A photosystem that contains a pheophytin-quinone reaction center with associated accessory pigments and electron carriers. In cyanobacteria and chloroplasts, in the presence of light, PSII functions as a water-plastoquinone oxidoreductase, transferring electrons from water to plastoquinone, whereas other photosynthetic bacteria carry out anoxygenic photosynthesis and oxidize other compounds to re-reduce the photoreaction center. [ GOC:mah https://www.worldcat.org/search?q=bn%3A0943088399 GOC:ds http://www.ncbi.nlm.nih.gov/pubmed/9821949 ]