In all cells, glucose catabolism begins with glycolysis, chemiosmosis, fermentation, pyruvate oxidation, and the citric acid cycle. The organelle responsible for generating the majority of ATP in the cell is the mitochondria. During glycolysis, one six-carbon molecule is converted into two three-carbon molecules. The citric acid cycle occurs along the mitochondrial inner membrane in the mitochondrial matrix. In the first reaction of glycolysis, glucose receives a phosphate group from ATP. This reaction is exergonic. Overall, four molecules of ATP are produced during glycolysis. However, at the end of glycolysis, there are two ATP molecules, two pyruvate molecules, and two NADH molecules present. The ATP numbers do not match because two ATP are used to produce NADH. Glucose metabolism is inefficient. Glucose phosphate is formed from fructose phosphate, and two ATP are invested in the process of glycolysis. Glucose is degraded to CO2.