How is the proton gradient across the thylakoid membrane established for ATP synthesis?

How is the proton gradient across the thylakoid membrane established for ATP synthesis?

• A. The Calvin cycle consumes protons in the stroma
• B. The electron transport chain pumps protons into the thylakoid lumen as electrons are transferred
• C. ATP hydrolysis pumps protons into the lumen
• D. Water diffusion creates the gradient

Answer: (B)

The main idea is that the buildup of a proton gradient across the thylakoid membrane comes from actively moving protons into the lumen during the light reactions. As electrons flow through the photosynthetic electron transport chain, the cytochrome b6f complex uses part of that energy to pump protons from the stroma into the thylakoid lumen. In addition, splitting water at Photosystem II releases protons directly into the lumen. These two processes raise the proton concentration inside the lumen relative to the stroma, creating a proton-motive force. ATP synthase then uses the flow of protons back into the stroma to drive ATP formation from ADP and Pi. Other options don’t fit because they don’t generate the proton gradient: the Calvin cycle occurs in the stroma and doesn’t establish a gradient, ATP hydrolysis pushing protons into the lumen would oppose the gradient, and proton movement by simple water diffusion isn’t a mechanism to create this stored energy across the membrane.

The main idea is that the buildup of a proton gradient across the thylakoid membrane comes from actively moving protons into the lumen during the light reactions. As electrons flow through the photosynthetic electron transport chain, the cytochrome b6f complex uses part of that energy to pump protons from the stroma into the thylakoid lumen. In addition, splitting water at Photosystem II releases protons directly into the lumen. These two processes raise the proton concentration inside the lumen relative to the stroma, creating a proton-motive force. ATP synthase then uses the flow of protons back into the stroma to drive ATP formation from ADP and Pi.

Other options don’t fit because they don’t generate the proton gradient: the Calvin cycle occurs in the stroma and doesn’t establish a gradient, ATP hydrolysis pushing protons into the lumen would oppose the gradient, and proton movement by simple water diffusion isn’t a mechanism to create this stored energy across the membrane.