Photosystem I. Molecular model of the photosystem I complex from the cyanobacterium Synechococcus elongatus. Shown here are beta-carotene, alpha-chlorophyll and reaction centre subunits. Photosystems are protein complexes involved in photosynthesis. They are enzymes that absorb light and use it to reduce (add electrons to) other molecules. Photosystems I and II work together to form a photosynthetic chain that removes electrons from water, creating oxygen as a byproduct.