Final answer:
The source of the hydrogen ions that accumulate in the thylakoid space during the light-dependent reactions of photosynthesis is water (H2O), which is split into oxygen, electrons, and protons in the photosystem II (PSII) reaction center.
Step-by-step explanation:
Original Source of Hydrogen Ions in Light-Dependent Reactions
The light-dependent reactions of photosynthesis drive the formation of ATP through the process of chemiosmosis. During these reactions, hydrogen ions accumulate within the thylakoid space of the chloroplast. The source of these hydrogen ions is the splitting of water (H2O) molecules, which occurs in the photosystem II (PSII) reaction center.
Under the influence of sunlight, the PSII complex uses light energy to extract electrons from water. This photo-induced oxidation of water generates oxygen, electrons, and protons (H+ ions). It's these protons that contribute to the electrochemical gradient across the thylakoid membrane. Since the membrane is impermeable to ions, these accumulated protons can only pass back into the stroma through the enzyme ATP synthase, which uses the potential energy from the movement of hydrogen ions to convert ADP to ATP.
The accumulation of protons in the thylakoid space not only comes from water splitting but also as a result of the proton-pumping activity of the electron transport chain. This creates a significant concentration gradient and electrochemical potential across the thylakoid membrane, referred to as the proton motive force. The exergonic movement of hydrogen ions from high to low proton concentration powers the ATP synthase to produce ATP, a process similar to the mitochondrial electron transport chain during cellular respiration.