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O and NADH: energy-releasing reactions

The electrons from NADH and FADH are transferred to oxygen (O) and hydrogen (protons) in several steps via an electron transport chain. NADH and FADH molecules are produced within the matrix via the citric acid cycle and in the cytoplasm by glycolysis. Reducing equivalents from the cytoplasm can be imported via the malate-aspartate shuttle system of antiporter proteins or fed into the electron transport chain using a glycerol phosphate shuttle.