BIOLOGICAL TRANSPORT 



Figure 10 presents a model of carrier transport (Patlak, 1957), 

 which we can use to explain the phenomenon of counter-transport 

 as observed by Heinz and Walsh. The acceptor site, or "gate" 

 (Gi), receives a solute molecule A at either face and, as a result, 

 is reorientated so that when it dissociates it releases A to the other 

 phase (G 3 ). If the gate can return freely to the first side to accept 

 additional A molecules, the solute can eventually reach a steady- 

 state distribution between the two phases. We shall assume for the 

 moment that the system is unable to produce uphill transport. Obvi- 

 ously, the gate will then operate freely in both directions; in fact, at 

 the steady state, equal fluxes will continue in both directions, and 

 only exchange diffusion will be occurring. 



A, + 



k- 4 



k_, 



+ A r 



Figure 10 A model for membrane transport. The gate G x receives 

 the solute A t from the left. A reorganization is triggered orienting the 

 gate toward the other phase (G 3 ) where the unchanged solute is released 

 to the right. The gate can then be returned to its original orientation. 

 If uphill transport is to be achieved, energy must be supplied to drive 

 either k x , k 2 , k 3 , or k A . For example, energy may be supplied at k± to 

 prepare the site so that the entry of Aj from the left will trigger a relaxa- 

 tion to the condition G 3 . [From Patlak, C. (1957), Bull. Math. Biol., 

 19, 209; with permission.] 



