Which of the following processes occurs first during cellular respiration in eukaryotic cells?

• A. Citric acid cycle (Krebs cycle)
• B. Oxidative phosphorylation
• C. Glycolysis
• D. Fermentation

Answer: (C)

Verified step by step guidance

1. Understand the concept of cellular respiration: Cellular respiration is the process by which cells convert glucose into energy (ATP). It consists of three main stages: glycolysis, the citric acid cycle (Krebs cycle), and oxidative phosphorylation. Fermentation is an alternative pathway used when oxygen is not available.
2. Identify the sequence of events in cellular respiration: Glycolysis is the first step in cellular respiration, occurring in the cytoplasm. It breaks down glucose into two molecules of pyruvate, producing a small amount of ATP and NADH.
3. Clarify the role of glycolysis: Glycolysis does not require oxygen and serves as the initial step to prepare glucose for further energy extraction in the mitochondria during the citric acid cycle and oxidative phosphorylation.
4. Explain why fermentation is not part of aerobic cellular respiration: Fermentation occurs only when oxygen is absent, allowing cells to regenerate NAD+ for glycolysis. It is not a step in the aerobic cellular respiration pathway.
5. Summarize the correct sequence: Glycolysis occurs first, followed by the citric acid cycle and oxidative phosphorylation in aerobic respiration. Fermentation is an alternative pathway used under anaerobic conditions.