Photosystem I

P700 is the core reaction center of photosystem I, similar in function to P680 in photosystem II. When P700 absorbs light energy, it becomes excited and transfers a high-energy electron to ferredoxin, the unique electron acceptor of PSI. This electron transfer is essential for reducing NADP+ into NADPH, a critical energy molecule for the Calvin Cycle.

Electrons move from PSII to PSI through an electron transport chain on the thylakoid membrane. The pathway includes plastoquinone, a cytochrome complex, and plastocyanin. As electrons transfer between these carriers, they lose energy and must be re-energized by light absorption at PSI, allowing continuous electron flow through both photosystems.

Ferredoxin is the unique electron acceptor in photosystem I that receives excited electrons from the reduced P700 reaction center. Once ferredoxin accepts these high-energy electrons, it transfers them to ferredoxin-NADP+ reductase, which catalyzes the formation of NADPH, the high-energy product essential for photosynthetic carbon fixation.

As electrons move through the electron transport chain between PSII and PSI, energy is released and used to pump protons into the thylakoid space via the cytochrome complex. This creates a proton gradient that ATP synthase uses to phosphorylate ADP into ATP through chemiosmosis, providing energy for carbohydrate synthesis.

Photosystem I captures light energy to reduce NADP+ into NADPH, a high-energy electron carrier. The excited P700 transfers electrons through ferredoxin to ferredoxin-NADP+ reductase, which catalyzes NADPH formation. This molecule provides reducing power for the Calvin Cycle and other biosynthetic pathways in the chloroplast stroma.

Plastocyanin is a mobile electron carrier that transfers electrons from the cytochrome complex to the oxidized P700 reaction center of PSI. Once plastocyanin is reduced by the electron transport chain, it delivers electrons to PSI, enabling the sequential activation of both photosystems and maintaining continuous electron flow through the thylakoid membrane.

Although electrons arrive at PSI from PSII, they lose energy during transport through the electron transport chain and must be re-energized to reach the high energy level required for NADP+ reduction. Light absorption by PSI's antenna complex re-excites these electrons, allowing P700 to transfer them to ferredoxin with sufficient energy to ultimately form NADPH.