The Krebs cycle (citric acid cycle) is the second step in cellular respiration after glycolysis. During the Krebs cycle, pyruvate from glycolysis enters the mitochondria and is broken down, producing carbon dioxide, NADH, FADH2, and ATP. For each glucose molecule, the Krebs cycle occurs twice, producing a total of 6 carbon dioxide molecules, 2 ATP, 8 NADH, and 2 FADH2 to be used in the electron transport chain to power aerobic respiration. The Krebs cycle is critical for producing electron carriers that generate most of the ATP from cellular respiration.