One photon excites an electron from photosystem II, which is passed down an electron transfer chain to photosystem I. A second photon excites an electron from photosystem I, which is then used in the reduction of NADP+ to NADPH.
When light is absorbed by pigments in photosystem II, it raises the energy level of an electron within a chlorophyll molecule
When light is absorbed by pigments in photosystem II, it raises the energy level of an electron within a chlorophyll molecule. This energized electron is then passed along a series of proteins in an electron transfer chain, which leads to photosystem I.
Upon reaching photosystem I, another photon of light excites an electron within this system. This energized electron is then captured by an electron carrier called ferredoxin, which shuttles the electron to a different enzyme called NADP+ reductase.
NADP+ reductase is responsible for the reduction of NADP+, which is an electron carrier molecule, to NADPH. In this process, the energized electron from ferredoxin is passed onto NADP+ along with a hydrogen ion (H+), resulting in the formation of NADPH. The NADPH molecule is an essential component for energy-requiring reactions in the Calvin cycle, the process of carbon fixation in plants.
In summary, the excitation of an electron by a photon in photosystem II initiates the movement of electrons in an electron transfer chain. The second excitation by a photon in photosystem I leads to the reduction of NADP+ to NADPH, a vital electron carrier in photosynthesis.
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