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

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

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 Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius 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 Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 terahe ...

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, tro ...

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, 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 A proton pump is an integral membrane protein pump that builds up a proton gradient across a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membrane that separates the interior of a cell f ...

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 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 te ...

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 meta ...

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 exc ...

of P700, one of its electrons is passed on to an electron acceptor, 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 conven ...

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 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 Iron–sulfur proteins (or iron–sulphur proteins in British spelling) are proteins characterized by the presence of iron–sulfur clusters containing sulfide-linked di-, tri-, and tetrairon centers in variable oxidation states. Iron–sulfur cl ...

. 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, 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 The cytochrome ''b''6''f'' complex (plastoquinol—plastocyanin reductase; ) is an enzyme found in the thylakoid membrane in chloroplasts of plants, cyanobacteria, and green algae, that catalyzes the transfer of electrons from plastoquinol to ...

(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 th ...

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 Plastocyanin is a copper-containing protein that mediates electron-transfer. It is found in a variety of plants, where it participates in photosynthesis. The protein is a prototype of the blue copper proteins, a family of intensely blue-colored m ...

, which regenerates P700.

References

{{Reflist, 2 Photosynthesis Light reactions