FIGURE 12-45 Cyclic electron flow in plants, which generates a proton-motive force and ATP but no oxygen or NADPH. In cyclic electron flow, light energy is used by PSI to transport electrons in a cycle to generate a proton-motive force and ATP without oxidizing water or generating NADPH. High-energy electrons are transferred via the ferredoxin of PSI either to a PGR5-PGRL1 heterodimer (red, left pathway) or to the NADH dehydrogenase–like complex (blue, right pathway), where they then reduce plastoquinone (Q) to QH₂. Each of these two electron acceptors forms independent supercomplexes with PSI via light harvesting complex (LHC) subunits (yellow). (The PGR5-PGRL1 heterodimer and NADH dehydrogenase–like complex are not found together in the same supercomplex, but here are drawn together with only one PSI to emphasize the similarities of the two mechanisms of cyclic electron flow.) QH₂ then transfers the electrons to the cytochrome bf complex, then to plastocyanin, and finally back to PSI, as is the case for the linear electron flow pathway (see Figure 12-44).