Photodissociation, photolysis, or photodecomposition is a
chemical reactionin which a chemical compoundis broken down by photons. Photodissociation is not limited to visible light, but to have enough energyto break up a molecule, the photon is likely to be an electromagnetic wavewith the energy of visible light or higher, such as ultraviolet light, x-rays and gamma rays. The direct process is defined as the interaction of one or more photons interacting with one target molecule.
Photolysis in photosynthesis
Photolysis is a part of the
light-dependent reactions of photosynthesis. The general reaction of photosynthetic photolysis can be given as:
H2A + 2 photons (light) 2e- + 2H+ + A
The chemical nature of "A" depends on the type of organism. For example in
purple sulfur bacteria, hydrogen sulfide(H2S) is oxidized to sulfur (S). In oxygenic photosynthesis, water (H2O) serves as a substrate for photolysis resulting in the generation of free oxygen (O2). This process is responsible for generating the majority of breathable oxygen in earth's atmosphere. Photolysis of water occurs in the thylakoids of cyanobacteria and the chloroplasts of green algaeand plants.
Energy transfer models
The conventional, semi-classical, model describes the photosynthetic energy transfer process as one in which excitation energy hops from light-capturing pigment molecules to reaction center molecules step-by-step down the molecular energy ladder.
The effectiveness of photons of different wavelengths depends on the absorption spectra of the
photosynthetic pigments in the organism. Chlorophylls absorb light in the violet-blue and red parts of the spectrum, while accessory pigments capture other wavelengths as well. The phycobilins of red algae absorb blue-green light which penetrates deeper into water than red light, enabling them to photosynthesize in deep waters. Each absorbed photon causes the formation of an exciton(an electron excited to a higher energy state) in the pigment molecule. The energy of the exciton is transferred to a chlorophyllmolecule ( P680, where P stands for pigment and 680 for its absorption maximum at 680 nm) in the reaction center of photosystem IIvia resonance energy transfer. P680 can also directly absorb a photon at a suitable wavelength.
Photolysis during photosynthesis occurs in a series of light-driven oxidation events. The energized electron (exciton) of P680 is captured by a primary electron acceptor of the photosynthetic
electron transfer chainand thus exits photosystem II. In order to repeat the reaction, the electron in the reaction center needs to be replenished. This occurs by oxidation of water in the case of oxygenic photosynthesis. The electron-deficient reaction center of photosystem II (P680*) is the strongest biological oxidizing agent known on earth, which allows it to break apart molecules as stable as water.cite book | last = Campbell| first = Neil A. | coauthors = Reece, Jane B. | title = Biology, 7th Edition | publisher = Pearson - Benjamin Cummings | date = 2005 | location = San Francisco | pages = 186-191 | isbn = 0-8053-7171-0]
The water-splitting reaction is catalyzed by the
oxygen evolving complexof photosystem II. This protein-bound inorganic complex contains four manganese ions, plus a calcium and chloride ion as cofactors. Two water molecules are complexed by the manganese cluster, which then undergoes a series of four electron removals (oxidations) to replenish the reaction center of photosystem II. At the end of this cycle, free oxygen (O2) is generated and the hydrogen of the water molecules has been converted to four protons released into the thylakoid lumen.
These protons, as well as additional protons pumped across the thylakoid membrane coupled with the electron transfer chain, form a
proton gradientacross the membrane that drives photophosphorylationand thus the generation of chemical energy in the form of adenosine triphosphate(ATP). The electrons reach the P700reaction center of photosystem Iwhere they are energized again by light. They are passed down another electron transfer chain and finally combine with the coenzymeNADP+ and protons outside the thylakoids to NADPH. Thus, the net oxidation reaction of water photolysis can be written as:
2H2O + 2NADP+ + 8 photons (light) 2NADPH + 2H+ + O2
The free energy change (ΔG) for this reaction is 102 kilocalories per mole. Since the energy of light at 700 nm is about 40 kilocalories per mole of photons, approximately 320 kilocalories of light energy are available for the reaction. Therefore, approximately one-third of the available light energy is captured as NADPH during photolysis and electron transfer. An equal amount of ATP is generated by the resulting proton gradient. Oxygen as a byproduct is of no further use to the reaction and thus released into the atmosphere.cite book | last = Raven | first = Peter H. | coauthors = Ray F. Evert, Susan E. Eichhorn | title = Biology of Plants, 7th Edition | publisher = W.H. Freeman and Company Publishers | date = 2005 | location = New York | pages = 115-127 | isbn = 0-7167-1007-2]
In 2007 a quantum model was proposed by Graham FlemingGregory S. Engel Tessa R. Calhoun, Elizabeth L. Read, Tae-Kyu Ahn, Tomás caron Manc caronal, Yuan-Chung Cheng, Robert E. Blankenship and Graham R. Fleming, "Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems", in Nature 446, 782-786 (12 April 2007)] , which includes the possibility that photosynthetic energy transfer might involve quantum oscillations, explaining its unusually high efficiency.
