Energy production in cells is centered in the mitochondria, which act as the cell’s powerhouses. In aerobic conditions, most ATP comes from mitochondria, where the citric acid cycle and oxidative phosphorylation take place on the inner mitochondrial membrane. Here, electrons are carried by NADH and FADH2 to the electron transport chain, creating a proton gradient that powers ATP synthase to produce ATP—the cell’s energy currency. Glycolysis in the cytoplasm also yields ATP, but that is a relatively small amount compared with what mitochondria generate. Mitochondria even contain their own DNA and ribosomes, underscoring their specialized role in energy metabolism. In contrast, the nucleus stores genetic material and regulates activity, ribosomes synthesize proteins, and the endoplasmic reticulum processes and transports proteins and lipids. Therefore, the primary site for energy production is the mitochondria.