BIOLOGICAL TRANSPORT 



to perform simultaneously and in a single test tube the hydrogenation 

 of a fat and the oxidation of acetic acid. These and dozens of other 

 reactions must be kept apart. But, if barriers are placed between 

 them, how can the product of one reaction be made to serve as the 

 reactant for perhaps two others and still not be consumed by a 

 third? How can each metabolic reaction be made to keep pace with 

 others? 



In the laboratory, the investigator establishes the course of a 

 metabolic sequence by reproducing it one step at a time; first, he 

 places the reactants and the specific enzyme for the first step in the 

 test tube and then isolates the product; he presents the product to 

 the second enzyme and makes the conditions favorable for the sec- 

 ond reaction. Perhaps, under optimal circumstances, he may get two 

 or three or more consecutive reactions to take place in a single solu- 

 tion in ordered sequence. But before long he must isolate an inter- 

 mediate from the bulk phase and transfer it to another tube con- 

 taining both the next enzyme and the coreactants for the following 

 step of the desired sequence. In the cell, what machinery takes the 

 place of the biochemist in shepherding a substance through an ex- 

 tended and complex sequence in the presence of perhaps hundreds 

 of other related or unrelated sequences? Because many chemical re- 

 actions must be segregated, segregating and desegregating processes, 

 that is, barriers and transport processes, are necessary. 



One might suppose that a great deal of this necessary segrega- 

 tion of reactants and reactions could be accomplished by having 

 different cells serve for different reactions. The nonbiologist would 

 certainly suspect this from looking at the cellular structure of an 

 organism and, particularly, at the division of higher animals into 

 distinct organs. Here, of course, a degree of cellular specialization 

 of chemical function is achieved because some cells synthesize, or 

 modify or transport, particular metabolites much faster than others 

 do. Urea synthesis or serum albumin synthesis and degradation come 

 to mind. Such specialization of function requires that the plasma 

 membranes around the cells be able to deliver a precursor from one 

 cell into the intervening extracellular fluid, that the receptor cell be 

 able to receive this precursor and release its product, and so on. In 

 these cases, the transport problems have been met at or above, and 

 not below, the cellular level. Differences in transport properties of 

 cells underlie much cellular specialization. 



Some of the most conspicuous cellular specializations are actu- 



4 



