Practice question

Intestinal glucose absorption across absorptive enterocytes lining duodenum and jejunum illustrates elegant coupling of electrochemical gradients for nutrient uptake against concentration difference. Luminal sodium concentration remains high around 140 millimolar due to continuous activity of Na+/K+ ATPase on basolateral membrane pumping three Na+ out and two K+ in per ATP hydrolyzed, creating inward sodium electrochemical gradient with both chemical and electrical components. Sodium glucose cotransporter 1, SGLT1, SLC5A1 gene, located apically in brush border with fourteen transmembrane segments, exploits this gradient by symporting two Na+ ions together with one D-glucose molecule in same direction across apical membrane. Transport is classified secondary active because glucose is driven uphill against its concentration gradient without direct ATP hydrolysis by transporter itself, but energy originates indirectly from primary active Na+/K+ ATPase maintaining Na+ gradient. After accumulation, glucose exits basolaterally via GLUT2 facilitated diffusion uniporter down its gradient into interstitial fluid and portal blood. Similar mechanism concentrates glucose in kidney proximal tubule S1 segment. Symport stoichiometry of 2:1 allows concentrative capacity over thirtyfold and coupling ratio determines efficiency, demonstrating indirect energy coupling and vectorial transport for efficient absorption and preventing loss of calories.