BIOLOGICAL TRANSPORT 



1956; Rosenberg and Wilbrandt, 1957). This behavior cannot result 

 by simple diffusion under any reasonable assumptions (LeFevre and 

 McGinnis, 1960), nor can it result with the single, fixed, symmetri- 

 cally placed site of Figure 7. It could conceivably result if two 

 entirely different fixed sites served, one for entry and one for 

 exodus (Figure 7). Such an improbable arrangement could permit 

 the added second sugar to lower by competition the entry of the 

 first at site Z, without a corresponding effect on the exodus by 

 site Y. 



Most investigators feel it more likely that the entering solute 

 causes the entry site to be reorientated to become, in effect, an 

 exit site. This behavior implies that the transport site is mobile, or 

 serves as a carrier. It would also permit the second sugar added to 

 the external phase to compete with the original sugar and slow its 

 entry, without having a corresponding effect on its exit; this effect 

 would account for the original observation of Park et al., in 1956. 

 For sugar migration across the red blood cell membrane, this ex- 

 planation appears to be sufficient. 



In another case, however, namely, the uphill transport of amino 

 acids into Ehrlich ascites tumor cells, separate estimates of the two 

 opposed fluxes show that the effect is not adequately explained as 

 a lowering of the parallel flux. Heinz had observed in 1954, without 

 explanation, that a prior glycine accumulation in these tumor cells 

 accelerated the subsequent uptake of labeled glycine. In 1958, Heinz 

 and Walsh supplied further results showing that the glycine influx 

 increased linearly with the magnitude of the cell glycine content 

 (see Figure 9). Assuming that the influx had indeed been measured 

 separately from the efflux, this result indicates that a flux can stimu- 

 late a counterflux, and not simply slow a parallel flux. 



In the meantime Rosenberg and Wilbrandt (1957) had observed 

 a corresponding outpouring of one sugar from erythrocytes when 

 a second sugar was added, and coined the terms flow driven by 

 counterflow and (Wilbrandt and Rosenberg, 1961) counter-trans- 

 port for the phenomenon. 



The results of Heinz and Walsh (1958), assuming that their 

 separation of the fluxes was adequate, identified the behavior for 

 certain amino acids as a mass-action acceleration of exchange dif- 

 fusion, a phenomenon discussed by Ussing in 1949. If a chemical 

 structure mediates transport across a membrane, it can obviously 

 work in both directions, and much of the flux in one direction will 



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