(d) : Last step of aerobic respiration is oxidation of reduced co-enzymes, i.e., NADH ‌2 and FADH2 by molecular oxygen through FAD, CoQ (ubiquinone), Cyt. b, Cyt. c1, Cyt. c, Cyt. a and Cyta3. Two hydrogen atoms or electrons move from NADH ‌2 and travel through this ETS chain and finally combine with half molecule of O2 to form water. During this electron transport FAD and Fe of different cytochromes are successively reduced and oxidised and at certain points, enough energy is released which is used to bind ADP with Pi to form ATP.