A small molecule X has a higher concentration in the cytosol than in the extracellular space. A second small molecule Y has a higher concentration in the extracellular space than in the cytosol. 

Y is transported out of cells (against its concentration gradient) by binding to a transmembrane protein that hydrolyses ATP. In turn, X is transported into cells (against its concentration gradient) by binding, together with Y, to the extracellular side of a transmembrane protein and then being released into the cytosol.

The movement of X into cells is an example of: