If oxygen is not available, the muscle fiber will produce ATP by __________.

• A. the citric acid cycle
• B. oxidative phosphorylation
• C. the electron transport chain
• D. lactic acid fermentation

Answer: (D)

Verified step by step guidance

1. Understand the context: The problem is asking about how muscle fibers produce ATP when oxygen is not available. This is a situation where aerobic respiration cannot occur, so the cell must rely on an anaerobic process.
2. Recall the key processes: Aerobic respiration includes the citric acid cycle, oxidative phosphorylation, and the electron transport chain, all of which require oxygen. Without oxygen, these processes cannot function.
3. Identify the alternative pathway: In the absence of oxygen, muscle cells switch to anaerobic respiration, specifically lactic acid fermentation, to produce ATP. This process does not require oxygen and allows glycolysis to continue by regenerating NAD⁺.
4. Describe lactic acid fermentation: During this process, pyruvate (produced from glycolysis) is converted into lactic acid, and NADH is oxidized back to NAD⁺. This ensures that glycolysis can continue to produce small amounts of ATP.
5. Conclude the reasoning: Since the citric acid cycle, oxidative phosphorylation, and the electron transport chain all require oxygen, the correct answer is lactic acid fermentation, which is the anaerobic pathway used by muscle fibers to produce ATP when oxygen is unavailable.