<P> A reaction center comprises several (> 10 or> 11) protein subunits, that provide a scaffold for a series of cofactors . The cofactors can be pigments (like chlorophyll, pheophytin, carotenoids), quinones, or iron - sulfur clusters . </P> <P> For oxygenic photosynthesis, both photosystems I and II are required . Oxygenic photosynthesis can be performed by plants and cyanobacteria; cyanobacteria are believed to be the progenitors of the photosystem - containing chloroplasts of eukaryotes . Photosynthetic bacteria that cannot produce oxygen have a single photosystem called BRC, bacterial reaction center . </P> <P> The photosystem I was named "I" since it was discovered before photosystem II, but this does not represent the order of the electron flow . </P> <P> When photosystem II absorbs light, electrons in the reaction - center chlorophyll are excited to a higher energy level and are trapped by the primary electron acceptors . To replenish the deficit of electrons thus created, electrons are extracted from water by a cluster of four Manganese ions in photosystem II and supplied to the chlorophyll via a redox - active tyrosine . </P>

Where do the electrons from photosystem ii end up