Glycolysis, the initial stage of both aerobic and anaerobic respiration, begins when glucose breaks down in a cell's cytoplasm. During glycolysis, a single glucose molecule (6-carbon) is converted into two pyruvate molecules (3-carbon), yielding a small amount of ATP. The subsequent fate of pyruvate hinges on oxygen availability: in aerobic respiration, pyruvate enters the mitochondria and transforms into carbon dioxide and water; conversely, in anaerobic respiration, pyruvate is converted into lactic acid (in animals) or ethanol and carbon dioxide (in yeast). Pyruvate is thus the initial molecule formed.