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symporter A transport protein that couples the movement of a substance across a cell membrane with the simultaneous movement of ions in the same direction. The substance is ‘pulled’ uphill against its concentration gradient by the flow of the ions down their pre-existing concentration gradient. Hence, it is a form of active transport, powered by the energy stored in the ionic concentration gradient, which is maintained by cellular mechanisms. For example, kidney tubule cells contain a sodium/glucose symporter that exploits the gradient of sodium ions (Na+) across the plasma membrane; for every two Na+ that flow inwards it imports one glucose molecule. Compare antiporter; uniporter.