P680, or photosystem II primary donor, is the
reaction-center chlorophyll ''a'' molecular dimer associated with
photosystem II in plants, algae, and cyanobacteria, and central to oxygenic
photosynthesis
Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
.
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 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 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 particles that can ...
of suitable frequency or indirectly from other chlorophylls within photosystem II, thereby exciting an electron 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 ...
. 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 common reducing agents include hydrogen, carbon ...
.
Charge separation
Following excitation, the loosened electron of P680* is taken up by the primary electron acceptor, a
pheophytin 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 known, with an estimated redox potential of ~1.3 V. This makes it possible to oxidize water during
oxygenic photosynthesis
Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metaboli ...
. P680
+ recovers its lost electron by oxidizing
water
Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
via the
oxygen-evolving complex, which regenerates P680.
See also
*
P700
*
Photosystem I
*
Photosystem II
References
Bibliography
*
*
Photosynthesis
Light reactions
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