KINETIC APPROACH TO TRANSPORT 



Systems with concentrative capabilities 



Uphill transport can be produced by withdrawing the carrier 

 at one side of the membrane and supplying it at the other, as is 

 considered to occur when a flow is driven by a counterflow. Or 

 it can be visualized as being caused by a change in the affinity pro- 

 duced through a modification in the chemical structure of the carrier 

 at one face of the membrane, as illustrated later in Figure 23. In 

 either case, the effective K m , the level of solute producing half- 

 saturation of transport, is modified. The appropriate form of Equa- 

 tion (4) then becomes 



/ Sj S 2 \ 

 V=V ma J — J (6) 



The ratio of the two values of K m defines the maximal distribution 

 ratio that the transport can produce. 



Obviously, if the affinity of the carrier is to be increased in 

 such a way that uphill transport will result, energy must be re- 

 ceived from an exergonic reaction. Part of this energy is then 

 presumed to become available, as the affinity is again decreased, to 

 permit dissociation of the modified carrier-solute complex to gen- 

 erate elevated levels of the solute. The increase in the K m of the 

 carrier may be produced equally well before or after the dissocia- 

 tion; if it occurs afterward, the reversal of the dissociation is 

 decreased to a corresponding degree. 



When the kinetics of escape of solutes from cells are approxi- 

 mately linear, the assumption has often been made that the uphill 

 transport is opposed by diffusion and that the steady state finds 

 the active transport exactly balanced by leakage. Wilbrandt and 

 Rosenberg (1961) have also derived kinetic equations for this case. 

 The implication of this assumption is that the carrier has been re- 

 moved completely or modified in a way to make the affinity negli- 

 gible, or the value of K m infinite. But if K m has instead been in- 

 creased only moderately, the cellular levels tested may still not have 

 been high enough to demonstrate the mediation of exit. The general 

 equation [see Equation (6)] should remain applicable to this case. 



49 



