This molecule, whatever it is, passes electrons to plastoquinone, which is closely related to the ubiquinone of respiration. From there the electrons go to b- and c-type cytochromes, to a copper protein (plastocyanin), and finally to Photocenter I. (The cytochrome f of photosynthesis actually is a c-type protein. It was labeled f for the Latin "frons," or leaf.) As before, ATP is generated during the passage of electrons down the chain, although how much ATP is not known with certainty. Like the sulfur bacteria, green plants also can carry out part of their photosynthetic process cyclically, passing electrons from the flavoprotein near the end of the chain, back to the middle of the electron-transport chain, and making ATP but not NADPH.

Bacteria that photosynthesize do not also respire (with the exception of the purple nonsulfur bacteria), so there is no confusion as to the use of NADH. Green plants carry out both photosynthesis and respiration, and there might be the possibility that the reduced dinucleotides produced by photosynthesis would be used immediately as fuel for the respiratory chain, even though these reactions are carried out in two different organelles within the cell-chloroplasts and mitochondria. It may be that this is why green-plant photosynthesis has come to operate with a dinucleotide labeled with an extra phosphate group, NADP⁺ (nicotine adenine dinucleotide phosphate), instead of NAD⁺.