<P> In aerobic respiration, oxygen serves as the recipient of electrons from the electron transport chain . Aerobic respiration is thus very efficient because oxygen is a strong oxidant . Aerobic respiration proceeds in a series of steps, which also increases efficiency - since glucose is broken down gradually and ATP is produced as needed, less energy is wasted as heat . This strategy results in the waste products H O and CO being formed in different amounts at different phases of respiration . CO is formed in Pyruvate decarboxylation, H O is formed in oxidative phosphorylation, and both are formed in the citric acid cycle . The simple nature of the final products also indicates the efficiency of this method of respiration . All of the energy stored in the carbon - carbon bonds of glucose is released, leaving CO and H O. Although there is energy stored in the bonds of these molecules, this energy is not easily accessible by the cell . All usable energy is efficiently extracted . </P> <P> Anaerobic respiration is done by aerobic organisms when there is not sufficient oxygen in a cell to undergo aerobic respiration as well as by cells called anaerobes that selectively perform anaerobic respiration even in the presence of oxygen . In anaerobic respiration, weak oxidants like sulfate and nitrate serve as oxidants in the place of oxygen . </P> <P> Generally, in anaerobic respiration sugars are broken down into carbon dioxide and other waste products that are dictated by the oxidant the cell uses . Whereas in aerobic respiration the oxidant is always oxygen, in anaerobic respiration it varies . Each oxidant produces a different waste product, such as nitrite, succinate, sulfide, methane, and acetate . Anaerobic respiration is correspondingly less efficient than aerobic respiration . In the absence of oxygen, not all of the carbon - carbon bonds in glucose can be broken to release energy . A great deal of extractable energy is left in the waste products . Anaerobic respiration generally occurs in prokaryotes in environments that do not contain oxygen . </P> <P> Fermentation is another process by which cells can extract energy from glucose . It is not a form of cellular respiration, but it does generate ATP, break down glucose, and produce waste products . Fermentation, like aerobic respiration, begins by breaking glucose into two pyruvate molecules . From here, it proceeds using endogenous organic electron receptors, whereas cellular respiration uses exogenous receptors, such as oxygen in aerobic respiration and nitrate in anaerobic respiration . These varied organic receptors each generate different waste products . Common products are lactic acid, lactose, hydrogen, and ethanol . Carbon dioxide is also commonly produced . Fermentation occurs primarily in anaerobic conditions, although some organisms such as yeast use fermentation even when oxygen is plentiful . </P>

What are the two waste products of respiration