A photosynthetic course of exists inside crops and sure micro organism the place electrons cycle particularly by photosystem I (PSI). As an alternative of following the everyday non-cyclic route involving each photosystems I and II, electrons excited by gentle vitality in PSI are handed to ferredoxin (Fd). Fd, somewhat than decreasing NADP+ to NADPH, transfers these electrons again to the cytochrome b6f advanced. This advanced then pumps protons (H+) into the thylakoid lumen, contributing to a proton gradient used to generate ATP. Finally, the electrons return to PSI, finishing the cycle. This course of doesn’t produce NADPH, nor does it contain photosystem II; due to this fact, oxygen just isn’t developed.
This different electron pathway serves an important perform in regulating the stability of ATP and NADPH manufacturing throughout the chloroplast. In conditions the place ATP demand is excessive, this course of permits the cell to generate further ATP to fulfill its vitality wants. Furthermore, it performs a photoprotective position, stopping harm to the photosynthetic equipment underneath excessive gentle circumstances. By diverting electrons away from NADPH manufacturing, it reduces the chance 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 by biochemical and spectroscopic analyses.
