Practice question

Facilitated diffusion accelerates equilibration of membrane-impermeant polar molecules without expending cellular ATP, harnessing existing chemical gradients and thermal motion. Integral membrane proteins such as glucose uniporters GLUT1-4, amino acid transporters LAT1, urea transporter and aquaporins provide hydrophilic pathways lowering activation barrier for crossing hydrophobic core. Two hallmarks define this process: selectivity and saturability. Selectivity emerges from specific binding pockets, steric fit or selectivity filter discriminating D-glucose over L-glucose, glucose over galactose, or water over ions via NPA motifs and size exclusion. Saturability occurs because finite number of transporters exists in membrane, so flux follows Michaelis-Menten-like kinetics approaching plateau Vmax when all binding sites continuously occupied and cycling at maximal turnover, unlike simple diffusion which increases linearly with gradient indefinitely. Transport remains thermodynamically passive, does not use ATP directly, cannot move solute against its electrochemical gradient, though gradient itself may be maintained by active pumps. Regulation can occur via transporter trafficking, as insulin-stimulated GLUT4 recruitment to plasma membrane increases Vmax, or allosteric gating. This distinguishes facilitated diffusion from primary and secondary active transport and simple lipid diffusion, providing controlled uptake of polar nutrients while conserving energy and allowing rapid response to metabolic demands during feeding and fasting cycles.