Unfortunately it's not one molecule -- it's this insane Heath-Robinson contraption with a total atomic weight measured in megaDaltons and about thirty different enzymes and peptides playing pass-the-parcel with electrons. There are two different metabolic pathways for photosynthesis in chlorophyl-based plants, the common C3 pathway and the more recently evolved, more efficient C4 pathway (used by Maize, among other plants), but the difference in relative efficiency is bollixed up by the grotesque inefficiency of the first step.