In the process of photosynthesis, ATP synthesis occurs through photo-phosphorylation, which can be cyclic or non-cyclic. In non-cyclic photo-phosphorylation, both Photosystem I (PSI) and Photosystem II (PSII) work sequentially to generate ATP and NADPH from light energy. This process follows a specific pathway known as the Z-scheme, where electrons are high in energy and move from PSII to PSI before being transferred to NADP+, resulting in the synthesis of NADPH and a proton gradient necessary for ATP generation.
In contrast, during cyclic photo-phosphorylation, only PSI is involved, and the excited electron cycles back to the PSI complex after its energy has been harvested. This results in only ATP production without generating NADPH. The production of ATP through these mechanisms relies on the chemiosmotic hypothesis, which posits that a proton gradient across the thylakoid membrane drives ATP synthesis by allowing protons to flow back into the stroma through ATP synthase, thus producing ATP. The section concludes by discussing the implications of these processes in the broader context of photosynthesis and energy conversion.