FIGURE 11-2 Overview of membrane transport proteins. Gradients are indicated by triangles with the tip pointing toward lower concentration, electric potential, or both. 1 Channels permit movement of specific ions (or water) down their electrochemical gradient. 2 Transporters, which fall into three groups, facilitate movement of specific small molecules or ions. Uniporters transport a single type of molecule down its concentration gradient 2A. Cotransport proteins (symporters, 2B, and antiporters, 2C) catalyze the movement of one molecule against its concentration gradient (black circles), driven by movement of one or more ions down an electrochemical gradient (red circles). 3 Pumps use the energy released by ATP hydrolysis to power movement of specific ions or small molecules (red circles) against their electrochemical gradient. Differences in the mechanisms of transport by these three major classes of proteins account for their varying rates of solute movement.