NADH and FADH2 are the forms of NAD and FAD. The four stages of aerobic cellular respiration take place in the mitochondria and the of the cell. Cellular respiration that takes place in the presence of oxygen is called and is the more efficient form of respiration. At the end of glycolysis, a six-carbon glucose molecule has been converted into two three-carbon molecules of . Acetyl-CoA combines with to form citric acid and start the Krebs cycle. At the end of the electron transport chain, the electrons combine with to form water.