Which stage of cellular respiration can occur in human cells both in the presence and absence of oxygen?

• A. Krebs cycle (Citric Acid Cycle)
• B. Electron Transport Chain
• C. Oxidative Phosphorylation
• D. Glycolysis

Answer: (D)

Verified step by step guidance

1. Understand the stages of cellular respiration: Cellular respiration consists of glycolysis, the Krebs cycle (citric acid cycle), and oxidative phosphorylation (which includes the electron transport chain). Each stage has specific requirements for oxygen availability.
2. Recall the role of oxygen in cellular respiration: Oxygen is required for the Krebs cycle and oxidative phosphorylation because these processes rely on aerobic conditions to function efficiently. The electron transport chain specifically uses oxygen as the final electron acceptor.
3. Identify the process that occurs in both aerobic and anaerobic conditions: Glycolysis is the first stage of cellular respiration and occurs in the cytoplasm. It does not require oxygen and can proceed under both aerobic and anaerobic conditions.
4. Understand the products of glycolysis: Glycolysis breaks down one molecule of glucose into two molecules of pyruvate, producing a net gain of 2 ATP and 2 NADH molecules. Under anaerobic conditions, pyruvate can be further processed through fermentation.
5. Conclude why glycolysis is the correct answer: Since glycolysis does not depend on oxygen and can occur in human cells regardless of oxygen availability, it is the stage of cellular respiration that functions in both aerobic and anaerobic conditions.