The recently determined structure of plant photosystem I (PSI) provides the first relatively high-resolution structural model of a supercomplex containing a reaction center and its peripheral antenna. Large amounts of highly purified PSI were required to get enough crystals amenable for structural determination by X-ray crystallography. In addition, a deep biochemical understanding of the large supercomplex was vital for achieving the goal. The stability of PSI was analyzed by sucrose gradient centrifugation and gel electrophoresis. Small amounts of LHCI were detached from PSI following a 12 day incubation under crystallization conditions. The interaction between the reaction center and the peripheral antenna of PSI (LHCI) as well as the interactions among the LHCI monomers are flexible. Nevertheless, the pure and homogeneous preparation of PSI allows for relatively tight crystal packing, which holds promise for obtaining atomic resolution in the future.