CONCEPTS AND TERMS 



20 40 60 



Intracellular glycine, mM 



Figure 9 Influx of glycine into Ehrlich cells as a function of the 

 intracellular glycine concentration. (Three different experiments are 

 illustrated.) [From Heinz, E., and Walsh, P. O. (1958), /. Biol. Chem., 

 233, 1490; with permission.} 



be cancelled out by flux in another, so that net flux may be only a 

 small fraction of the total flux. The influx of potassium ion into 

 cells is ordinarily so high as to appear to require more energy than 

 the total made available by cellular metabolism. It became necessary 

 to assume that, for uphill transports, a considerable part of the total 

 flux must also represent merely an exchange of the solute from the 

 low-energy side of the barrier for other molecules from the high- 

 energy side, without energy cost. Ussing warned that the isotopic 

 method can lead to erroneous conclusions concerning the rate of 

 active transport and its energy requirement, if part of the move- 

 ment occurs by such an exchange of an external molecule for an- 

 other of the same species already concentrated into the cell. 



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