A cotransporter is an integral membrane protein that is involved in secondary active transport. It works by binding to two molecules or ions at a time and using the gradient of one solute's concentration to force the other molecule or ion against its gradient.
It is sometimes equated with symporter, a word made up of 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 (i.e. up the concentration gradient of sucrose).
In August 1960, in Prague, Robert K. Crane presented for the first time his discovery of the sodium-glucose cotransport as the mechanism for intestinal glucose absorption. Crane's discovery of cotransport was the first ever proposal of flux coupling in biology.