CONCEPTS AND TERMS 



B 



Figure 15 Diagram to show how the uptake processes of epithelial 

 cells might produce secretion. A and B are two extracellular phases 

 separated by a layer of cells. The larger arrows for solute entry and 

 the smaller arrows for exit serve to illustrate that the steady-state con- 

 centration of the solute will be higher in the cells than in either extra- 

 cellular phase. Any process that will accelerate entry process 1, or slow 

 entry process 2, will cause uphill transport from phase A to phase B. 

 [From Oxender, D. L., and Christensen, H. N. (1959), /. Biol. Chem., 

 234, 2323; with permission^ 



uphill transport of amino acids and sugars from the mucosal to the 

 serosal side may well arise from the presence of an entry by active 

 transport and an exit by facilitated diffusion; or it may arise instead 

 from the presence of many more transport sites or carriers for up- 

 hill transport in the increased surface of the brush border, which 

 mucosal cells expose to the lumen, than on the opposite poles of 

 these cells; or from greater delivery of energy to the sites at the brush 

 border. For the amino acids at least, the latter explanation seems 

 more likely. Figure 16 represents the brush border. 



Among the amino acids, a strong similarity in the specificity of 

 the intestinal transport system, on the one hand, and of the systems 

 of the red blood cell and the Ehrlich cell, on the other, is illustrated 



37 



