A membrane-bound complex in plants and photosynthetic bacteria, responsible for light-harvesting and primary electron transfer. Comprises light-harvesting pigments such as chlorophyll; a primary electron-transfer centre, and secondary electron carriers. In green plant photosynthesis, Photosystem I transfers electrons from plastocyanin to a [2Fe-2S] ferredoxin, and contains iron-sulfur proteins. Photosystem II transfers electrons from the oxygen-evolving complex to plastoquinone, and contains an iron centre.