PROTOPLASMIC STRUCTURE iii 



the plasma membranes. The most significant general 

 fact is that apparently all substances with solubilities 

 or solvent properties characteristic of organic compounds 

 (rather than of water-soluble compounds or water) enter 

 cells with special readiness. A relation between the 

 presence of organic solvents in protoplasm and the 

 permeability of the plasma membrane is thus indicated. 

 Overton, who first investigated in detail this connection 

 between organic solubility and power of penetration, 

 drew the conclusion that the plasma membrane consisted 

 essentially of the so-called "lipoid" compounds, espe- 

 cially lecithin and cholesterol, which are universally 

 present in protoplasm. He showed that all members of 

 homologous series, such as alcohols, ethers, esters, normal 

 and substituted hydrocarbons, ketones, aldehydes, 

 amides, and similar compounds, readily enter living 

 cells; such compounds dissolve, or are dissolved by, the 

 lipoids or solutions of lipoids in organic solvents; and 

 their ready entrance is a result of this solubility. On 

 the other hand, sugars and polyatomic alcohols (pentites, 

 hexites, etc.), with molecules containing many hydroxyl 

 groups, are highly soluble in water, but not in lipoids, 

 and do not enter cells readily. In Overton's original 

 experiments, acid and basic dyes also showed a relation 

 between lipoid-solubility and power of penetration; but 

 the conditions are complex in this case and many excep- 

 tions to this rule are now known. ^ In the case of neutral 

 salts of alkali and alkali earth metals (especially Na, 

 K, and Ca) there is also Kttle or no evidence of penetra- 



^Cf. Ruhland, Jahrh. wiss. Botanik, XL VI (1908), i; cf. also 

 Hober's discussion, Physikalische Chemie der Zelle und der Gewebe, 

 pp. 426 ff. 



