A basic issue of biomembranes is their ability to facilitate specific transport of selected molecules. This transport is catalyzed by carriers which are membrane proteins and form, analogous to enzymes, carrier-substrate complexes. The ADP,ATP carrier of mitochondria is highly suitable for elucidating the mechanism of this catalysis due to its unique qualities such as great abundance in higher cells, easy isolation in native state by detergents, existence of inhibitors specific for either the in- or outward looking binding site and direct observation of a carrier-substrate complex. As central catalytic steps, the reorientation of the substrate-binding site at the carrier during translocation across the membrane could be demonstrated at the intact membrane and the isolated protein. The results are interpreted by the gated-pore mechanism where two subunits form a gate with a central binding site which radically change conformation and specificity on transition from one to the other side of the membrane.