Watching the Pulleys Turn while the Elevator Moves in a Secondary Transporter

Biological membranes are constantly crossed by a crowd of inorganic ions and small molecules thanks to a panoply of interdependent transport proteins. Primary active transporters are the bedrock of this ecosystem. They use chemical (for instance, ATP) or physical energy (light) to pump in, or out, solutes against their electrochemical gradients. The resulting gradients (for instance, Na+ or H+) serve as energy currencies for two other types of transport protein: secondary transporters and channels.