As the change in free energy is equal to the maximum useful work which can be accomplished by the reaction, then a negative ΔG associated with a reaction indicates that it can happen spontaneously. About three protons flow through the F0F1-ATPase complex per ATP synthesized, which corresponds to a free energy input of 14.4 kcal/mol of ATP, but in which only 7.3 kcal are stored in the ATP molecule, a yield of about 50%. No ATP is synthesized if the pH gradient is less than two units because the gradient force is then too small. The newly synthesized ATP is released into the stromal space. Likewise, NADPH formed by PSI is released into the stromal space. Thus ATP and NADPH, the products of light reactions of photosynthesis, are appropriately positioned for the subsequent light-independent reactions, in which CO2 is converted into carbohydrates. The overall reaction can be expressed as:
This equation implies that each H2O is split in the thylakoids under the influence of the light to give off 1/2O2 molecule, and that the two electrons so freed are then transferred to two molecules of NADP+, along with H+s, to produce the strong reducing agent NADPH. Two molecules of ATP can be simultaneously formed from two ADP and two inorganic phosphates (Pi) so that the energy is stored in high energy compounds. NADPH and ATP are the assimilatory power required to reduce CO2 to carbohydrates in the light-independent phase. The generation of ATP following this route is termed non-cyclic phosphorylation because electrons are just transported from water to NADP+ and do not come back.
An alternative pathway for ATP production is cyclic phosphorylation, in which electrons from PSI cycle in a closed system through the phosphorylation sites and ATP is the only product formed. Electron arising from P700 are transferred to ferredoxin and then to the cytochrome b6f complex. Protons are pumped by this complex as electrons return to the oxidized form of reaction center P700 through plastocyanine. This cyclic phosphorylation takes place when NADP is unavailable to accept electrons from reduced ferredoxin because of a very high ratio of NADPH to NADP+. The electrochemical potential of the proton gradient drives the synthesis of ATP through an ATP-synthase situated, as we have seen, anisotropically in the thylakoid membrane.
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