Final answer:
At least eight photons of light are required to produce one molecule of O2 and two molecules of NADPH during the light-dependent reactions of photosynthesis, with four photons used in photosystem II for oxygen production, and an additional four as a conservative estimate for the two NADPH produced by photosystem I.
Step-by-step explanation:
The question is asking how many photons of light are needed to produce one molecule of O2 and two molecules of NADPH. This process is related to the light-dependent reactions of photosynthesis, where light energy is captured by photosystems and converted into chemical energy.
In photosynthesis, water is split (photolysis) to provide electrons, which result in oxygen as the byproduct and protons that will be used in forming NADPH. This occurs in photosystem II when it absorbs a photon. Then, in photosystem I, more photons are absorbed to provide the energy for the reduction of NADP+ to NADPH. The complete process to fix one CO2 requires two NADPH and three ATP. Given that for every six CO2 fixed, twelve NADPH are needed, it means that each CO2 fixation requires two NADPH.
To generate one O2 molecule, four electrons must be moved through the photosystems, which in turn means four photons are used in photosystem II. As a conservative estimate, considering that NADPH generation also requires photons absorbed by Photosystem I, you'd need at least eight photons in total: four photons for the O2 and about four photons (as a minimal estimate) for the two NADPH (since each NADPH could require at least one photon for their production in the photosystem I).