BIOLOGICAL TRANSPORT 



another, being in the same initial and final states in both phases. 

 This terminologic question is considered further in Appendix 1. 



Although we should intuitively like the term transport to imply 

 that something is conveyed from one place to another, the more 

 closely we describe a given biological transport the less able we 

 may be to see an actual act of transport occurring at any stage. 

 Transport could possibly occur simply by an association reaction 

 followed by the diffusion of the product into the cell and its even- 

 tual dissociation to yield the original solute. If neither the associa- 

 tion nor the dissociation reaction has a vectorial character, migration 

 may occur solely by diffusion of the solute-carrier complex; only 

 by considering that the sequence occurs consecutively to produce 

 a result we are trying to explain does it appear to us as a specific 

 transport. An example that comes to mind concerns the concentra- 

 tion of iodide by a sea weed. Shaw (1959) proposed that iodide is 

 oxidized to iodine extracellularly, and that elementary iodine or 

 HIO diffuses into the cells, where it is again reduced to iodide to 

 be trapped there. As in this instance, transport as we understand 

 it intuitively may tend to disappear when we look at the process 

 stage by stage. As in the proverb, we see the golden eggs only if 

 we retain the goose intact. Perhaps only if it should be proved 

 that solutes are pulled through the barrier by a mechanical change 

 in the shape of a protein molecule should we be satisfied that a 

 crucial transport step has been identified. Unless transport is of such 

 a mechanical character, analysis may cause any identifiable transport 

 step to disappear. 



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