Apoprotein for the two 4Fe-4S centers FA and FB of photosystem I (PSI); essential for photochemical activity. FB is the terminal electron acceptor of PSI, donating electrons to ferredoxin. The C-terminus interacts with PsaA/B/D and helps assemble the protein into the PSI complex. Required for binding of PsaD and PsaE to PSI. PSI is a plastocyanin-ferredoxin oxidoreductase, converting photonic excitation into a charge separation, which transfers an electron from the donor P700 chlorophyll pair to the spectroscopically characterized acceptors A0, A1, FX, FA and FB in turn. hnu + oxidized [2Fe-2S]-[ferredoxin] + reduced [plastocyanin] = oxidized [plastocyanin] + reduced [2Fe-2S]-[ferredoxin] Binds 2 [4Fe-4S] clusters. Cluster 2 is most probably the spectroscopically characterized electron acceptor FA and cluster 1 is most probably FB. The eukaryotic PSI reaction center is composed of at least 11 subunits. Young tissue from this organism is photosynthetic and contains some thylakoids, although the photosynthetic activity does not exceed the light compensation point.