ATP-ADP translocase mediates the exchange of ATP and ADP and this favored by the proton gradient across the inner mitochondrial membrane. But is this transport process an active process? I mean, are ATP and ADP transported against their concentration gradient or not? I don't understand if the function of the proton gradient is just to further favor an already-favorable process (passive transport) or to make possible an unfavorable process (active transport).

Thanks in advance!

Think about where the content of ADP and ATP. Where do you think is more in each compartment and in which direction does it got? Another keyword to check is antiporter.