Cyanobacteria like Synechocystis sp. PCC6803 can use plastocyanin and cytochrome c6 to transfer electrons within their cells. In the dark, both electron carriers can bind to photosystem I (PSI) and help quickly reduce P700, a key molecule in photosynthesis. When the cytochrome c oxidase is blocked, the electron transport chain gets over-reduced, causing a delay in P700 oxidation when light is turned on. This delay happens with both plastocyanin and cytochrome c6, showing that the balance between respiration and photosynthesis is controlled similarly regardless of the electron carrier used.