There are three main steps of cellular respiration: glycolysis; the citric acid (TCA) or the Krebs cycle; and the electron transport chain, where oxidative phosphorylation occurs. The TCA cycle and oxidative phosphorylation require oxygen, while glycolysis can occur in anaerobic conditions.  Glycolysis is the initial breakdown of glucose to pyruvate, a three carbon structure, in the cytoplasm. The pyruvate then moves into the mitochondrial matrix where a transition step called pyruvate oxidation takes place. In this process, pyruvate dehydrogenase converts the three-carbon pyruvate to the two-carbon acetyl-CoA. The TCA cycle begins when acetyl-CoA combines with a four-carbon oxaloacetate in order to form the six-carbon citrate. Because each molecule of glucose produces 2 pyruvate molecules, it takes two turns through the Krebs cycle to completely break down the original glucose.  Finally, the electron transport chain is a series of redox reactions powered by high energy electrons that pumps protons across the membrane, creating a proton gradient. Together, an electrochemical gradient is created. At the end of the electron transport chain, the final electron acceptor, O2, combines with protons to produce water (H2O). Meanwhile, ATP synthase uses the movement of protons back into the mitochondrial matrix for ATP synthesis.