30 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



permeability are unknown and it is hoped that investigations of those condi- 

 tions producing such effects will ultimately lead to a better understanding of 

 what makes a cell permeable or impermeable. 



The concept of a permeable membrane poses certain problems, however. 

 During synthesis, the intermediates of the cell are not lost by diffusion. With 

 E. coli there is no evidence that any large concentration of intermediates, such 

 as amino acids, is built up in the medium before protein synthesis occurs. 

 When an exponentially growing culture of cells is washed and transferred to 

 fresh medium, immediate growth occurs and the growth rate shows no altera- 

 tion. Accordingly, there is a paradox; small molecules are free to diffuse into 

 and out of the cell, but in most cases metabolic intermediates do not diffuse out. 



One possible hypothesis to explain this paradox is to assume that even 

 though the cellular membrane is permeable, certain regions of the cell are 

 surrounded by impermeable or selectively permeable membranes. This hy- 

 pothesis permits arbitrary limitations on the access of exogenous compounds 

 to the reactive centers of the protoplasm, and provides a mechanism for hold- 

 ing endogenous metabolites within desired channels. When examined more 

 critically, however, this hypothesis of 'accessibility barriers' becomes untenable. 



For example, exogenous C^^-threonine used to supplement a medium con- 

 taining C^^-glucose provides radiocarbon for both threonine and glycine of 

 the proteins (24). Endogenous threonine, derived from glucose in the absence 

 of exogenous threonine, does not contribute carbon to glycine. If it is assumed 

 that the region where threonine is synthesized from glucose, together with the 

 regions where threonine is used for protein synthesis, is surrounded by an 

 accessibility barrier, this membrane must have very peculiar properties. 

 Exogenous threonine must be able to penetrate this membrane, as exogenous 

 threonine can be used for protein synthesis. Threonine formed within the 

 membrane from glucose (in the absence of exogenous threonine) must not leak 

 out or it would be acted upon by the enzymes which can convert it to glycine. 

 The region enclosed must include the entire group of reaction sites at which all 

 proteins are formed, because threonine is needed at all of them. 



A similar region would be required for lysine as the same distinction exists 

 between endogenous and exogenous lysine. These two regions must be over- 

 lapping since both threonine and lysine are required for protein synthesis. 

 As such a situation is clearly absurd, internal permeability barriers must be 

 discarded as mechanisms for keeping endogenous amino acids within the cell. 



Most of the difhculties arise in attempting to explain why compounds sup- 

 plied externally are not identical in behavior to endogenous compounds. These 

 difficulties disappear as soon as it is realized that no proof exists that the en- 

 dogenous compounds are in fact identical. Furthermore, there are many 

 examples which show that endogenous material is carried by, or attached to, 

 other molecules which may profoundly alter its original chemical behavior. 



