Anaerobic respiration occurs when oxygen is not available. Cells undergo fermentation when this happens, meaning the first stage of cellular respiration -glycolysis- occurs to produce 2 ATP and NAD+. Regular cellular respiration cannot occur because there's no oxygen at the end of the electron transport chain to receive the electrons. Aerobic respiration occurs when oxygen is available, and the process of cellular respirations runs as normal to produce the 36-38 ATP the body needs to function.