Which of the following occurs only in anaerobic metabolism of glucose?

• A. Lactic acid fermentation
• B. Electron transport chain
• C. Citric acid cycle
• D. Oxidative phosphorylation

Answer: (A)

Verified step by step guidance

1. Understand the difference between aerobic and anaerobic metabolism. Aerobic metabolism requires oxygen and involves processes like the citric acid cycle, electron transport chain, and oxidative phosphorylation. Anaerobic metabolism occurs in the absence of oxygen and relies on alternative pathways like fermentation.
2. Review the process of lactic acid fermentation. This is a type of anaerobic metabolism where pyruvate, produced during glycolysis, is converted into lactic acid to regenerate NAD⁺, which is essential for glycolysis to continue.
3. Analyze the options provided: The electron transport chain, citric acid cycle, and oxidative phosphorylation are all processes that require oxygen and are part of aerobic metabolism. These cannot occur in anaerobic conditions.
4. Identify that lactic acid fermentation is unique to anaerobic metabolism of glucose. It allows cells to produce ATP in the absence of oxygen by continuing glycolysis and regenerating NAD⁺.
5. Conclude that the correct answer is 'Lactic acid fermentation,' as it is the only process listed that occurs exclusively in anaerobic metabolism of glucose.