Step 1: In linear or non-cyclic electron flow, light energy drives electrons from water through PSII, pheophytin, plastoquinone, the cytochrome b6f complex, plastocyanin, PSI and finally to ferredoxin and NADP reductase to form NADPH.
In cyclic electron flow, only PSI operates. Excited electrons from P700 go to ferredoxin, then loop back through the cytochrome b6f complex and plastoquinone before returning to P700, generating ATP without making NADPH.
P680 and pheophytin function only within Photosystem II, which is absent from the cyclic pathway, so they cannot be the shared component. Plastoquinone ferries electrons to the cytochrome b6f complex in both the cyclic loop around PSI and the linear chain from PSII to PSI, making it the common link. Answer: Plastoquinone.