Answer:
The four phases of cellular respiration are glycolysis, pyruvate oxidation, the Krebs cycle (also known as the citric acid cycle or TCA cycle), and oxidative phosphorylation (which includes the electron transport chain).
Of these four phases, only oxidative phosphorylation requires oxygen. This is because oxidative phosphorylation relies on the presence of oxygen as the final electron acceptor in the electron transport chain. Oxygen molecules act as the "terminal electron acceptor," accepting electrons at the end of the electron transport chain and combining with hydrogen ions to form water. This process generates a large amount of ATP through the process of chemiosmosis.
While glycolysis, pyruvate oxidation, and the Krebs cycle do not require oxygen directly, they do require the presence of certain enzymes and coenzymes that are produced during oxidative phosphorylation. Therefore, the overall process of cellular respiration is dependent on the presence of oxygen to sustain the production of ATP through oxidative phosphorylation.