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antiporter A membrane protein that effects the active transport of a substance across a cell membrane while transporting ions in the opposite direction. An antiporter is a type of cotransporter; the ions, typically hydrogen ions (H+) or sodium ions (Na+), flow down their concentration gradient, and in so doing provide the energy for the transport of another substance in the other direction. For example, heart-muscle cells have a Na+/Ca+ antiporter, which is driven by the inward flow of sodium ions to pump calcium ions (Ca+) out of the cell. Hence, the energy for antiporters derives ultimately from the energy-consuming mechanisms that establish the concentration gradient of the driving ions. Compare symporter; uniporter.