CHAPTER VII 



GENERAL CONDITIONS DETERMINING THE 

 PROPERTIES OF PROTOPLASMIC MEMBRANES 



Overton's and later researches have shown that the 

 permeability of plasma membranes to lipoid-solvent 

 substances is a universal property, that the permeability 

 to water-soluble substances of low molecular weight, not 

 soluble in organic solvents, is relatively very slight, and 

 that the ability of substances to penetrate most forms of 

 protoplasm has a close dependence on their relative solu- 

 bility in the two classes of solvents; i.e., on their par- 

 tition-ratios between organic (oil-like) solvents and water. 

 The permeability for the two classes of substances, 

 water-soluble and '^organo-soluble," thus depends on 

 different conditions. It is further significant that the 

 permeabiHty for lipoid- soluble substances is much less 

 variable than that for the lipoid-insoluble substances 

 and water; the latter form of permeability shows wide 

 variations in the same cell under different physiological 

 conditions, while the former appears to undergo Httle 

 change. The characteristic semi-permeability of the 

 living plasma membranes thus relates to the lipoid- 

 insoluble group of substances; this fact, when con- 

 sidered in connection with the universal permea- 

 bility to the Hpoid-soluble group, suggests that the 

 normal semi-permeabiHty depends on the presence in 

 the membranes of water-insoluble compounds pos- 

 sessing the solvent properties of organic solvents. 

 The characteristic water-insolubiHty of the surface- 



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