Final answer:
In cellular respiration, oxygen (O₂) is used directly only in the Electron Transport Chain (option C). Glycolysis (option A), the Krebs Cycle (option B), and Pyruvate Decarboxylation (option D) do not use oxygen directly.
Step-by-step explanation:
The student has asked which stages of cellular respiration directly utilize oxygen (O₂) as an input. The summary equation of cellular respiration is presented as C6H12O6 + O2 → CO2 + H2O, which indicates the overall process involving glucose and oxygen to produce carbon dioxide, water, and ATP (energy).
Breaking down the process, glycolysis is the first stage and takes place in the cytoplasm. It does not require oxygen, instead it uses glucose to produce pyruvate, ATP, and NADH. Hence, option A (Glycolysis) does not use oxygen directly. The next stage is the transformation of pyruvate into Acetyl CoA, also known as pyruvate decarboxylation, which is an intermediate step before the Krebs cycle, but it does not directly involve oxygen either, so option D is excluded.
Option B, the Krebs Cycle (Citric Acid Cycle), occurs in the mitochondrial matrix and utilizes oxygen indirectly for the oxidation of Acetyl CoA, but oxygen does not directly participate in the reactions of the cycle itself. Lastly, the Electron Transport Chain (ETC), which takes place on the inner mitochondrial membrane, indeed uses oxygen directly. Oxygen serves as the final electron acceptor to form water.
Therefore, the only stage that directly uses O₂ as part of cellular respiration is option C, the Electron Transport Chain.