In chloroplasts, how does the thylakoid lumen become acidic?

In chloroplasts, how does the thylakoid lumen become acidic?

• A. Protons are pumped from the stroma into the thylakoid lumen.
• B. Protons are pumped from the lumen into the stroma.
• C. Protons are generated by ATP hydrolysis in the lumen.
• D. Protons diffuse out of the chloroplast.

Answer: (A)

The thylakoid lumen becomes acidic because protons are pumped from the stroma into the lumen as electrons are transported through the photosynthetic electron transport chain. As light drives the flow of electrons, the cytochrome b6f complex pumps H+ from the stroma into the lumen, and water splitting at photosystem II also releases protons into the lumen. This accumulation of H+ lowers the lumen’s pH, creating a proton gradient that stores energy used by ATP synthase to make ATP. If protons were pumped the other way, or if protons were simply generated by ATP hydrolysis in the lumen, or if they diffused out, the gradient would not form. The key is moving protons into the lumen to build the proton motive force.

The thylakoid lumen becomes acidic because protons are pumped from the stroma into the lumen as electrons are transported through the photosynthetic electron transport chain. As light drives the flow of electrons, the cytochrome b6f complex pumps H+ from the stroma into the lumen, and water splitting at photosystem II also releases protons into the lumen. This accumulation of H+ lowers the lumen’s pH, creating a proton gradient that stores energy used by ATP synthase to make ATP.

If protons were pumped the other way, or if protons were simply generated by ATP hydrolysis in the lumen, or if they diffused out, the gradient would not form. The key is moving protons into the lumen to build the proton motive force.