<P> The reaction center is in the thylakoid membrane . It transfers light energy to a dimer of chlorophyll pigment molecules near the periplasmic (or thylakoid lumen) side of the membrane . This dimer is called a special pair because of its fundamental role in photosynthesis . This special pair is slightly different in PSI and PSII reaction center . In PSII, it absorbs photons with a wavelength of 680 nm, and it is therefore called P680 . In PSI, it absorbs photons at 700 nm, and it is called P700 . In bacteria, the special pair is called P760, P840, P870, or P960 . Where "P" means pigment, and the number following it is the wavelength of light absorbed . </P> <P> If an electron of the special pair in the reaction center becomes excited, it cannot transfer this energy to another pigment using resonance energy transfer . In normal circumstances, the electron should return to the ground state, but, because the reaction center is arranged so that a suitable electron acceptor is nearby, the excited electron can move from the initial molecule to the acceptor . This process results in the formation of a positive charge on the special pair (due to the loss of an electron) and a negative charge on the acceptor and is, hence, referred to as photoinduced charge separation . In other words, electrons in pigment molecules can exist at specific energy levels . Under normal circumstances, they exist at the lowest possible energy level they can . However, if there is enough energy to move them into the next energy level, they can absorb that energy and occupy that higher energy level . The light they absorb contains the necessary amount of energy needed to push them into the next level . Any light that does not have enough or has too much energy cannot be absorbed and is reflected . The electron in the higher energy level, however, does not want to be there; the electron is unstable and must return to its normal lower energy level . To do this, it must release the energy that has put it into the higher energy state to begin with . This can happen various ways . The extra energy can be converted into molecular motion and lost as heat . Some of the extra energy can be lost as heat energy, while the rest is lost as light . (This re-emission of light energy is called fluorescence .) The energy, but not the e - itself, can be passed onto another molecule . (This is called resonance .) The energy and the e - can be transferred to another molecule . Plant pigments usually utilize the last two of these reactions to convert the sun's energy into their own . </P> <P> This initial charge separation occurs in less than 10 picoseconds (10 seconds). In their high - energy states, the special pigment and the acceptor could undergo charge recombination; that is, the electron on the acceptor could move back to neutralize the positive charge on the special pair . Its return to the special pair would waste a valuable high - energy electron and simply convert the absorbed light energy into heat . In the case of PSII, this backflow of electrons can produce reactive oxygen species leading to photoinhibition . Three factors in the structure of the reaction center work together to suppress charge recombination nearly completely . </P> <Ul> <Li> Another electron acceptor is less than 10 Å away from the first acceptor, and so the electron is rapidly transferred farther away from the special pair . </Li> <Li> An electron donor is less than 10 Å away from the special pair, and so the positive charge is neutralized by the transfer of another electron </Li> <Li> The electron transfer back from the electron acceptor to the positively charged special pair is especially slow . The rate of an of electron transfer reaction increases with its thermodynamic favorability up to a point and then decreases . The back transfer is so favourable that it takes place in the inverted region where electron - transfer rates become slower . </Li> </Ul>

Where do the different reactions of photosynthesis take place