Mitochondria can directly use adenosine diphosphate (ADP) and inorganic phosphate (Pi) to synthesize adenosine triphosphate (ATP).
During cellular respiration, particularly in the process of oxidative phosphorylation, the electrons from nutrients are transferred through a series of protein complexes in the inner mitochondrial membrane. This electron transport creates a proton gradient across the membrane, leading to the production of ATP.
As protons flow back into the mitochondrial matrix through ATP synthase, ADP and Pi combine to form ATP. Therefore, both ADP and inorganic phosphate are essential substrates for the direct synthesis of ATP in the mitochondria.