[1][2][3][4] Its name is derived from the word “pigment” (P) and the presence of a major bleaching band centered around 695-700 nm in the flash-induced absorbance difference spectra of P700/ P700+•.
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, like Fe-S complex, Ferridoxyn(FD), which have a higher redox potential i.e. greater affinity to electron.
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 -1.2V .
[10] Consequently, in most cases, the electrons transferring within photosystem I follow a linear pathway, from the excitation of the P700 special pair to the production of NADPH.
[11] Utilizing the energy of P700+, the cyclic pathway creates a proton gradient useful for the production of ATP, while no NADPH is produced, since the protein ferredoxin does not become reduced.