1. When a runner's oxygen supply is limited, which metabolic pathway primarily allows her cells to continue producing ATP?

• A. The Calvin cycle
• B. Anaerobic glycolysis (lactic acid fermentation)
• C. Oxidative phosphorylation in the mitochondria
• D. Beta-oxidation of fatty acids

Answer: (B)

Verified step by step guidance

1. Understand the context: The problem involves identifying the metabolic pathway that allows ATP production when oxygen supply is limited. This situation is referred to as anaerobic conditions, where cells must rely on pathways that do not require oxygen.
2. Review the options: The Calvin cycle is part of photosynthesis and occurs in plants, not relevant to human ATP production. Oxidative phosphorylation requires oxygen, so it cannot function under limited oxygen supply. Beta-oxidation of fatty acids also requires oxygen and is part of aerobic metabolism.
3. Focus on anaerobic glycolysis: This pathway allows cells to produce ATP without oxygen. It involves the breakdown of glucose into pyruvate, followed by conversion of pyruvate into lactic acid to regenerate NAD⁺, which is essential for glycolysis to continue.
4. Explain the role of lactic acid fermentation: During anaerobic glycolysis, lactic acid fermentation occurs to ensure that glycolysis can proceed by recycling NAD⁺. This process is critical for ATP production under oxygen-limited conditions.
5. Conclude: The correct metabolic pathway under limited oxygen supply is anaerobic glycolysis (lactic acid fermentation), as it enables ATP production without relying on oxygen.