Pasteur effect is

Options

A. Stoppage of fermentation in presence of oxygen
B. Increase of fermentation in presence of oxygen
C. Decrease in fermentation in presence of oxygen
D. No effect on fermentation.

Answer

**Step-by-Step Text Solution:**

1. **Understanding the Pasteur Effect**: The Pasteur effect refers to the observation that the presence of oxygen inhibits the fermentation process in yeast. This phenomenon was discovered by Louis Pasteur in 1857.

2. **Role of Yeast**: Yeast, specifically Saccharomyces cerevisiae, are facultative anaerobes. This means they can switch between aerobic respiration (using oxygen) and anaerobic fermentation (without oxygen) depending on the availability of oxygen.

3. **Fermentation Process**: Under anaerobic conditions (low oxygen), yeast converts pyruvate (the end product of glycolysis) into ethanol and carbon dioxide. This process is less efficient in terms of energy production, yielding only 2 ATP molecules per glucose molecule.

4. **Aerobic Conditions**: When oxygen is present, yeast prefers to undergo aerobic respiration. In this case, pyruvate is converted into acetyl-CoA, which enters the citric acid cycle (Krebs cycle). This process is much more efficient, producing 30 to 32 ATP molecules per glucose molecule.

5. **Conclusion**: Therefore, the Pasteur effect can be summarized as the inhibition of fermentation in the presence of oxygen, leading to a preference for aerobic respiration, which is more energy-efficient.

6. **Correct Answer**: The correct option regarding the Pasteur effect is the "stoppage of fermentation in the presence of oxygen."

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