P680, or photosystem II primary donor, is the reaction-center chlorophyll ''a'' molecular dimer associated with

photosystem II Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Within the photosystem ...

in plants, algae, and cyanobacteria, and central to oxygenic

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

Etymology

Its name is derived from the word “pigment” (P) and the presence of a major bleaching band centered around 680-685 nm in the flash-induced absorbance difference spectra of P680/ P680+•.Shigeru Itoh, S; Iwaki, M; Tomo, T; Satoh, K (1996). Dibromothymoquinone (DBMIB) replaces the function of QA at 77 K in the isolated photosystem II reaction center (Dl-D2-cytochrome 6559) complex: Difference spectrum of the P680+ (DBMIB") state. Plant Cell Physiol. 37(6): 833-839.

Components

The structure of P680 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 ...

of two distinct chlorophyll molecules, referred to as P and P. This “special pair” forms an excitonic dimer that functions as a single unit, excited by

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 ...

as if they were a single molecule.

Action and function

Excitation

P680 receives excitation energy either by directly absorbing a

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...

of suitable frequency or indirectly from other chlorophylls within photosystem II, thereby exciting an electron 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 ...

. The resulting P680 with a loosened electron is designated as P680*, 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 ...

Charge separation

Following excitation, the loosened electron of P680* is taken up by the primary electron acceptor, a

pheophytin Pheophytin or phaeophytin is a chemical compound that serves as the first electron carrier intermediate in the electron transfer pathway of Photosystem II (PS II) in plants, and the type II photosynthetic reaction center (RC P870) found in pur ...

molecule located within photosystem II near P680. During this transfer, P680* is ionized and oxidized, producing

cation 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 ...

ic P680.

Recovery of P680

P680⁺ is the strongest biological

oxidizing agent An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or "Electron acceptor, accepts"/"receives" an electron from a (called the , , or ). In ot ...

known, with an estimated redox potential of ~1.3 V. This makes it possible to oxidize water during

oxygenic photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...

. P680⁺ recovers its lost electron by oxidizing

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...

via the

oxygen-evolving complex The oxygen-evolving complex (OEC), also known as the water-splitting complex, is the portion of photosystem II where photo-oxidation of water occurs during the light reactions of photosynthesis. The OEC is surrounded by four core protein subuni ...

, which regenerates P680.

References

Bibliography

* * Photosynthesis Light reactions {{biochem-stub