Final answer:
The difference in hydrogen ion concentration across the thylakoid membrane during the light-dependent reactions of photosynthesis is used to generate ATP through a process known as chemiosmosis.
Step-by-step explanation:
In the light-dependent reactions of photosynthesis, the difference in hydrogen ion (H+) concentration across the thylakoid membrane is used to generate ATP. This process is known as photophosphorylation and involves ATP synthase, a protein complex that allows hydrogen ions to pass through the thylakoid membrane. The energy from the hydrogen ions moving down their electrochemical gradient is harnessed by ATP synthase to add a third phosphate to ADP, thus forming ATP.
The initial source of electrons for the chloroplast electron transport chain comes from water molecules. These electrons are excited by sunlight at photosystem II (PSII) and pass through a series of proteins in the electron transport chain, creating an energy source to pump H+ into the thylakoid lumen. As a result, a high concentration of hydrogen ions is built up inside the thylakoid space. The passive movement of these ions through ATP synthase during chemiosmosis generates ATP, which, along with NADPH, is used to fuel the Calvin cycle for sugar synthesis.