The mitochondrial outer membrane is freely permeable to most small solute molecules, but the inner membrane is impermeable to H⁺, OA⁻, K⁺, and also many other ionic solutes. How, then, can the ADP³ and  produced in the cytosol enter the matrix and how can the newly formed ATP⁴⁻ leave again, since oxidative phosphorylation takes place within the inner matrix space? Two of the many specific transport systems are present in the inner mitochondrial membrane. They are adenine nucleotide translocase system and phosphate translocase system.