In chemiosmotic synthesis of ATP, H+ diffuses through ATP-synthetase -

Options

A. from the stroma into thylakoid lumen/space
B. from thylakoid space into stroma
C. from the cytoplasm into stroma
D. from the periplastidal space to stoma

Answer

**Step-by-Step Solution:**

1. **Understanding Chemiosmosis**: Chemiosmosis is a process that uses the energy stored in the form of a proton gradient across a membrane to drive the synthesis of ATP. In chloroplasts, this occurs in the thylakoid membranes during photosynthesis.

2. **Location of ATP Synthesis**: ATP synthesis occurs in the thylakoid membranes of chloroplasts where ATP synthase is located. This enzyme facilitates the conversion of ADP and inorganic phosphate (Pi) into ATP.

3. **Proton Gradient Formation**: During the light-dependent reactions of photosynthesis, water is split (photolysis) to release oxygen, protons (H+ ions), and electrons. The protons accumulate in the thylakoid lumen, creating a high concentration of H+ ions inside the lumen compared to the stroma (the fluid-filled space surrounding the thylakoids).

4. **Diffusion of H+ Ions**: The protons (H+) then diffuse back into the stroma through ATP synthase. This movement of H+ ions from the thylakoid lumen (where they are in higher concentration) to the stroma (where they are in lower concentration) is driven by the concentration gradient.

5. **ATP Production**: As H+ ions pass through ATP synthase, the enzyme catalyzes the formation of ATP from ADP and Pi. This process is known as photophosphorylation.

6. **Conclusion**: Therefore, in chemiosmotic synthesis of ATP, H+ ions diffuse from the thylakoid space (lumen) into the stroma, leading to ATP production.

**Final Answer**: In chemiosmotic synthesis of ATP, H+ diffuses from the thylakoid space into the stroma.

---