Mitochondria play a key role in energy production through oxidative phosphorylation. The electron transport chain located in the inner mitochondrial membrane transfers electrons from NADH and FADH2 to oxygen to create a proton gradient. ATP synthase uses this proton gradient to generate ATP from ADP and phosphate. The number of ATP produced depends on whether NADH or FADH2 donate electrons, with NADH producing more ATP. Different tissues use different shuttles like the glycerol phosphate or malate-aspartate shuttle to transport electrons from glycolysis into the mitochondria. Muscle and brain produce 30 ATP per glucose, while heart and liver produce 32 ATP per glucose.