Which process directly produces ATP from ADP during cellular respiration?

During cellular respiration, the process that directly produces ATP from ADP is the electron transport chain. In cellular respiration, the electron transport chain occurs in the inner mitochondrial membrane and involves a series of protein complexes that transfer electrons derived from NADH and FADH2, which are produced in earlier stages of respiration like glycolysis and the Krebs cycle.

As electrons move through the chain, they create a proton gradient across the mitochondrial membrane, which stores potential energy. This gradient is utilized by ATP synthase, an enzyme that harnesses the flow of protons back into the mitochondrial matrix to convert ADP and inorganic phosphate into ATP in a process called oxidative phosphorylation.

While glycolysis and the Krebs cycle also produce some ATP, they do so through substrate-level phosphorylation, a process that does not directly involve the electron transport chain. Fermentation, on the other hand, primarily occurs in anaerobic conditions and generates ATP through a different pathway that does not utilize the electron transport chain at all. Therefore, the electron transport chain is the crucial step that provides the majority of ATP during cellular respiration by directly converting ADP to ATP.