In this process of photosynthesis, which takes place in green plants which undergoes reduction?

Options

A. Water
B. `CO_(2)`
C. Protons
D. Photons

Answer

To determine which substance undergoes reduction in the process of photosynthesis, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Photosynthesis**:
Photosynthesis is the process by which green plants convert light energy into chemical energy, using carbon dioxide (CO2) from the atmosphere and water (H2O) from the soil to produce carbohydrates (sugars) and oxygen (O2).

2. **Chemical Equation of Photosynthesis**:
The overall reaction can be summarized as:
\[ 6CO_2 + 6H_2O \xrightarrow{\text{light energy}} C_6H_{12}O_6 + 6O_2 \]
Here, glucose (C6H12O6) is produced along with oxygen.

3. **Identifying Oxidation States**:
- In carbon dioxide (CO2), carbon has an oxidation state of +4.
- In glucose (C6H12O6), carbon has an oxidation state of 0.
- In water (H2O), oxygen has an oxidation state of -2.
- In molecular oxygen (O2), oxygen has an oxidation state of 0.

4. **Analyzing Changes in Oxidation States**:
- **Carbon**:
- In CO2, the oxidation state of carbon is +4.
- In glucose, the oxidation state of carbon is 0.
- The change in oxidation state for carbon is from +4 to 0, indicating a reduction (gain of electrons).
- **Oxygen**:
- In H2O, the oxidation state of oxygen is -2.
- In O2, the oxidation state of oxygen is 0.
- The change in oxidation state for oxygen is from -2 to 0, indicating oxidation (loss of electrons).

5. **Conclusion**:
Since the oxidation state of carbon decreases (from +4 to 0), carbon dioxide (CO2) is the substance that undergoes reduction in the process of photosynthesis. Therefore, the answer is CO2.

### Final Answer:
The substance that undergoes reduction in photosynthesis is **carbon dioxide (CO2)**.

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