When do cells switch from cellular respiration to fermentation?

Options

A. when electron acceptors are not available
B. ewhen NADH and FADH2 supplies are low
C. when the proton-motive force runs down
D. when pyruvate is not available

Answer

### Step-by-Step Solution

1. **Understanding Cellular Respiration**:
- Cellular respiration is the process by which cells convert glucose and oxygen into energy (ATP), carbon dioxide, and water. This process primarily occurs in the presence of oxygen (aerobic respiration).

2. **Role of Oxygen**:
- Oxygen serves as the final electron acceptor in the electron transport chain (ETC) during aerobic respiration. The presence of oxygen allows for the complete oxidation of glucose, leading to maximum ATP production.

3. **What is Fermentation?**:
- Fermentation is an anaerobic process that occurs in the absence of oxygen. It allows cells to produce energy when oxygen is not available. In fermentation, glucose is partially broken down, resulting in products like lactic acid or ethanol, depending on the organism.

4. **Conditions for Switching to Fermentation**:
- Cells switch from cellular respiration to fermentation primarily when oxygen is not available. However, there are specific conditions that can trigger this switch:
- **When NADH and FADH2 supplies are low**: These molecules are crucial for the continuation of the electron transport chain. If their levels drop, the ETC cannot function properly, leading to a decrease in ATP production and prompting the cell to switch to fermentation to regenerate NAD+.

5. **Incorrect Options**:
- **When electron acceptors are not available**: This is not a prominent condition since electron acceptors are fixed in the ETC.
- **When the Proton Motif Force runs down**: This is related to the functioning of the ETC, and its depletion does not directly trigger fermentation.
- **When pyruvate is not available**: Pyruvate is produced during glycolysis, and its availability is not a limiting factor for fermentation.

6. **Conclusion**:
- Cells primarily switch from cellular respiration to fermentation when there is a lack of NADH and FADH2, which are essential for the electron transport chain. This switch allows cells to continue producing ATP in anaerobic conditions.