This chapter describes the preparation of NaBr-treated chloroplasts, purification of photosystem I reaction center, preparation of antibodies to individual subunits, and methods for the study of the biogenesis of the Adenosine-5'-triphosphate (ATP) synthase and photosystem I reaction center. Multisubunit protein complexes are vital for the function of biological membranes. The chloroplast inner membrane contains four protein complexes that together with single subunit proteins catalyze light-dependent oxygen evolution, nicotinamide adenine dinucleotide phosphate (NADP) reduction, and adenosine diphosphate ADP photophosphorylation. The four protein complexes are photosystem I reaction center, photosystem II reaction center, cytochrome b6-f complex, and ATP synthase. A protein complex is the minimal structure that catalyzes a characterized biochemical reaction. Photosystem I reaction center is the minimal structure that catalyzes plastocyanin photooxidation and ferredoxin photoreduction. The purified photosystem I reaction center is active in NADP photoreduction.