A photosynthetic process exists within plants and certain bacteria where electrons cycle specifically through photosystem I (PSI). Instead of following the typical non-cyclic route involving both photosystems I and II, electrons excited by light energy in PSI are passed to ferredoxin (Fd). Fd, rather than reducing NADP+ to NADPH, transfers these electrons back to the cytochrome b6f complex. This complex then pumps protons (H+) into the thylakoid lumen, contributing to a proton gradient used to generate ATP. Ultimately, the electrons return to PSI, completing the cycle. This process does not produce NADPH, nor does it involve photosystem II; therefore, oxygen is not evolved.
This alternative electron pathway serves a vital function in regulating the balance of ATP and NADPH production within the chloroplast. In situations where ATP demand is high, this process allows the cell to generate additional ATP to meet its energy needs. Moreover, it plays a photoprotective role, preventing damage to the photosynthetic machinery under high light conditions. By diverting electrons away from NADPH production, it reduces the risk of over-reduction of the electron transport chain, minimizing the formation of damaging reactive oxygen species. Its presence was inferred from early observations of photosynthesis and has since been confirmed through biochemical and spectroscopic analyses.
