Both these processes produce ATP for the cells energy needs. Cellular respiration is the process that occurs in the presence of oxygen. This process occurs in the cells mitochondrion, the organelle nicknamed the "powerhouse" of the cell. Glucose is oxidized by oxygen, the result is the production of energy, in the form of ATP, and carbon dioxide and water as waste products. Of the two processes, cellular respiration is the more efficient of the two, producing 34 to 38 molecules of ATP, which stands for adenosine triphosphate, as compared to the two molecules of ATP produced by fermentation. Fermentation is a process of energy production that occurs in the absence of oxygen. It involves the transfer of electrons to NADH+ to form NADH. Glucose is still involved, but a different set of products ensue with this reaction. It is limited in its energy production because there is a limited amount of NADH available in the cell to participate in the electron transport process. The end waste product is an accumulation of lactic acid, which is what causes the "burn" we feel when we have overexerted a muscle, indicating we need to take a rest and allow oxygen levels to restore themselves.