Topic overview

Start with the big picture

Transport across the membrane can be passive, active, or vesicular. Simple diffusion moves suitable molecules down a gradient, while facilitated diffusion uses carriers or channels and remains gradient-driven. Primary active transport uses ATP to move solutes against gradients; secondary active transport couples one solute’s downhill movement to another’s uphill movement. Carriers may act as uniporters, symporters, or antiporters, and vesicles move larger materials through endocytosis or exocytosis. Ion gradients and selective permeability also contribute to membrane potential. The deeper lesson organizes these mechanisms by driving force, protein type, directionality, kinetics, and inhibition, with examples such as glucose transporters, ion pumps, and exchangers.