According to Fleminghttp://www.physorg.com/news95605211.html Quantum secrets of photosynthesis revealed] there is direct evidence that remarkably long-lived wavelike electronic quantum coherence plays an important part in energy transfer processes during photosynthesis, which can explain the extreme efficiency of the energy transfer because it enables the system to sample all the potential energy pathways, with low loss, and choose the most efficient one.
Photolysis in the atmosphere
Photolysis also occurs in the atmosphere as part of a series of reactions by which primary
pollutantssuch as hydrocarbonsand nitrogen oxidesreact to form secondary pollutants such as peroxyacyl nitrates. See photochemical smog.
The two most important photodissociaton reactions in the
:O3 + hν → O2 + O(1D) λ < 320 nm
which generates an excited oxygen atom which can go on to react with water to give the
:O(1D) + H2O → 2OH
The hydroxyl radical is central to
atmospheric chemistryas it initiates the oxidationof hydrocarbonsin the atmosphere and so acts like a detergent.
Secondly the reaction:
:NO2 + hν → NO + O
is a key reaction in the formation of
The formation of the
ozone layeris also caused by photodissociation. Ozonein the earth's stratosphereis created by ultraviolet light striking oxygenmolecules containing two oxygen atoms (O2), splitting them into individual oxygen atoms (atomic oxygen); the atomic oxygen then combines with unbroken O2 to create ozone, O3. In addition, photolysis is the process by which CFCs are broken down in the upper atmosphere to form ozone-destroying chlorine free radicals.
astrophysics, photodissociation is one of the major processes through which molecules are broken down (but new molecules are being formed). Because of the vacuumof the interstellar medium, molecules and free radicals can exist for a long time. Photodissociation is the main path by which molecules are broken down. Photodissociation rates are very important in the study of the composition of interstellar cloudsin which stars are formed.
Typical examples of photodissociation in the interstellar medium are ( is the scientific notation for light, specifically a
Multiple photon dissociation
In comparison to ultraviolet or other high energy photons, single photons in the
infraredspectral range usually are not energetic enough for direct photodissociation of molecules. However, after absorption of multiple infrared photons a molecule may gain internal energy to overcome its barrier for dissociation. Multiple photon dissociation (MPD) can be achieved by applying high power lasers, e.g. a Carbon dioxide laser, or a Free electron laser, or by long interaction times of the molecule with the radiation field without the possibility for rapid cooling, e.g. by collisions. The latter method allows even for MPD induced by black body radiation.
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photodissociation — [ fɔtodisɔsjasjɔ̃ ] n. f. • v. 1960; de photo et dissociation ♦ Chim. Dissociation (d une molécule) sous l effet d un rayonnement de photons. ● photodissociation nom féminin Dissociation d une molécule par absorption de photons. photodissociation … Encyclopédie Universelle
photodissociation — [fōt΄ō di sō΄shē ā′shən] n. the breaking up of a substance, esp. a chemical compound, into simpler components by the action of radiant energy … English World dictionary
photodissociation — fotodisociacija statusas T sritis fizika atitikmenys: angl. photodissociation vok. Photodissoziation, f rus. фотодиссоциация, f pranc. photodissociation, f … Fizikos terminų žodynas
photodissociation — fotocheminė disociacija statusas T sritis fizika atitikmenys: angl. photochemical dissociation; photolysis vok. photochemische Dissoziation, f; Photodissoziation, f; Photolyse, f rus. фотодиссоциация, f; фотолиз, m; фотохимическая диссоциация, f… … Fizikos terminų žodynas
photodissociation — noun Date: 1925 dissociation of the molecules of a substance (as water) caused by absorption of radiant energy • photodissociate verb … New Collegiate Dictionary
photodissociation — /foh toh di soh see ay sheuhn, shee /, n. the dissociation or breakdown of a chemical compound by radiant energy. [1920 25; PHOTO + DISSOCIATION] * * * … Universalium
photodissociation — noun the dissociation of a molecule following the absorption of a photon … Wiktionary
Photodissociation — Фотодиссоциация; фотолиз … Краткий толковый словарь по полиграфии
photodissociation — pho·to·dissociation … English syllables
photodissociation — pho•to•dis•so•ci•a•tion [[t]ˌfoʊ toʊ dɪˌsoʊ siˈeɪ ʃən, ʃi [/t]] n. chem. the dissociation of a chemical compound by radiant energy • Etymology: 1920–25 … From formal English to slang