BIOLOGICAL TRANSPORT 



G + 



ii 



(a) 



M 



M 



L 



(b) 



GC 



^Barrier 



Exchanger 



i 



w 



GC 



^\ 



Barrier 



i 



Figure 21 A visualization of the possible operation of dissimilar 

 mediators (C and C): (a) for entry and exodus of an amino acid such 

 as glycine; (b) for an amino acid such as methionine to permit it to 

 drive the uphill transport of, for example, leucine. 



60 



