Precisely controlled movements of ions into and out of cells and organelles

| 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. R E V I E W S 344 | MAY 2009 | VOLUME 10 www.nature.com/reviews/molcellbio Nature Reviews | Molecular Cell Biology Gate open Gate closed a Ion channel: single gate Other gate open One gate open Other gate open One gate open Both gates closed b Ion pump: alternating gates c Ion pump: alternating gates and occluded states