The complete oxidation of one glucose molecule yields 30 to 32 ATP molecules. It starts with glycolysis yielding two ATPs and two NADHs, followed by the oxidation of pyruvate producing two NADHs. Next, the citric acid cycle yields six NADHs, two FADH2s, and two ATPs. The NADH and FADH2 molecules are oxidized in the electron transport chain to produce 26 or 28 ATPs. This range of ATP is approximate for three reasons. First, the electron carriers indirectly produce ATP by pumping hydrogen ions into the intermembrane space, producing theoretical fractions of 2.5 and 1.5 ATP molecules. Second, NADH produced in glycolysis cannot pass through the mitochondrial membrane. So, high-energy electrons are transferred to other electron carriers within the mitochondria and, depending on the cell types, produce FADH2 or NADH, yielding 1.5 or 2.5 ATPs. Third, the energy produced by respiration is also used to power other activities, like the transport of pyruvate through the mitochondrial membrane.