P680
Encyclopedia
P680, or Photosystem II primary donor, (where P stands for pigment) refers to any of the 2 special chlorophyll
dimers (also named special pairs), PD1 or PD2. These 2 special pairs form an excitonic dimer, which means that they behave in function as a single entity; i.e., they are excited
as if they were a single molecule. The 680 number is its absorption maximum in the red part of the visible spectrum
(680 nm). The primary donor receives excitation energy either by absorbing a photon
of suitable frequency (colour) or by excitation energy transfer from other chlorophylls within photosystem II. During excitation, an electron is excited to a higher energy level
. This electron is subsequently captured by the primary electron acceptor, a pheophytin
molecule located within photosystem II near P680. The oxidized P680 (P680+) is subsequently reduced by an electron originating from water (via Oxygen evolving complex
).
Oxidized P680 (P680+) is the strongest biological oxidizing agent known. It has an estimated redox potential of ~1.3 V. This makes it possible to oxidize water
during oxygenic photosynthesis
.
Chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
dimers (also named special pairs), PD1 or PD2. These 2 special pairs form an excitonic dimer, which means that they behave in function as a single entity; i.e., they are excited
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
as if they were a single molecule. The 680 number is its absorption maximum in the red part of the visible spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
(680 nm). The primary donor receives excitation energy either by absorbing a photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
of suitable frequency (colour) or by excitation energy transfer from other chlorophylls within photosystem II. During excitation, an electron is excited to a higher energy level
Energy level
A quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...
. This electron is subsequently captured by the primary electron acceptor, a pheophytin
Pheophytin
Pheophytin or phaeophytin is a chemical compound that serves as the first electron carrier intermediate in the electron transfer pathway of photosystem II in plants, and the photosynthetic reaction center found in purple bacteria...
molecule located within photosystem II near P680. The oxidized P680 (P680+) is subsequently reduced by an electron originating from water (via Oxygen evolving complex
Oxygen evolving complex
The oxygen-evolving complex, also known as the water-splitting complex, is a water-oxidizing enzyme involved in the photooxidation of water during the light reactions of photosynthesis. Based on a widely accepted theory from 1970 by Kok, the complex can exist in 5 states: S0 to S4. Photons trapped...
).
Oxidized P680 (P680+) is the strongest biological oxidizing agent known. It has an estimated redox potential of ~1.3 V. This makes it possible to oxidize water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
during oxygenic photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
.