BIOLOGICAL TRANSPORT 



alkali metal providing the energy. These proposals seek to explain 

 the strong effects of alkali metal distribution on the transport of un- 

 charged organic solutes. 



In the case of amino acid uptake by cells, the presence of inter- 

 nal potassium ion and external sodium ion seems to be critical, and 

 potassium ion migrates outward in exchange for sodium ion during 

 amino acid uptake (Riggs et al., 1958). For transmucosal absorption 

 in the intestine, it is the presence of sodium ion that is critical 

 (Csaky and Thale, 1960); this is also the case for amino acid trans- 

 port into isolated nuclei (Allfrey et al., 1961). For the uphill trans- 

 port of galactose by kidney slices, both the sodium- and potassium- 

 ion concentrations are critical (Kleinzeller and Kotyk, 1961). 



We should examine once more the proposed phenomenon of 

 flow driving a counterflow to see how it works in an active trans- 

 port system. The cycle of Figure 10 can be caused to operate uphill 

 by introducing energy at any of the four stages. For example, reac- 

 tion k 4 may activate the gate so that it has a high affinity for A. 

 The entrance of A into the gate may then trigger a reorganization 

 that tends to orientate the gate to the right side, with the result that 

 A tends to be released on that side of the osmotic barrier. A large 

 part of the energy introduced may then come to be represented by 

 the higher concentration of A at the right. Accumulation will occur 

 until the level at the right becomes high enough to compensate for 

 the extra affinity of A for the gate at the left; if the membrane is free 

 of leaks by which A can otherwise return from right to left, this 

 steady state will show two equal and opposed fluxes through the 

 system, and no net transport, only exchange diffusion, would be 

 observed by separate tracer measurement of each flux. In some cases, 

 evidence has been noted for the presence of separate leaks, either 

 by diffusion or facilitated diffusion, in which case the transport sys- 

 tem at the steady state will produce exchange diffusion plus enough 

 uphill transport to compensate for the leakage. Notice that the ex- 

 change diffusion occurs without energy cost, i.e., without loss of 

 the energy inherent in the presence of a molecule on the high-con- 

 centration side of the membrane. 



In either case, the addition of a relatively large quantity of the 

 solute to the right side of the membrane could again be expected 

 to produce a counterflow, in accord with the results of Heinz and 

 Walsh (1958), by accelerating the reorientation of the gate from 

 right to left. 



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