Ion channels versus ion pumps: the principal difference, in principle

Abstract

Ions move across cell membranes through either ion channels or ion pumps. Recently, atomic-resolution structures and high-resolution functional measurements of examples from both channels and pumps have begun to suggest that these molecules need not be as different as was once thought. The incessant traffic of ions across cell membranes is controlled by two kinds of border guards: ion channels and ion pumps. Open channels let selected ions diffuse rapidly down electrical and concentration gradients, whereas ion pumps labour tirelessly to maintain the gradients by consuming energy to slowly move ions thermodynamically uphill. Because of the diametrically opposed tasks and the divergent speeds of channels and pumps, they have traditionally been viewed as completely different entities, as alike as chalk and cheese. But new structural and mechanistic information about both of these classes of molecular machines challenges this comfortable separation and forces its re-evaluation.