Practice question

Dietary carbohydrate absorption predominantly occurs in duodenum and jejunum where luminal concentrations fluctuate from millimolar after meal to micromolar between meals, requiring active accumulation. Enterocytes polarized with brush border apical membrane expressing SGLT1 coded by SLC5A1 high affinity sodium glucose cotransporter belonging to sodium substrate symporter family LeuT fold with 14 transmembrane helices, sodium binding sites Na1 Na2 coordinating carbonyls and glucose vestibule lined by aromatic residues conferring D-glucose galactose specificity. Stoichiometry two sodium per one glucose harnessing electrochemical sodium gradient established by basolateral Na+/K+ ATPase exporting three sodium importing two potassium per ATP maintaining low intracellular sodium about 12 millimolar and negative potential minus 60 millivolts, giving driving force about 10 kilojoules per mol. Concentrative uptake raises intracellular glucose to millimolar enabling exit via basolateral GLUT2 low affinity high Km about 17 millimolar facilitating diffusion into bloodstream. GLUT1 restricted to erythrocytes blood brain barrier basal, GLUT4 insulin dependent in muscle adipose, but intestinal apical uptake relies on SGLT1. Phlorizin competitive inhibitor blocks it, causing glucosuria and used therapeutically.