Cyclic Electron Flow

Suppose the level of NADP is low and the level of NADPH is high. Because reduced ferredoxin donates its electron to NADP, the electrons could possibly “back up” in the two photosystems. In other words, all the electron carriers would be fully reduced and no photosynthesis could take place. This situation doesn’t happen because an alternative pathway exists by which electrons can be donated to cytochrome bf instead of to NADP. The movement of electrons through this pathway pumps protons into the thylakoid lumen. These protons, along with the ones generated by the action of PSII, are used to make ATP.