A symporter, also known as a cotransporter, is an integral membrane protein that is involved in secondary active transport. It works by binding to two molecules at a time and using the gradient of one solute's concentration to force the other molecule against its gradient.

The word is a conjunction of the Greek syn- or sym- for "together, with" (cf. symphony, synonym) and -porter. Symporter is also sometimes misspelled simporter because of the simultaneous transport of molecules (and phonetic resemblance to symporter!).

In order for any protein to do work, it must harness energy from some source. In particular, symporters do not require the splitting of ATP because they derive the necessary energy for the movement of one molecule from the movement of the another. Overall, the movement of the two molecules still acts to increase entropy.

Integral membrane proteins | Transport phenomena