This system consists of a specific protein that extends across the inner membrane. It trans-locates one molecule of ADP inward in exchange for one molecule of ATP^4-coming out.

As the entrance of ADP is coupled to the exit of ATP, this system is better called ADP-ATP antiporter. Obviously, this transport system is moving more negative charges out than it is bringing in, so it is effectively discharging the outside positive electric potential across the inner membrane.

The adenine nucleotide transport system is specific since it carries ADP and ATP and not AMP or any other nucleotides such as GDP or GTP.