pigment molecules varies from organism to organism. For instance, the cyanobacterium Synechococcus elongatus (Thermosynechococcus elongatus) has about 100 chlorophylls and 20 carotenoids, whereas spinach chloroplasts have around 200 chlorophylls and 50 carotenoids. Located within the antenna complex of PSI are molecules of chlorophyll called P700 reaction centers. The energy passed around by antenna molecules is directed to the reaction center. There may be as many as 120 or as few as 25 chlorophyll molecules per P700. P700 reaction center The P700 reaction center is composed of modified chlorophyll a that best absorbs light at a wavelength of 700 nm. P700 receives energy from antenna molecules and uses the energy from each photon to raise an electron to a higher energy level (P700*). These electrons are moved in pairs in an oxidation/reduction process from P700* to electron acceptors, leaving behind P700. The pair of P700* - P700 has an electric potential of about −1.2 volts. The reaction center is made of two chlorophyll molecules and is therefore referred to as a dimer. The dimer is thought to be composed of one chlorophyll a molecule and one chlorophyll a′ molecule. However, if P700 forms a complex with other antenna molecules, it can no longer be a dimer. Modified chlorophyll A and A The two modified chlorophyll molecules are early electron acceptors in PSI. They are present one per PsaA/PsaB side, forming two branches electrons can take to reach F. A accepts electrons from P700*, passes it to A of the same side, which then passes the electron to the quinone on the same side. Different species seems to have different preferences for either A/B branch. Phylloquinone A phylloquinone, sometimes called vitamin K, is the next early electron acceptor in PSI. It oxidizes A in order to receive the electron and in turn is re-oxidized by F, from which the electron is passed to F and F. The reduction of Fx appears to be the rate-limiting step. Iron–sulfur complex Three proteinaceous iron–sulfur reaction centers are found in PSI. Labeled F, F, and F, they serve as electron relays. F and F are bound to protein subunits of the PSI complex and F is tied to the PSI complex. Various experiments have shown some disparity between theories of iron–sulfur cofactor orientation and operation order. In one model, F passes an electron to F, which passes it on to F to reach the ferredoxin. Ferredoxin