CONCEPTS AND TERMS 



may occur in the structure of the protein components of the pore 

 to assist in conveying molecules through the pore. But, if the polar 

 groups lining the pore function only passively, the behavior should 

 resemble that expected of a single fixed site. 



We should not assume that facilitated diffusion necessarily has 

 biological importance in every instance. The human red blood cell 

 seems unlikelv to need to receive glycerol at the unusual rate per- 

 mitted by the glycerol transport site. Mediation may occur when- 

 ever a molecule finds appropriate spacings in the membrane struc- 

 ture, without reference to need. Hober (1902) wrote: "Utilizability 

 or nonutilizability, usefulness or injuriousness, either in a recogniz- 

 able or more obscure form, needs not be at all decisive for the ques- 

 tion of uptake or non-uptake. . . ." 



The dimerizer theory. Stein (1961a) has recently suggested that 

 a pair of reactive sites receives two molecules of glycerol in such 

 relative positions that they dimerize by hydrogen bonding to pass 

 the lipid barrier in this lipophilic form. In this process, the usual 

 hydrogen bonding of the hydroxyl groups of glycerol with water 

 is temporarily interrupted, because hydrogen bonding to another 

 glycerol molecule is favored. The dimer dissociates spontaneously 

 on leaving the lipid phase. The kinetics of glycerol entry appear to 

 indicate that two molecules of glycerol react, rather than only 

 one. A similar proposal has been made for monosaccharide transport 

 (Stein, 1961b). Under this interesting concept, one molecule of the 

 solute serves as a carrier for another, and no interruption or polar 

 channel in the lipid barrier is required. 



The phenomenon of flow driven by counterflow. The dia- 

 gram of Figure 7 presents a fixed transport site equally subject to 

 mass-action effects from both sides. Suppose that we have a solute 

 at an equilibrium, 1 : 1 distribution between the two phases. If we 

 now add a high-affinity analog to one phase, we shall slow the flux 

 of the original solute in both directions, but its distribution will not 

 be changed. 



This behavior should be contrasted with what is actually seen in 

 certain instances. If a cell or tissue is permitted to come to a 1:1 

 equilibrium distribution with a given monosaccharide and a second 

 competing monosaccharide is then added to the suspending solution, 

 the first sugar may frequently be seen to move out of the cell to 

 create a concentration gradie?it while the second sugar is entering 

 the cell in the direction of its concentration gradient (Park et al., 



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