Final answer:
Photosystem II (PSII) is the main contributor to generating high-energy electrons for ATP synthesis in the light-dependent reactions of photosynthesis. It uses electrons from water to create a proton gradient for ATP production through ATP synthase, in a process known as photophosphorylation.
Step-by-step explanation:
The photosystem that's mainly responsible for generating high-energy electrons for the synthesis of ATP is Photosystem II (PSII). During the light-dependent reactions of photosynthesis, electrons are excited in PSII and then travel through the chloroplast electron transport chain.
This flow of electrons is used to pump hydrogen ions into the thylakoid lumen, thus creating a proton gradient. The potential energy stored in this gradient drives the synthesis of ATP as the protons flow back through the enzyme complex called ATP synthase.
The initial source of electrons for the chloroplast electron transport chain is water molecules, which are split by PSII in a process that also generates oxygen. As electrons move from PSII to Photosystem I (PSI), they lose energy, which is used to move protons across the thylakoid membrane and generate the electrochemical gradient required for ATP synthesis during photophosphorylation.
The term 'chemiosmosis' refers to the movement of ions across the semi-permeable thylakoid membrane to synthesize ATP.