Practice question

Water permeability of plasma membranes exceeds lipid diffusion requiring dedicated proteinaceous pores. Aquaporin family comprising thirteen human isoforms forms homotetramers in kidney proximal tubules, red cells, and brain astrocytes each monomer about twenty eight kilodaltons six transmembrane helices arrangement N-terminus cytosolic with two half-helices HB and HE entering membrane containing invariant NPA motifs meeting at center forming aqueous pathway hourglass shape. Extracellular vestibule leads to aromatic arginine constriction formed by Arg195 His180 Phe56 limiting diameter to two point eight angstroms permitting single file waters hydrogen bonded to carbonyl oxygens and asparagine side chains while excluding hydronium via electrostatic barrier and interrupting Grotthuss proton wire. Transport occurs passive facilitated diffusion rate up to three billion molecules per second per channel driven by osmotic gradient without conformational change measured by stopped-flow light scattering. Band 3 performs anion exchange, GPCR transduces signals, spectrin provides elasticity, establishing aquaporin uniquely as water channel with dual NPA filter ensuring high selectivity.