If there is not enough oxygen available for cellular respiration, cells must rely on anaerobic processes to generate energy. Cellular respiration typically involves three main stages: glycolysis, the citric acid cycle (Krebs cycle), and the electron transport chain, requiring oxygen as the final electron acceptor in the electron transport chain under aerobic conditions.
When oxygen is scarce, here’s what occurs:
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Glycolysis Continues: Glycolysis, the first step of cellular respiration, can still occur without oxygen. It breaks down glucose into pyruvate, yielding a small amount of ATP (2 ATP molecules per glucose).
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Fermentation: Instead of entering the citric acid cycle and electron transport chain, pyruvate is converted into other compounds through fermentation. There are two main types of fermentation:
- Lactic Acid Fermentation: In animal cells (including human muscle cells) and some bacteria, pyruvate is converted into lactic acid. This process regenerates NAD+, allowing glycolysis to continue, albeit with a limited ATP yield.
- Alcoholic Fermentation: In yeast and some bacteria, pyruvate is converted into ethanol and carbon dioxide. This also regenerates NAD+ so glycolysis can proceed.
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Reduced ATP Yield: Anaerobic respiration (fermentation) produces significantly less ATP than aerobic respiration. While aerobic respiration can yield about 36-38 ATP molecules per glucose molecule, fermentation produces only about 2 ATP.
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Accumulation of Byproducts: The accumulation of byproducts, such as lactic acid in muscles or ethanol in yeast, can lead to various effects, such as muscle fatigue in humans or inhibiting further yeast growth in fermentation processes.
In summary, insufficient oxygen leads to anaerobic respiration, which allows for some energy production, but at the cost of much lower ATP yield and potentially harmful byproducts.
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