130 PERMEABILITY 



hence could penetrate through the membrane at relatively slow rates. Oster- 

 hout (1936) considers that electrolytes must diffuse through this layer prin- 

 cipally in the form of molecules as their dissociation when dissolved in a fat- 

 like liquid would be very slight. 



Other authorities are not convinced that this theory presents a complete 

 picture of the basis for differential permeability in the cytoplasm. Observed 

 rates of penetration of some polar compounds seem too great to be accounted 

 for satisfactorily by the lipoid-solubility hypothesis. Modifications of this the- 

 ory have therefore been suggested. For instance Nathansohn (1904) has pro- 

 posed a theory which postulates a membrane composed of a mosaic of proteins 

 and lipoids. He assumed that polar (lipoid-insoluble) compounds could dif- 

 fuse through the more or less hydrated protein portion of the membrane. Per- 

 meability to the latter class of compounds might vary with changes in the 

 hydration of the proteins. Although this view is almost purely speculative, 

 some properties of the membrane, such as its elasticity, suggest the presence 

 of proteins as one of its constituents. 



3. Adsorption a?td Chemical Reaction Theories. — It has been suggested 

 that the permeability to non-polar compounds is correlated more closely with 

 the adsorptive capacity of the particles of the membrane for them than to their 

 solubility in membrane constituents. In brief this adsorption theory would 

 hold that the more strongly molecules are adsorbed, the more readily they 

 will pass through the membrane. The possible mechanics of the transport of 

 adsorbed molecules across a membrane is described later. 



Chemical reactions may be involved in the transport of certain types of 

 solutes across the cytoplasmic membranes. A penetrating molecule or ion, 

 according to such a chemical reaction theory, is supposed to combine with 

 some substance at the outer boundary of the membrane, thus forming an inter- 

 mediate compound, in the form of which it is presumed to move across the 

 membrane. Once the other boundary of the membrane has been reached a 

 second chemical reaction is supposed to occur resulting in release of the ion 

 or molecule on the interior of the membrane. 



Whether the penetration of a molecule depends upon solution, adsorption, 

 or chemical reaction the probability is strong that the cytoplasm itself actively 

 participates in some manner in the movement of substances through it. For 

 example, after molecules of a penetrating substance dissolve in, are adsorbed 

 by, or combine chemically with micelles of the membrane, their penetration 

 may be facilitated by kinetic activity of the membrane particles. A micelle 

 might act in the capacity of a ferry boat, transporting a cargo of one or more 

 molecules from one side of the membrane to the other. Once across the 

 membrane many of the passenger molecules would escape because of their 



