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General summary 



Cell function requires the presence of barriers. These barriers 

 permit the passage of some molecules but not of others. Their selec- 

 tivity does not arise entirely from the nature of the barrier material, 

 but also from the presence of mediating groups to which molecules 

 meeting certain structural and conformational requirements bind. 

 As a consequence, these molecules may be released into the phase 

 lying beyond the barrier, in some cases at a higher concentration 

 than existed in the phase of origin. The energy causing this result 

 may reach the membrane in the form of ATP, or it may con- 

 ceivably be represented at least partially by an established gradient 

 of another solute, possibly an analogous one. The system that ac- 

 cepts the energy from ATP may conceivably be the same system, 

 a transport system, that generates ATP when electrons are trans- 

 ported. 



The reactive sites that mediate transport can readily be detected 

 in intact cells, but means for recognizing them in simpler systems 

 are mainly lacking. Their modes of gripping the solute molecule 

 do not seem to leave a permanent mark on the transported solute. 

 The nature of transport specificity suggests that the several com- 

 ponents of each site are distributed in a three-dimensional matrix, 

 as if in a macromolecular structure; but the specificity patterns 

 tend to differ from those of known enzymes. For such reasons, 

 the proposed models from which we seek to explain uphill transport 

 by changes in the shapes of molecules forming part of the barrier 



105 



