176 PROTOPLASMIC ACTION AND NERVOUS ACTION 



if lipoids or other ester-like compounds are membrane- 

 components, the formation of soaps becomes a possi- 

 bihty; these vary in their solubilities and water- 

 combining properties; and variations in their proportions, 

 e.g., the substitution of Ca soaps for Na or K soaps, will 

 entail corresponding changes of permeability. 



The question of whether the protein or the hpoid 

 components of the living plasma membranes are chiefly 

 concerned in the variations of permeability underlying 

 toxic effects or normal physiological processes is an open 

 one ; but in all probability both compounds play a part, 

 although the present evidence seems to favor the view 

 that the proteins are physically the more stable of the 

 two and form the permanent structural substratum, 

 while variations of permeability depend chiefly on 

 changes in the Hpoid components, especially the soaps. 

 Parallels to the physiological salt antagonisms are wxll 

 known in non-Hving systems containing both classes of 

 compounds. Of chief biological interest is the antago- 

 nism between Na salts and Ca salts, which is apparently 

 universal in Kving organisms. Protein systems contain- 

 ing salts of these two metals show variations in properties 

 when the proportions of the salts are varied; such 

 properties as water-combining power (swelling or hydra- 

 tion), viscosity, osmotic pressure, susceptibility to 

 alteration by organic compounds (precipitation by 

 alcohol), and electrical polarization of particles are 

 affected; Loeb's recent investigations afford instances 

 of salt-antagonisms affecting all of these properties.^ 

 The inference from such facts would be that Ca and Na 



^Loeb, Journal of Biological Chemistry, XXXI (191 7), 3; XXXIV 

 (1918), 395, 489; XXXV (1918), 497. 



