Photosystem I (PS I) (or plastocyanin: ferredoxin oxidoreductase) is the second photosystem in the photosynthetic light reactions of algae, plants, and some bacteria. Photosystem I is named because it was discovered before photosystem II. Aspects of PS I were discovered in the 1950s, but the significances of these discoveries was not yet known. Louis Duysens first proposed the concepts of photosystems I and II in 1960, and, in the same year, a proposal by Fay Bendall and Robert Hill assembled earlier discoveries into a cohesive theory of serial photosynthetic reactions. Hill and Bendall’s hypothesis was later justified in experiments conducted in 1961 by Duysens and Witt groups.