Active transport, by definition, requires energy, which means that molecules are going to be moved up their concentration gradient.
Primary active transport refers to the movement of molecules that are directly coupled to cellular energy in the form of ATP, whereas with secondary active transport, the movement of a molecule is coupled to a second molecule's electrochemical gradient that is maintained with ATP. The proteins that function as primary active transporters are called pumps. They contain an enzyme within their structure that acts as an
ATPase to hydrolyze ATP for energy. Like carriers, these pumps have binding sites that can become saturated at high molecule concentrations.
Secondary active transport is mediated by two types of proteins characterized by the direction in which the molecules are moving relative to one another. Cotransporters (
symport) are proteins that move molecules across a membrane against their
electrochemical gradient by coupling that movement to the electrochemical gradient of a second molecule, both of which move in the same direction. Countertransporters (antiporters or exchangers) are proteins that move molecules in opposite directions (the electrochemical gradient of one molecule is used to move another molecule against its electrochemical gradient in the opposite direction to the first). Since these pumps, antiporters and symporters, all have a binding site, they will all demonstrate saturation kinetics, which means they will have a maximal transport rate. At the same time, the rate of transport could be increased by having more of these proteins present.
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