CONCEPTS AND TERMS 



The phenomenon of flow driving counterflow may then be 

 explained as follows: Suppose we add to the right-hand phase at a 

 high level A', an analog of A. This solute will have a great tendency 

 to migrate to the left through the system. As it does so, it may tend 

 to orientate more of the gates to the left than to the right. Accord- 

 ingly, some degree of increase in the flow of A to the right (against 

 the concentration gradient of A) will tend to accompany the large 

 flow of A' to the left. 



On the other hand, if the gate is constantly and freely oscillat- 

 ing from the G 4 to the G x state, at least as readily as the solute-gate 

 complex oscillates from the G 2 to the G 3 state, no influence may be 

 produced by a flux on a counterflux; in this case, counter-transport 

 can arise only from the inhibition of the parallel flux by an analog. 

 This is the situation considered to hold for sugar migrations in the 

 human red blood cell. 



Active transport. Note that we have been caused to intro- 

 duce active or uphill transport here in a section designed to consider 

 instead the phenomenon of one net migration accelerating an op- 

 posed one. The striking aspect of this phenomenon is that it per- 

 mits a transport, which does not in the usual biological context 

 function uphill, to be made experimentally to function uphill. In 

 the experiment of Park et al. (1956), it was shown that xylose moved 

 out of the red cell to establish a concentration gradient after glucose 

 had been added. The movement of one sugar on the addition of a 

 second is uphill only to the observer, who differentiates between the 

 two sugars; to the transport site, which does not distinguish be- 

 tween them, the total movement of sugar is, of course, in the direc- 

 tion of the over-all gradient. 



In contrast to this behavior, several of the monosaccharides are 

 normally transported uphill by the intestinal and renal tubular 

 mucosa, without benefit of the driving force of a gradient of an- 

 other sugar. We may consider, however, that the potential for up- 

 hill transport is built into transports that are mediated by a mobile 

 site or carrier, even if they do not ordinarily function uphill. It is 

 even possible that some uphill transports are driven naturally by the 

 downhill movement of another solute, which may be the one pri- 

 marily concentrated. This idea has been considered both for amino 

 acids (Riggs et al., 1958) and for sugars (Crane, 1960), with the 

 gradient arising from the primary concentrative transport of an 



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