Free ADP is transported from the cytoplasm to the mitochondrial matrix, while ATP produced from oxidative phosphorylation is transported from the mitochondrial matrix to the cytoplasm, thus providing the cell with its main energy currency.[4] ADP/ATP translocases are exclusive to eukaryotes and are thought to have evolved during eukaryogenesis.[5] Human cells express four ADP/ATP translocases: SLC25A4, SLC25A5, SLC25A6 and SLC25A31, which constitute more than 10% of the protein in the inner mitochondrial membrane.[6] These proteins are classified under the mitochondrial carrier superfamily.
^Klingenberg M (October 2008). "The ADP and ATP transport in mitochondria and its carrier". Biochimica et Biophysica Acta (BBA) - Biomembranes. 1778 (10): 1978–2021. doi:10.1016/j.bbamem.2008.04.011. PMID18510943.