P700, or photosystem I primary donor, is the reaction-center chlorophyll a molecular dimer associated with photosystem I in plants, algae, and cyanobacteria. Its name is derived from the word “pigment”, and the maximal wavelength of light it can absorb, 700 nm, at which the phenomenon of photobleaching would occur. Its absorption spectrumpeaks at 700 nm. The structure of P700 consists of a heterodimer with two distinct chlorophyll molecules, most notably, chlorophyll a and chlorophyll a’, giving it an additional name of “special pair”. Inevitably, however, the special pair of P700 behaves as if it were just one unit. This species is vital due to its ability to absorb light energy with a wavelength approximately between 430 nm-700 nm, and transfer high-energy electrons to a series of acceptors that are situated near it. Photosystem I operates with the intention of producing NADPH, the reduced form of NADP+, at the end of the photosynthetic reaction through electron transfer. When photosystem I absorbs light, an electron is excited to a higher energy level in the P700 chlorophyll. The resulting P700 with an excited electron is designated as P700*, which is a strong reducing agent due to its very negative redox potential of -1.2 V. Following the excitation of P700, one of its electrons is passed on to an electron acceptor, Ao, triggering charge separation producing an anionic Ao- and cationic P700+. Subsequently, electron transfer continues from Ao to a phylloquinone molecule known as A1, and then to three iron-sulfur clusters. Type I photosystems use iron-sulfur cluster proteins as terminal electron acceptors. Thus, the electron is transferred from Fx to another iron sulfur cluster, FA, and then passed on to the last iron-sulfur cluster serving as an electron acceptor, FB. Eventually, the electron is transferred to the protein ferredoxin, causing it to transform into its reduced form. The electron carrier, ferredoxin, finalizes the process by reducing NADP+ to NADPH, completing the initial purpose of photosystem I. The rate of electrons being passed from P700* to the subsequent electron acceptors is high, preventing the electron from being transferred back to the cationic form of the special pair P700. P700+ recovers its lost electron by oxidizing plastocyanin, which regenerates P700. In most cases, the electrons transferring within photosystem I follow a linear pathway, consisting of the excitation of the P700 special pair to the production of NADPH. However, in certain situations, it is vital for the photosynthetic organism to recycle the electrons being transferred, resulting in the electron from the terminal iron-sulfur cluster FB transferring back to the cytochrome b6f complex. The cyclic pathway creates a proton gradient useful for the production of ATP. However, it is important to note that no NADPH is produced through the cyclic electron-transferring pathway in photosystem I, since the protein ferredoxin does not become reduced.
