P700, or photosystem I primary donor, is a molecular dimer of chlorophyll ''a'' associated with the reaction-center of

photosystem I Photosystem I (PSI, or plastocyanin–ferredoxin oxidoreductase) is one of two photosystems in the Light-dependent reactions, photosynthetic light reactions of algae, plants, and cyanobacteria. Photosystem I is an integral membrane ...

in plants, algae, and cyanobacteria.

Etymology

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+•.

Components

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 In physics, and in particular as measured by radiometry, radiant energy is the energy of electromagnetic and gravitational radiation. As energy, its SI unit is the joule (J). The quantity of radiant energy may be calculated by integrating radian ...

with a

wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

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''.

Action and functions

Photosystem I operates with the functions of producing

NADPH Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle and lipid and nucleic acid syntheses, which require N ...

, the reduced form of NADP(''Fd^2-_red + NADH + 2 NADP⁺ + H⁺ = Fd_ox + NAD⁺ + 2 NADPH

', at the end of the photosynthetic reaction through

electron transfer Electron transfer (ET) occurs when an electron relocates from an atom, ion, or molecule, to another such chemical entity. ET describes the mechanism by which electrons are transferred in redox reactions. Electrochemical processes are ET reactio ...

, and of providing energy to
proton pump
and eventually ATP, for instance in cyclic electron transport.

Excitation

When photosystem I absorbs light, an electron is excited to a higher

energy level A quantum mechanics, quantum mechanical system or particle that is bound state, bound—that is, confined spatially—can only take on certain discrete values of energy, called energy levels. This contrasts with classical mechanics, classical pa ...

in the P700 chlorophyll. The resulting P700 with an excited electron is designated as P700*, which is a strong

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are common reducing agents include hydrogen, carbon ...

due to its very negative redox potential of -1.2V .

Electron transport chain

Following the excitation of P700, one of its electrons is passed on to an

electron acceptor An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. Electron acceptors are oxidizing agents. The electron accepting power of an electron acceptor is measured by its redox potential. In the ...

, A, triggering charge separation producing an

anionic An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

A and

cationic An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

P700. Subsequently, electron transfer continues from A to a phylloquinone molecule known as A, 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 F to another iron sulfur cluster, F, and then passed on to the last iron-sulfur cluster serving as an electron acceptor, F. Eventually, the electron is transferred to the protein

ferredoxin Ferredoxins (from Latin ''ferrum'': iron + redox, often abbreviated "fd") are iron–sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co. and applied t ...

, causing it to transform into its reduced form, which subsequently finalizes the process by reducing NADP to NADPH.

Linear electron transport

The rate of electrons being passed from P700* to the subsequent electron acceptors is high, preventing the electron from being transferred back to P700. 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.

Cyclic electron transport

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 F transferring back to the cytochrome b6f complex (adaptor between photosystems II and I). 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.

Recovery of P700

P700 recovers its lost electron by oxidizing plastocyanin, which regenerates P700.

References

{{Reflist, 2 Photosynthesis Light reactions