transporter
n. a protein complex that spans a cell membrane and conveys ions, neurotransmitters, or other substances between the exterior and interior of the cell. For example, at synapses between neurons, transporters in the presynaptic membrane recognize and bind to neurotransmitter molecules and return them to the presynaptic neuron for reuse (see reuptake). Transporters may utilize passive transport, in which a substance is transported into or out of a cell according to its concentration gradient across the cell membrane; or active transport, which is an energy-dependent process often relying on the hydrolysis of ATP to provide energy to facilitate movement of a substance from one side of the cell membrane to the other.