Well, because ADP contains the charge of negative three while ATP contained the charge of negative four. So both of these molecules are highly charged species. And what that means is they can't simply diffuse across the inner membrane of the mitochondria and their movement across the inner membrane of the mitochondria basically depends on the existence of a special type of antiporter transport system and exchange protein molecule known as ATP ATP translocase. And what this molecule does is it basically catalyzes the movement. It couples the import of the ATP into the matrix to the export of the ATP out of that matrix. And this is the neck reaction that is catalyzed by ATP ADP trams locates.