Researchers treat illuminated chloroplasts with a compound that lets protons diffuse freely across the thylakoid membrane. ATP production in the stroma rapidly collapses even though electron flow and NADPH formation continue. Which process did the compound most directly disrupt?

Core Concept

ATP synthesis in the light reactions depends on the H+ gradient concentrated in the thylakoid lumen by water splitting and by proton pumping at the cytochrome b6f complex. A leak lets protons slip back across the membrane without passing through ATP synthase, so the proton motive force drains even while photons are absorbed and electrons flow. With the gradient gone, the synthase stops rotating and ADP can no longer be phosphorylated.