BIOLOGICAL TRANSPORT 



in a sequence, not only can metabolism be the cause of transport, 

 but also transport can be the cause of metabolism. Thus, we 

 might be inclined to recognize that transport and metabolism, as 

 usually understood by biochemists, may be conceived advanta- 

 geously as different aspects of one and the same process of vec- 

 torial metabolism. 



In this hypothesis (Figure 27) the process of oxidative phos- 

 phorylation is seen as a transport, which may be linked to electron 

 transport but which, presumably, may equally well be linked to a 

 specific molecular transport, functioning instead as an ATP-splitting 

 system. 



Mitchell's hypothesis offers us a third possibility, beyond those 

 presented in the second and third paragraphs of this chapter, for 

 interpreting flux associations such as those seen between amino acids 

 and the alkali metal ions. Uphill transports may well not be p'aced 

 in series, e.g., with alkali metal transport driving amino acid trans- 

 port; but in parallel, with many or all uphill transports driven by 

 the response of membrane structure to ATP cleavage. The mainte- 



2H 2 0^ 



2(ADP + P) 



2ATP 



Figure 27 Mitchell's scheme for linkage of electron-transport sys- 

 tem to oxidative phosphorylation, the latter produced by a reversible 

 ATPase system. See text for discussion. {From Mitchell, P. (1961), Na- 

 ture, 191, 144; with permission^ 



86 



