Final answer:
The minimum number of photons needed to transfer electrons from water and make one molecule of O₂ and two molecules of NADPH during photosynthesis is 4.
Step-by-step explanation:
The minimum number of photons needed to transfer electrons from water and make one molecule of O₂ and two molecules of NADPH during photosynthesis can be determined by understanding the process of photosynthesis. In the light reactions of photosynthesis, light energy is absorbed by chlorophyll molecules, resulting in the excitation of electrons.
The excited electrons are then transferred from PSII to PSI through an electron transport chain. During this process, water molecules are split, and two electrons are released for every water molecule split. Therefore, for the formation of one molecule of O₂ and two molecules of NADPH, a minimum of 4 photons are needed, as two water molecules need to be split to release four electrons.