<P> 2H O + 2NADP + 3ADP + 3P → O + 2NADPH + 3ATP </P> <P> The two photosystems are protein complexes that absorb photons and are able to use this energy to create an electron transport chain . Photosystem I and II are very similar in structure and function . They use special proteins, called light - harvesting complexes, to absorb the photons with very high effectiveness . If a special pigment molecule in a photosynthetic reaction center absorbs a photon, an electron in this pigment attains the excited state and then is transferred to another molecule in the reaction center . This reaction, called photoinduced charge separation, is the start of the electron flow and is unique because it transforms light energy into chemical forms . </P> <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>

Where does the light dependent reaction of photosynthesis occur what is produced