Final answer:
In photosynthesis's light-dependent reactions, H+ ions are pumped into the thylakoid lumen creating a concentration gradient. These ions flow through ATP synthase during chemiosmosis to produce ATP, which, together with NADPH, fuels the Calvin cycle.
Step-by-step explanation:
During the light-dependent reactions of photosynthesis, energy from sunlight is absorbed by pigment molecules located in the photosystems within the thylakoid membranes of the chloroplasts. Specifically, photosystem II (PSII) uses this energy to extract electrons from water molecules. As electrons move from PSII to photosystem I (PSI) through the electron transport chain, their energy is used to pump hydrogen ions (H+) into the thylakoid lumen. This builds up a gradient of H+ ions, creating a high concentration inside the lumen relative to the stroma.
At the end of the light-dependent reactions, these H+ ions play a critical role in ATP formation. They flow back from the thylakoid lumen into the stroma through ATP synthase, a process known as chemiosmosis. The energy released from the flow of H+ ions through ATP synthase is harnessed to convert ADP into ATP, which, along with NADPH, is used to power the light-independent reactions (Calvin cycle) for synthesizing organic molecules.