What role does adenosine diphosphate (ADP) play in cellular respiration?

Adenosine diphosphate (ADP) plays a critical role in cellular respiration, particularly in the process of oxidative phosphorylation, where it is converted into adenosine triphosphate (ATP). This conversion is a key step in energy production within the cell.

During oxidative phosphorylation, which occurs in the mitochondria, electrons are transferred through a series of protein complexes in the electron transport chain. As these electrons move through the chain, they create a proton gradient across the inner mitochondrial membrane. This gradient is then used by ATP synthase, an enzyme that synthesizes ATP, by adding an inorganic phosphate group to ADP. This process effectively captures the energy released from the electron transport chain and stores it in the form of ATP, the primary energy carrier in cells. Thus, ADP acts as a substrate that, when phosphorylated, produces ATP, enabling the cell to use energy for various metabolic processes.

The other options do not accurately represent the function of ADP in cellular respiration. For instance, while glycolysis produces a small amount of ATP and does involve energy transfer, ADP is not considered a waste product; rather, it is separated from ATP as the latter is consumed. Additionally, while ADP can influence metabolic pathways,