The second step in the machinery is much more efficient in extracting energy. Instead of being reduced to lactate or ethanol, pyruvate enters the citric acid cycle, where it is broken down to C0₂, with hydrogen atoms being used to reduce NAD⁺ and FAD to NADH and FADH₂. Some additional ATP also is made along the way. The NADH and FADH₂ from the citric acid cycle, plus the NADH from fermentation, which now is not needed to convert pyruvate into something else, all flow into the third process, the respiratory chain. Here they are reoxidized to NAD⁺ and FAD and are recycled. The hydrogen atoms ultimately are added to 0₂ to make water, and the free energy that is liberated is stored in the form of ATP. The overall process - the combustion of glucose with oxygen - is carried out in a series of small steps so that the maximum amount of energy from the reaction can be saved.