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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.

It is sometimes equated with symporter, but the term "cotransporter" refers both to symporters and antiporters (though not uniporters).

The word "symporter" 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 the 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.

Proton-sucrose cotransporters are common in plant cell membranes. An ATP molecule in the cell phosphorylates a carrier protein, causing a conformational change that shuttles a proton across the membrane. The proton binds with sucrose in the extracellular fluid, then undergoes passive transport down its concentration gradient (ie. up the concentration gradient of sucrose).

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