BIOLOGICAL TRANSPORT 



Case 3. A tissue (let us assume) incorporates an amino acid into 

 protein at the plasma membrane. Eventually, part of this amino 

 acid appears free (we shall suppose) at a high level in the cells by 

 intracellular proteolysis, perhaps with an average delay of several 

 days. 



Case 4. Succinate reacts with an enzyme in the plasma mem- 

 brane, and the succinyl group is then donated to a series of succes- 

 sive acceptors, some of which are potentially able to yield free 

 succinate ion at a higher level than it exists in the extracellular 

 phase. Let us assume that, in actuality, only very low levels of free 

 succinate are realized, and that the interest is placed in the transfer 

 of the succinyl group. 



Case 5. Inorganic phosphate enters a given cell, we shall sup- 

 pose, by forming ATP and, successively, one or two other phos- 

 phoryl compounds, appearing seconds later as inorganic phosphate 

 perhaps at a substantially higher level than that present in the ex- 

 ternal medium. 



Case 6. Glucose enters the red blood cell at a rate so fast as 

 to be measurable only with difficulty. So small an amount of an 

 intermediate exists at any instant that it has defied detection up to 

 the present time. Nevertheless, indirect evidence shows that an 

 intermediate must be formed in the process. 



These examples range in nature from the first two cases, which 

 as arbitrarily set forth obviously are not transports, to the last two, 

 which almost everyone will accept as falling within the meaning 

 of the term. By creating the concept of functional compartmental- 

 ization, Case 2 can be brought rather artificially within the scope 

 of our subject; but surely no merit can be ascribed to calling this 

 a functional transport. 



In between these extremes we have more or less hypothetical 

 examples that do not meet the above definition for one reason or 

 another. In the third case, the factor of time lag makes it rather 

 absurd to think of the interrupted sequence as a transport. 



In Case 4 the interest is in the transfer of a chemical group 

 and not of a molecular species. This process is known as a group 

 translocation. The means by which groups are transferred vec- 

 torially in this process, as well as from donor to receptor, will un- 

 doubtedly receive companion interest to transport and may yet 

 prove to be intrinsic to that subject as the means by which the de- 

 cisive vectorial step may occur in transport. 



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