If oxygen is not available, what happens to the end product of glycolysis, pyruvate, in most eukaryotic cells?

• A. It is directly excreted from the cell without further modification.
• B. It enters the mitochondria and is fully oxidized in the citric acid cycle.
• C. It is converted into glucose by gluconeogenesis.
• D. It is converted into lactate through lactic acid fermentation.

Answer: (D)

Verified step by step guidance

1. Understand the context of glycolysis: Glycolysis is the metabolic pathway that breaks down glucose into pyruvate, producing ATP and NADH. This process occurs in the cytoplasm and does not require oxygen.
2. Recognize the role of oxygen: In the presence of oxygen, pyruvate typically enters the mitochondria to be fully oxidized in the citric acid cycle. However, when oxygen is unavailable, cells must find an alternative pathway to regenerate NAD+ for glycolysis to continue.
3. Learn about anaerobic conditions: In most eukaryotic cells, under anaerobic conditions (lack of oxygen), pyruvate cannot proceed to the citric acid cycle. Instead, it undergoes fermentation to regenerate NAD+.
4. Understand lactic acid fermentation: During lactic acid fermentation, pyruvate is reduced to lactate by the enzyme lactate dehydrogenase. This process uses NADH and regenerates NAD+, allowing glycolysis to continue producing ATP.
5. Conclude the fate of pyruvate: In the absence of oxygen, pyruvate is converted into lactate through lactic acid fermentation, ensuring the cell can maintain energy production under anaerobic conditions.