Where would the pH be lowest in a chloroplast?

Where would the pH be lowest in a chloroplast?

• A. The lumen of the thylakoid.
• B. The stroma.
• C. The cytosol.
• D. The inner membrane space.

Answer: (A)

During the light reactions of photosynthesis, energized electrons pump protons from the stroma into the space inside the thylakoid membranes, creating a proton gradient across the thylakoid membrane. This buildup of protons inside the thylakoid lumen makes that compartment more acidic (lower pH) than the surrounding stroma, which becomes relatively more alkaline as protons are removed from it. The cytosol is outside the chloroplast and not directly involved in this proton-pumping gradient, and the term “inner membrane space” isn’t a standard chloroplast compartment like the lumen is. Because pH is a measure of H+ concentration, the lumen ends up with the highest H+ concentration and thus the lowest pH.

During the light reactions of photosynthesis, energized electrons pump protons from the stroma into the space inside the thylakoid membranes, creating a proton gradient across the thylakoid membrane. This buildup of protons inside the thylakoid lumen makes that compartment more acidic (lower pH) than the surrounding stroma, which becomes relatively more alkaline as protons are removed from it. The cytosol is outside the chloroplast and not directly involved in this proton-pumping gradient, and the term “inner membrane space” isn’t a standard chloroplast compartment like the lumen is. Because pH is a measure of H+ concentration, the lumen ends up with the highest H+ concentration and thus the lowest pH.