Photosystem II; it is also called water-plastoquinone oxido-reductase as it functions to oxidize water to molecular oxygen and reduce plastoquinone to plastoquinol. (See Govindjee and Coleman, 1990, Scientific American 262, 50-58 for a basic picture; for further details, see Debus, 1992, Biochimica et Biophysica Acta 1102, 269-352; and see a special issue of "Photosynthesis Research"(Dec.,1993):"How Plants and Cyanobacteria Make Oxygen: 25 Years of Period Four Oscillations", edited by G. Renger and Govindjee). Our diagram does not show other chlorophyll-containing light-harvesting complexes "feeding" excitonic energy to PSII reaction center. The structure of the major chlorophyll a- chlorophyll b containing light harvesting complex, known as LHCIIb is now available (see Kuehlbrandt et al.,1994, Nature, 367, 614-621).