BIOLOGICAL TRANSPORT 



If we could succeed in identifying a characteristic mark left by 

 transport on a molecule, we could detect a catalytic action of 

 the transport site in broken-cell preparations and perhaps monitor 

 the isolation of the mediating structure. 



Associated physical fluxes 



So far, transport has not yielded many of its chemical secrets. 

 Despite many efforts to detect such a substance, no compound is 

 recognized that shows anywhere near as large a preference for either 

 sodium or potassium ion as is required to explain the biological dis- 

 crimination. The failure of any of the procedures discussed so far 

 to identify a single carrier or reactive site for the transport of any 

 solute has stimulated investigators to try even more indirect ap- 

 proaches. One of these approaches seeks to observe whether an- 

 other substance moves into or out of cells more rapidly whenever 

 a transport is caused to occur. Such a behavior might have one of 

 several meanings. 



First, unless the carrier is strictly confined to the membrane 

 phase, one might anticipate that rapid net transport would produce 

 or accelerate an uptake of the carrier (or of one of its precursors) 

 at least slightly at the receptor face of the membrane, or accelerate 

 the release of it at the donor side of the membrane. This accelera- 

 tion could be far less than stoichiometric to the flux of the solute 

 under consideration. Detection of such an acceleration requires that 

 the background flux of the carrier-related molecule be not too 

 large. For example, a tetracoordinate metal ion could serve to join 

 two amino acid molecules together (or a hexacoordinate metal, three 

 amino acid molecules) so as to mask their polar properties and per- 

 mit passage of a barrier. If so, one might anticipate an accelerated 

 flux of the particular metal ion involved. An acceleration of Mn(II) 

 uptake by Ehrlich cells, but not of Cu(II), Zn(II), or Fe(III), 

 has been observed in association with increased amino acid loads (Pal 

 and Christensen, 1959). Of course, the amino acids might serve in 

 such experiments as metal-ion carriers, rather than the metal as an 

 amino acid carrier, particularly considering that the basic amino 

 acids are more conspicuously active in stimulating manganese up- 

 take. 



A second possible relationship may cause two fluxes to tend to 

 occur together, a possibility we shall illustrate with the alkali metal 



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