Source of protons within the chloroplasts is

Options

A. Water
B. Excited chlorophyll
C. Carbon dioxide
D. Rubisco.

Answer

**Step-by-Step Solution:**

1. **Understanding Photosynthesis:** Photosynthesis consists of two main stages: light reactions and dark reactions (Calvin cycle). The light reactions occur in the thylakoid membranes of the chloroplasts and require light energy.

2. **Role of Chlorophyll:** In the light reactions, chlorophyll absorbs light energy in the form of photons. This energy excites the chlorophyll molecules, leading to their activation.

3. **Photolysis of Water:** The activation of chlorophyll triggers the process known as photolysis of water. This process involves the splitting of water molecules (H₂O) into oxygen (O₂), protons (H⁺), and electrons (e⁻).

4. **Source of Protons:** The protons (H⁺ ions) produced during the photolysis of water are the source of protons within the chloroplasts. Therefore, the correct answer to the question is water, as it is the molecule that, when split, releases protons.

5. **Elimination of Incorrect Options:**
- **Excited Chlorophyll:** While excited chlorophyll is crucial for initiating the photolysis, it does not directly provide protons.
- **Carbon Dioxide:** Carbon dioxide is involved in the dark reactions but does not contribute to the proton pool in the chloroplasts.
- **Rubisco:** Rubisco is an enzyme involved in the Calvin cycle and is not a source of protons.

6. **Conclusion:** The source of protons within the chloroplasts is water, as it undergoes photolysis during the light reactions of photosynthesis.

**Final Answer:** The source of protons within the chloroplasts is **water**.

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