INVOLVEMENT OF PHOTOSYNTHETIC LIGHT REACTIONS
The energy used by FₒF₁ to drive the synthesis of ATP from ADP and Pi is derived from the capture of light by light-harvesting and reaction center protein complexes in thylakoid membranes (1). Once absorbed, light energy is transferred to Photosystem I (PSI) or Photosystem II (PSII) where it induces oxidation of the reaction center chlorophyll and reduction of an electron acceptor that is far more electronegative than the ground state chlorophyll. Since molecules with a more electronegative redox potential store more energy upon reduction, the reduction of the initial electron acceptors in PSI and PSII can be considered to be the step in which light energy is first captured as chemical energy during photosynthesis. Due to the instability of this initial product, a series of sequential redox reactions occurs to prevent the recombination of charges in the reaction center chlorophyll and the initial electron acceptor. These reactions result in the reduction of a molecule that is less electronegative than the initial acceptor, but still substantially more electronegative than the reaction center chlorophyll. The oxidized PSII reaction center abstracts electrons from water to release molecular oxygen and protons into the thylakoid lumen.
Reduction of plastoquinone by PSII consumes two protons from the stroma. The reduced, lipid soluble quinol is then oxidized by the cytochrome b₆/f protein complex in the thylakoid which results in the deposition of the protons in the thylakoid lumen. The reducing equivalents then transfer to oxidized PSI reaction center chlorophyll. The reducing equivalents generated by PSI reduce the iron-sulfur cluster of ferredoxin, a water-soluble protein in the stroma. Ferredoxin in turn reduces NADP⁺ via the enzyme ferredoxin NADP⁺ reductase (FNR), a peripheral membrane protein on the stromal side of the thylakoid that is associated with an intrinsic protein in the thylakoid. The net result of the photosynthetic electron transfer reactions is that electrons from water with a redox potential of +0.81V are used to reduce NADP⁺ that has a redox potential of –0.32V that accompanies vectorial movement of protons from the stroma to the thylakoid lumen (1).