The sodium/potassium pump is a good example of active transport of molecules across a membrane. In this example, active transport is coupled to ATP hydrolysis to obtain enough free energy to transport the ions against their concentration gradient. This ion pump is an example of antiport membrane transport, where the transported molecules are pumped across the membrane in opposite directions (as opposed to synport). The sodium gradient is generated for use by cotransport systems, such as the active transport of glucose from the extracellular environment into the inside of the cell. The non-equilibrium state of the sodium gradient is essentially free energy to be used for the import of molecules against their own concentration gradient.