text: what specific mechanism regulates cyclic vs noncyclic electron transfer during light reactions of photosynthes i suspect that is reglated by a feedback mechanism based on relative concentrations of nadph and atp during the calvin cycle, but i can not find specific enzymes or mechanisms that regulate the processes. in plants, the light-dependent reactions occur in the thylakoid membranes of the chloroplasts and use light energy to synthesize atp and nadph. the photons are captured in the antenna complexes of photosystem i and ii by chlorophyll and accessory pigments (see diagram at right). when a chorophyll a molecule at a photosystem's reaction center absorbs energy, an electron is excited and transferred to an electron-acceptor molecule through a process called photoinduced charge separation. these electrons are shuttled through an electron transport chain that initially functions to generate a chemiosmotic potential across the membrane, the so called z-scheme shown in the diagram. an atp synthase enzyme uses the chemiosmotic potential to make atp during photophosphorylation while nadph is a product of the terminal redox reaction in the z-scheme.  [edit] water photolysis the nadph is the main reducing agent in chloroplasts, providing a source of energetic electrons to other reactions. its production leaves chlorophyll with a deficit of electrons (oxidized), which must be obtained from some other reducing agent. the excited electrons lost from chlorophyll in photosystem i are replaced from the electron transport chain by plastocyanin. however, since photosystem ii includes the first steps of the z-scheme, an external source of electrons is required to reduce its oxidized chlorophyll a molecules. this role is played by water during a reaction known as photolysis and results in water being split to give electrons, oxygen and hydrogen ions. photosystem ii is the only known biological enzyme that carries out this oxidation of water. initially, the hydrogen ions from photolysis contribute to the chemiosmotic potential but eventually they combine with the hydrogen carrier molecule nadp+ to form nadph. oxygen is a waste product of light-independent reaction, but oxygen is used in cellular respiration for a majority of organism on earth