When sunlight is on the chloroplast, pH is lowest in the

Options

A. Stroma
B. Cytosal
C. Space enclosed by thylakoid membrane
D. Space enclosed by the inner and outer membrane

Answer

To answer the question "When sunlight is on the chloroplast, pH is lowest in the," we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Chloroplast Structure**:
- Chloroplasts have several compartments, including the stroma (the fluid-filled space surrounding the thylakoids), the thylakoid lumen (the space inside the thylakoids), and the cytosol (the fluid part of the cytoplasm).

2. **Role of Sunlight in Photosynthesis**:
- When sunlight hits the chloroplast, it initiates the process of photosynthesis, which involves the absorption of light energy by chlorophyll.

3. **Electron Transport Chain**:
- The absorbed light energy is used to excite electrons, which are then transferred through a series of proteins known as the electron transport chain located in the thylakoid membrane.

4. **Proton Pumping**:
- As electrons move through the electron transport chain, some of their energy is used to actively transport protons (H⁺ ions) from the stroma into the thylakoid lumen.

5. **pH Changes**:
- The active transport of protons into the thylakoid lumen causes the concentration of protons to increase in that space, leading to a decrease in pH (making it more acidic). The pH in the thylakoid lumen can drop to as low as 4.

6. **Comparison of pH Levels**:
- In contrast, the pH in the stroma remains relatively higher, around pH 8, due to the lower concentration of protons.

7. **Conclusion**:
- Therefore, when sunlight is on the chloroplast, the pH is lowest in the space enclosed by the thylakoid membrane, which is the thylakoid lumen.

### Final Answer:
The pH is lowest in the **space enclosed by the thylakoid membrane** (thylakoid lumen).

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