Excited electrons are captured by primary electron acceptor of the Photosystem II electron transport chain. This system also splits molecules of water into 1/2 O2, 2H+ and 2 electrons. This process is termed noncyclic because the electrons that pass through here do not return to the original photosystem. Photolysis, an ultraviolet light-mediated nonenzymatic reaction, generates H+ ions replacing those lost in the photosystems. This is explained through the chemiosmosis theory which states that as electrons are transported to the electron transport chain some energy is released which are used to pump protons across the thyalakoid membrane from the stroma of the chloroplast. This produces a proton gradient or what is called, proton motive force. This same force is used to generate ATP from ADP and Pi.
What are the roles of NADH and NADPH in cellular respiration and photosynthesis?
NADH, together with FADH2 is produced from three processes, namely: glycolysis, fatty acid oxidation and citric acid cycle. They serve as electron donors and cofactors for the protein complexes that participate in the electron transport chain.
During cellular respiration, NADH is the first electron donor in Complex I which results to 4 protons and transfer of 2 electrons and 2 protons. During oxygenic photosynthesis two stages occur: first, the light-dependent reactions convert light energy to chemical which produces ATP and NADPH; second, the light-independent reactions use the generated ATP and NADPH of the first stage to reduce CO2. It then converts the energy to chemical bond energy in carbohydrates.
A.L. Lehninger, et. al., Principles of Biochemistry, 2nd ed. Worth Publishers, NY, 1993.
R.K. Murray, et. al., Harpers Biochemistry, 24th ed. Appleton & Lange, 1993. http://student.ccbcmd.edu/~gkaiser/biotutorials/photosyn/fg4.html