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Photosynthesis is a recurrent process that produces NADPH by reducing the power supplied by the oxidation of ${\mathrm{H}}_2\mathrm{O}$. Two photosynthetic reaction centers are involved in this process.

• Photosystem I (PS I) 鈥� It reduces${\mathrm{NADP}}^{+}$
• Photosystem II (PS II) 鈥� It oxidizes ${\mathrm{H}}_2\mathrm{O}$

The cyclic electron flow in photosynthesis utilizes PS I and it will not increase the production of NADPH. However, ATP production is increased, thus leading to an increase in ${\mathrm{O}}_2$. At least 8 photons are required to evolve one molecule of ${\mathrm{O}}_2$or CO₂ reduced. During, the cyclic electron flow, PS I move the electrons more freely. This photoactivation of PSI is the driving factor of ETC and it is not depending on the electron flow that is derived from water. Hence, oxidation of water by PS II is not related to the number of photons utilized by PS I.