ATP (adenosine triphosphate) is the energy-carrying molecule typically produced in cellular energy metabolism. In most organisms, including humans, the process begins with glycolysis in the cytosol, proceeds through the mitochondria with the pyruvate dehydrogenase step forming acetyl-CoA, and culminates in the citric acid cycle (Krebs cycle) and oxidative phosphorylation (electron transport chain). The end result is the production of ATP, NADH, and FADH2, with oxygen acting as the final electron acceptor to drive most ATP synthesis. The majority of ATP is generated by oxidative phosphorylation in the mitochondrial inner membrane, where protons pumped across the membrane create a proton gradient that drives ATP synthase to phosphorylate ADP to ATP. This ATP then serves as the immediate energy currency used by cellular processes.