to examine some of these issues , we have developed a model that allows for spatial variations in the concentrations of co2 , hco3- , and h2co3 within the tubule lumen and cell cytoplasm , passive transport of these substances across cell membranes , carbonic anhydrase-catalyzed interconversion of hco3- and co2 within the cell and at the luminal membrane surface , and the corresponding uncatalyzed reactions in lumen and cell .
C0007603
cell membranes
C0007634
cell