ground state molecule will be excited, and the process will continue between molecules all the way to the reaction center. At the reaction center, the electrons on the special chlorophyll molecule will be excited and ultimately transferred away by electron carriers. (If the electrons were not transferred away after excitation to a high energy state, they would lose energy by fluorescence back to the ground state, which would not allow plants to drive photosynthesis.) The reaction center will drive photosynthesis by taking light and turning it into chemical energy that can then be used by the chloroplast. Two families of reaction centers in photosystems can be distinguished: type I reaction centers (such as photosystem I (P700) in chloroplasts and in green-sulfur bacteria) and type II reaction centers (such as photosystem II (P680) in chloroplasts and in non-sulfur purple bacteria). The two photosystems originated from a common ancestor, but have since diversified. Each of the photosystem can be identified by the wavelength of light to which it is most reactive (700 nanometers for PSI and 680 nanometers for PSII in chloroplasts), the amount and type of light-harvesting complex present, and the type of terminal electron acceptor used. Type I photosystems use ferredoxin-like iron-sulfur cluster proteins as terminal electron acceptors, while type II photosystems ultimately shuttle electrons to a quinone terminal electron acceptor. Both reaction center types are present in chloroplasts and cyanobacteria, and work together to form a unique photosynthetic chain able to extract electrons from water, creating oxygen as a byproduct. Structure of PSI and PSII A reaction center comprises several (about 25-30) protein subunits, which provide a scaffold for a series of cofactors. The cofactors can be pigments (like chlorophyll, pheophytin, carotenoids), quinones, or iron-sulfur clusters. Each photosystem has two main subunits: an antenna complex (a light harvesting complex or LHC) and a reaction center. The antenna complex is where light is captured, while the reaction center is where this light energy is transformed into chemical energy. At the reaction center, there are many polypeptides that are surrounded by pigment proteins. At the center of the reaction center is a special pair of chlorophyll molecules. Each PSII has about 8 LHCII. These contain about 14 chlorophyll a and chlorophyll b molecules, as well as about four carotenoids. In the reaction center of PSII of plants and cyanobacteria, the light energy is used to split water into oxygen, protons, and