I02 PROTOPLASMIC ACTION AND NERVOUS ACTION 



is effected. During life it behaves as if it were semi- 

 permeable and water-insoluble; yet it is evident that 

 it allows passage, at least at certain times, to water 

 and many dissolved substances; e.g., food-substances. 

 Because of its special character as an intermediary 

 structure or organ, its properties require special con- 

 sideration in any study of the properties of protoplasm. 

 In some respects the properties of the plasma 

 membrane appear difficult to explain on known physical 

 grounds or even paradoxical. If a diffusible food- 

 substance passes through the membrane into the cell it 

 may there undergo metabolism, but it is difficult to 

 see how any such substance can enter without other 

 important substances leaving. If simple diffusion alone 

 is the chief factor in the entrance and exit of dissolved 

 materials, we should expect that any increase of per- 

 meability sufficient to allow the entrance of substances 

 like sugar would also involve a loss of diffusible cell 

 constituents to the exterior. It seems necessary to 

 assume that the conditions determining the normal 

 transport of substances across the cell boundary are of a 

 special kind, and that diffusion is only one of a number 

 of factors. In fact we know that in secretion and 

 absorption special physiological mechanisms of transport 

 are concerned, by which water and dissolved substances 

 are actively conveyed into and out of the cell, fre- 

 quently against concentration-gradients. This process 

 requires the performance of work by the cell, just as 

 does the process of muscular contraction, although 

 its exact conditions are at present unknown. Many 

 substances (sugar, salts, amino-acids, urea) may thus 

 be transported from regions of lower to regions of higher 



