P700, or photosystem I primary donor, is the reaction-center chlorophyll ''a'' molecular dimer associated with photosystem I 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 hetero dimer 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 In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...

approximately between 430 nm-700 nm, and transfer high-energy electrons to a series of acceptors that are situated near it.

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 NADP ...

, the reduced form of NADP, at the end of the photosynthetic reaction through

electron transfer Electron transfer (ET) occurs when an electron relocates from an atom or molecule to another such chemical entity. ET is a mechanistic description of certain kinds of redox reactions involving transfer of electrons. Electrochemical processes ar ...

, and of providing energy to a proton pump and eventually

ATP ATP may refer to: Companies and organizations * Association of Tennis Professionals, men's professional tennis governing body * American Technical Publishers, employee-owned publishing company * ', a Danish pension * Armenia Tree Project, non ...

, for instance in cyclic electron transport.

Excitation

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

energy level A quantum mechanical system or particle that is bound—that is, confined spatially—can only take on certain discrete values of energy, called energy levels. This contrasts with classical particles, which can have any amount of energy. The ...

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 commonly reducing agents include the Earth m ...

due to its very negative

redox potential Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respe ...

of -1.2 V.

Electron transport chain

Following the

excitation Excitation, excite, exciting, or excitement may refer to: * Excitation (magnetic), provided with an electrical generator or alternator * Excite Ballpark, located in San Jose, California * Excite (web portal), web portal owned by IAC * Electron ...

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. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process. Electron acceptors are sometimes mist ...

, 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 conve ...

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 conve ...

P700. Subsequently, electron transfer continues from A to a

phylloquinone Phytomenadione, also known as vitamin K1 or phylloquinone, is a vitamin found in food and used as a dietary supplement. It is on the World Health Organization's List of Essential Medicines. As a supplement it is used to treat certain bleeding ...

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 ...

, 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 An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and t ...

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