Complete glucose breakdown requires the presence of oxygen to keep the working so that ATP is produced. When oxygen is absent, an anaerobic process called fermentation occurs instead, producing fewer ATP molecules overall. During this process, glycolysis proceeds as normal with glucose splitting into 2 molecules, which are then phosphorylated into 2 molecules, and finally converted into 2 molecules. However, at this point, the 2 molecules are , freeing NAD⁺ to return to the glycolytic pathway. The results of are ATP molecules and 2 molecules in animals and bacteria or 2 molecules in plants and yeast. Because these molecules enter the citric acid cycle or the electron transport chain, the net gain for fermentation is ATP molecules, compared to ATP molecules in cellular respiration.