Water is the by-product of cellular respiration. The water is produced as a result of -

Options

A. Conversion of pyruvate to acetyl CoA
B. Conversion of glucose to pyruvate
C. Combining carbon dioxide with protons
D. The reduction of oxygen at the end of electron transport chain

Answer

### Step-by-Step Solution:

1. **Understanding Cellular Respiration**: Cellular respiration is a biochemical process where glucose is broken down in the presence of oxygen to produce energy, carbon dioxide, and water.

2. **Identifying the By-products**: The main by-products of cellular respiration are carbon dioxide (CO2) and water (H2O).

3. **Chemical Equation**: The general equation for cellular respiration can be written as:
\[ \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy} \]
Here, glucose (C6H12O6) reacts with oxygen (O2) to produce carbon dioxide (CO2), water (H2O), and energy.

4. **Formation of Water**: Water is specifically produced during the final stages of cellular respiration, particularly during the electron transport chain (ETC).

5. **Reduction of Oxygen**: In the electron transport chain, oxygen acts as the final electron acceptor. It combines with electrons and protons (H+) to form water:
\[ \text{O}_2 + 4\text{e}^- + 4\text{H}^+ \rightarrow 2\text{H}_2\text{O} \]
This reaction shows that water is formed as a result of the reduction of oxygen.

6. **Conclusion**: Therefore, the water produced as a by-product of cellular respiration is primarily due to the reduction of oxygen at the end of the electron transport chain.

### Final Answer:
Water is produced as a by-product of cellular respiration due to the reduction of oxygen at the end of the electron transport chain.

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