Cotransporters are found in all organisms and fall under the broader category of transport proteins, a diverse group of transmembrane proteins that includes uniporters, symporters, and antiporters. Each of them are responsible for providing a means of movement for water-soluble molecules that otherwise would not be able to pass through lipid-based plasma membrane. The simplest of these are the uniporters, which facilitate the movement of one type of molecule in the direction that follows its concentration gradient. In mammals, they are most commonly responsible for bringing glucose and amino acids into cells.
Symporters and antiporters are more complex because they move more than one ion and the movement of one of those ions is in an energetically unfavorable direction. As multiple molecules are involved, multiple binding processes must occur as the transporter undergoes a cycle of conformational changes to move them from one side of the membrane to the other. The mechanism used by these transporters limits their functioning to moving only a few molecules at a time. As a result, symporters and antiporters are characterized by a slower transport speed, moving between 10 and 10 molecules per second. Compare this to ion channels that provide a means for facilitated diffusion to occur and allow between 10 and 10 ions pass through the plasma membrane per second.
Though ATP-powered pumps also move molecules in an energetically unfavorable direction and undergo conformational changes to do so, they fall under a different category of membrane proteins because they couple the energy derived from ATP hydrolysis to transport their respective ions. These ion pumps are very selective, consisting of a double gating system where at least one of the gates is always shut. The ion is allowed to enter from one side of the membrane while one of the gates is open, after which it will shut. Only then will the second gate open to allow the ion to leave on the membrane's opposite side. The time between the alternating gate opening is referred to as the occluded state, where the ions are bound and both gates are shut. These gating reactions limit the speed of these pumps, causing them to function even slower than transport proteins, moving between 10 and 10 ions per second.