Practice question

Coupled transport nomenclature systematically describes stoichiometry and directionality of substrates moved by single transmembrane protein complex operating via alternating access. Uniport category transports single molecular species in one direction without coupling to another solute, exemplified by GLUT1 glucose uniporter moving glucose down gradient, voltage-gated potassium channel Kv allowing potassium efflux, and mitochondrial ADP/ATP translocase albeit antiport. Symport, traditionally called cotransport, moves two or more different molecules simultaneously in same direction across membrane, such as Na+/glucose cotransporter SGLT1 carrying two sodium ions together with one glucose into enterocyte from lumen, H+/peptide symporter PEPT1 absorbing dipeptides via proton gradient in gut, and Na+/2Cl-/K+ cotransporter NKCC in thick ascending limb. Symport can be secondary active if one substrate moves down its electrochemical gradient providing energy to drive uphill movement of co-substrate. Antiport, or exchanger, moves substrates in opposite directions across membrane, like Na+/H+ exchanger NHE1 exporting H+ while importing Na+ maintaining pH, chloride bicarbonate antiport Band 3 in erythrocyte facilitating CO2 transport, and Na+/Ca2+ exchanger. Distinguishing directionality predicts net charge movement, electrogenicity, stoichiometry and overall energetics of nutrient absorption, pH regulation, and cell volume control.