PERMEABILITY OF SURFACE LAYER OF CELLS. 1 93 



The process probably requires the performance of work on the 

 part of cells, just as does the separation of a secretion by a gland. 

 The kidney performs osmotic work in concentrating its secretion, 

 and the cells of the intestinal mucosa absorb many substances 

 against concentration gradients, a capability also implying per- 

 formance of work. The rapidity with which salts and food- 

 materials are absorbed seems indeed incompatible with the exist- 

 ence of an impermeable plasma- membrane ; yet whenever resting 

 cells are tested with regard to their physical permeability the 

 above unequivocal results appear. Evidently the decisive factors 

 in absorption as also in secretion are largely independent of physi- 

 cal diffusion. 1 



Whatever the actual case may be and the problem remains 

 unsolved for the present -- there is no doubt that impermeability 

 to the diffusion of many dissolved substances of low molecular 

 weight is a highly distinctive and even necessary characteristic of 

 living cells. The following considerations will make this clear. 

 Substances of relatively low molecular weight and high diffusi- 

 bility form an important part of the living protoplasmic complex. 

 In the specific metabolism of any animal the protein and carbo- 

 hydrate food materials are split respectively to amino-acids and 

 sugars, both highly diffusible substances ; and many other diffu- 

 sible products important in metabolism are formed by oxidation 

 or hydrolysis. These substances must not be lost from the cell. 

 It is plain that any specific organism must exhibit a constant and 

 specific metabolism is indeed the product or the manifestation 

 of this. Now the existence of specific metabolic processes in any 

 cell requires the presence of many interacting substances in pro- 

 portions that must not vary widely from a constant mean ; in 

 other words, constancy in the character of its metabolic processes 

 is essential to the specificity of a particular cell. Its protoplasm 

 may be regarded as a mixture of diverse yet constantly present 

 substances in an approximate chemical equilibrium of a highly 

 complex order. Any such constancy of composition, implying 

 constancy in the conditions of equilibrium, would be impossible in 

 a system not very completely isolated from its surroundings. In 



1 Cf. Asher, Biochemische Zeitschrift, 1908, XIV., p. I, " Untersuchungen iiber 

 die physiologische Permeabilitat der Zellen." 



