 Symporter

A symporter is a membrane protein coupling the movement of different substances toward the same side of a cell membrane. The favored movement of one substrate can help drive another against its own gradient, a form of secondary active transport. Same-direction transport does not by itself say whether the combined movement carries net charge.

A functional experiment identifies coupled transport rather than merely a promising sequence. The 1994 kidney-cotransporter study expressed cloned transporters in Xenopus laevis oocytes and tested their transport activity. Related proteins differed in their potassium requirement and inhibitor response, connecting molecular identity to distinct transport functions.^[ref]

A deliberately simplified energy example uses two neutral solutes, $A$ and $B$, coupled 1:1 from outside to inside. Assume dilute ideal solutions, a tenfold larger concentration of $A$ outside, and a fivefold larger concentration of $B$ inside. Their combined free energy change iswhich is negative. Favorable inward movement of $A$ can therefore pay for inward movement of $B$ against its concentration gradient. Charged substrates require additional electrical terms; the toy calculation is not a measured stoichiometry of the kidney transporters.