Although structurally similar to photosystem II (PSII), photosystem I (PSI) is has a different electron supplier and electron acceptor.

Both these photosystems work in concert. An excited electron from PSII is relayed to PSI via an electron transport chain in the thylakoid membrane of the chloroplast, which is comprised of the carrier molecule plastoquinone, the dual-protein cytochrome complex, and plastocyanin. As electrons move between PSII and PSI, they lose energy and must be re-energized by PSI. As a result, another photon is absorbed by the pigments of the PSI antenna complex and relayed to the PSI reaction center called P700. P700 is oxidized and sends a high-energy electron to NADP⁺ to form NADPH. Just as PSII captures the energy to create proton gradients to make ATP, PSI captures the energy to reduce NADP⁺ into NADPH.

After the energy from the sun is converted into chemical energy in the form of ATP and NADPH molecules, the cell has the fuel needed to build carbohydrate molecules for long-term energy storage in the stroma of the chloroplast.