Going against the flow

摘要

To function properly, living cells must maintain ions and small molecules at concentrations that are far different from those in their local environments. The electrochemical gradients formed in this way can be used by cells as a source of energy to drive other processes, for example membrane transport or the generation of electrical signals in specialized cells — such as those in nerves and muscle. Internal sodium ions are kept at roughly one-tenth of their external concentration, and the reverse is true for potassium ions. These ion gradients are maintained by a transmembrane protein (the sodium-potassium pump, an enzymic transporter), which actively pumps out sodium ions and pumps in potassium ions. Then, as an electrical signal (action potential) propagates along a nerve cell, sodium and potassium channels open, allowing the rapid flow of sodium ions into and potassium ions out of the cell. From many such examples, the concept emerges that transporters generate ionic gradients and channels dissipate them: transporters and channels seem as different as oil and water.