Final answer:
Bicarbonate crosses the apical membrane of the proximal tubule by conversion to carbon dioxide, facilitated by the enzyme carbonic anhydrase. It then diffuses across the membrane, is re-formed into bicarbonate inside the cell, and co-transported out alongside sodium ions via Na+/HCO3- cotransporters.
Step-by-step explanation:
Bicarbonate (HCO3-) transport in the kidney's proximal tubule is essential for maintaining acid-base balance. The question pertains to how bicarbonate crosses the apical membrane of the proximal tubule. This occurs through conversion to carbon dioxide (CO2). Bicarbonate combines with hydrogen ions to form carbonic acid (H2CO3), which is catalyzed by the enzyme carbonic anhydrase (CA). The carbonic acid then rapidly dissociates into water (H2O) and CO2, which both diffuse across the cell's apical membrane. Within the cell, the reverse reaction happens to re-form bicarbonate ions, which are ultimately transported out of the cell alongside sodium ions (Na+) through Na+/HCO3- cotransporters located at the basolateral membrane.
Simultaneously, a Na+/H+ antiporter on the apical membrane actively excretes protons (H+) into the tubular lumen, preserving the sodium gradient created by the Na+/K+ pump.