Where does the energy for ATP synthesis come from in photosynthesis?

The energy for ATP synthesis in photosynthesis primarily comes from the electron transport chain. When light energy is absorbed by chlorophyll, it excites electrons, initiating the light-dependent reactions of photosynthesis. These energized electrons are then transferred through a series of proteins in the thylakoid membrane known as the electron transport chain.

As the electrons move through this chain, the energy they release during their transfer is used to pump protons (H+ ions) from the stroma into the thylakoid lumen, creating a proton gradient. This gradient is crucial because it stores potential energy. The protons flow back into the stroma through ATP synthase, a protein complex that couples this flow to the synthesis of ATP from ADP and inorganic phosphate. This process is known as chemiosmosis.

In this context, while chlorophyll absorbs light energy and is essential for the initial excitation of electrons, and while the splitting of water provides the electrons needed to replace those lost by chlorophyll, it is the electron transport chain that ultimately harnesses light energy to create the proton gradient necessary for ATP production. This highlights the critical role of the electron transport chain in converting light energy into chemical energy in the form of ATP.