

 acids and alkalies. Why does salicylic acid enter instantly and citric 

 acid only after 40 minutes, when both are 30 per cent dissociated? 

 Why does ammonia enter instantly and NaOH not at all, when am- 

 monia is a many times weaker base? Why is the effect of ammonia 

 on cells so much greater than NaOH when, in virtue of its strength, 

 NaOH will saponify and catalyse more rapidly, and replace the weaker 

 NH 4 OH in combination? 



Without a doubt the greater physiological efficiency of ammonia is 

 due to its greatest power of penetrating the cell, so that it acts not only 

 on the cell-surface but the cell-interior as well. I know of no types of 

 membranes which allow NH 4 OH to pass through more readily than 

 NaOH. Collodion, parchment, and protein films will not. Many 

 types of protein crystals and granules and starch grains, if stained in 

 neutral red, are found to be entered equally readily or more readily 

 by NaOH than NH 4 OH. I repeat that the cell behaves toward acids 

 and alkalies much more as if it were a drop of benzol or xylol than a sol 

 or gel of any ordinary protein. This unexpected fact must be explained 

 by the assumption of some fat-like body at the cell-surface, perhaps a 

 fat protein or a lecitho-protein, but certainly not a true fat or lecithin. 



'Harvey, Am. Journ. Physiol., 6, p. 341, 1913. 



