= 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.<Molecular cloning, primary structure, and characterization of two members of the mammalian electroneutral sodium-(potassium)-chloride cotransporter family expressed in kidney by G. Gamba and others>[Abstract: oocyte expression and functional comparison]{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 is
$$
\Delta G=RT\ln(1/10)+RT\ln5=RT\ln(1/2),
$$
which 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.
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