The photosynthesis is the process Bioénergétique which makes it possible the Plante S to synthesize to them Organic matter by exploiting the solar energy. It is the manufacture of carbonaceous matter organic starting from water and mineral carbon (CO2) in the presence of light. The nutritive needs for the plant are the Carbon dioxide of the air, the Eau and the minerals of the ground. The plants are known as autotrophic S for carbon. An important consequence is the release of molecules of Dioxygène.
During the night, photosynthesis is suspended, but the plant breathes in a continuous way the day and the night. The day, the exchanges out of carbon dioxide resulting from breathing are less important than those in dioxygene resulting from photosynthesis. Thus one can say that the plant produces dioxygene.
The discovery of the mechanism
- In Antiquity, Aristote thought that the ground provided to the plant the elements which it needed.
- At the 17th century, Jan Baptist van Helmont shows that a willow planted out of vat grows bigger of 77 kg in 5 years whereas the ground contained in the vat decreases only by 57 G; it allots the difference to the action of water.
- At the 18th century, several scientists highlight the concepts of breathing and production of oxygen by the plants and the importance of the light in this last phenomenon. They are initially two English chemists: Stephen Haul in 1727, which thinks that the air and the light contribute to the growth of the plants, and Joseph Priestley between 1771 and 1777 which highlights the oxygen rejection. With their continuation, Jan Ingen-Housz, doctor and botanist Dutch, establishes in 1779 the role of the light in the production of oxygen by the plants. Then Jean Senebier, Swiss Pasteur, starting from work of Antoine Lavoisier on the composition of the air, understands that the plants consume carbonic gas and reject oxygen at the time of this phase.
- At the beginning of the 19th century, Nicolas Theodore de Saussure shows the water consumption during photosynthesis. The Chlorophylle is insulated by French chemists in 1817, Pierre Joseph Pelletier and Joseph Bienaimé Caventou.
- In the middle of the XIXe century the broad outlines of the mechanism are included/understood, luminous energy conversion, carbonic gas and water consumption, production of starch and oxygen rejection.
- It is during the 20th century that the more detailed explanation of the process is established. The beginning of the century sees the description of the chemical structure of chlorophyl then the discovery of the existence of the types has and B . Emerson, Robert establishes into 1932 that 600 chlorophyls has are necessary for emmettre 1 molecule of O ². In the Years 1930, work of Robert Hill makes it possible to see there more clearly. At the conclusion of its experiments, photosynthesis appears as a Réaction of oxydoreduction during which carbon passes from a form oxidized to a reduced form CO2 → HCHO and the oxygen of a form reduced to an oxidized form H2O → O2.
General sight
Photosynthesis uses luminous energy (solar or artificial origin), to manufacture Sucre (Glucose).This process is represented by the following equation:
One simplifies often wrongly, this equation in this manner, which denies the fact that the atoms of produced dioxygene come only from water:
Photosynthesis can be done in the Plante S, the Algue S and the Bactérie S (at the Cyanobactérie S), and some Protiste S.
Note: certain photosynthetic organizations (of the Bacterium S) do not produce O2 and CO2 is not the single source of carbon. These molecules of O2 and CO2 would thus not be the common denominators of photosynthesis. It would be then preferable to define photosynthesis as being " a series of process in which electromagnetic energy is converted into chemical energy used for the biosynthesis of the material cellulaire" as Gest (2002) proposes it.
The support of photosynthesis
See also: Chloroplast, Photosystème
The whole of the phases of photosynthesis is in a specific organoid: the Chloroplast. This Organoid present of very rich membrane structures in which several types of Protéine S. are.
Among those, the antennas fix a great quantity of pigments whose most known Chlorophylle S. These antennas are the increase the cross section of capture of luminous energy and make it possible to feed the flow of energy forwarding to other membrane proteins: the reactional centers which transform luminous energy into chemical energy.
The whole of the collecting antennas and the reactional centers is called Photosystème . The photosystèmes intervene in the first phase of photosynthesis by capturing the first electrons initiating the photochemical reaction.
Pigments (for example: chlorophyl has , B and carotenoids) contents in the antennas present various absorption spectra.
Two phases of photosynthesis
If photosynthesis can be studied in a total way with:
This process actually proceeds in two quite distinct phases:
-
the photochemical reactions , commonly called “clear phase”, which can be summarized as follows:
2. The Cycle of Calvin , also called phase of Fixing of carbon or not-photochemical phase, or improperly “dark phase”:
What is noted chemical energy corresponds to 12 molecules of NADPH +H+ and ATP. It will have been noticed that the 2nd phase uses the chemical energy provided by the 1e photochemical phase. The 2nd phase thus depends also on the light, although indirectly. This is why the expression “phase sinks” often used in the past, is in fact inappropriate.
Photochemical reactions or clear phase
The light reaches us in the form of photons. These photons have a different natural energy according to the speed to which they travel. The light travels like a wave. One calculates the speed to which voyage the light according to the distance between 2 peaks. The energy transported by a photon is inversely proportional to the wavelength. A red photon of light has less energy than a blue photon of light.The pigments absorb better certain wavelengths. For example, chlorophyl absorbs well the red light and the light blue, but it does not absorb the green light well what gives them this color. The carotenoids absorb the green light best but not well the yellow light or the orange light what gives them this color.
When a pigment collects a photon corresponding to its capacity for absorption one of its electrons passes in an excited state. This energy can be transmitted by 3 ways: maybe by spreading it in the form of photon or of heat. These two ways make lose energy. The third consists in transmitting energy by resonance and there is almost no loss of energy.
The light harvesting complex (LHC) are whole of pigments (chlorophyls, carotenoids and phycoérythrobiline). The LHC provide energy necessary to the reactional centers (CR).
CR are the place or all the energy of the LHC. They are composed of a chlorophyl molecule (P680 or P700) related to a primary education acceptor of electron.
The photosystèmes are composed of the LHC which surround CR, CR and several molecules being used to transport electrons and protons. Except for some conveyers of electrons all the molecules which compose the photosystèmes are connected the ones to the others.
Good one can start! It Photosystème II (called thus because he was discovered in 2nd) and the complexes of the cytochromes are responsible for the oxygen release in the atmosphere and he produces ATP starting from ADP and of a phosphate.
1 - A pigment of the LHC collects a photon which corresponds to a wavelength that it can absorb. An electron of this pigment passes in an excited state. Energy is transmitted by resonance to another pigment. 2 - Energy is transmitted thus until the reactional center of the PS II. 3 - P680 is a chlorophyl molecule located at the center of CR of the PS II. It is connected to a pheophytin (phéo). P680 collects very well the photons a wavelength in the neighborhood of 680 Nm. When this molecule receives energy coming from the LHC or that it collects itself a photon, one of its electrons passes from the fundamental state in an excited state. This electron does not have time to turn over at the fundamental state, because it is collected by Phéo. 4 - Phéo is a chlorophyl molecule without central magnesium atom. This atom is replaced by 2 hydrogen atoms. This molecule collects the excited electron of P680. 5 - Let us retrogress a little: P680 has just lost an electron, it must find some to become again stable. Tyrosin Z (tyr Z) is the primary education donor of electron of the PSII. This molecule will give an electron to P680. This molecule has a grouping hydroxide. To remain stable it will lose the hydrogen of radical OH this hydrogen will become a proton, because its electron was yielded to P680.
6 - Tyr Z must also become again stable so that the process continues. A agrénat of 4 manganeses is the enzyme which separates water. This molecule is located side on the interior wall of the thylakoïde. Two water molecules are hung there. The tyrosin which lost its electron will seize an atom of hydrogen and it will be again stable. This stage occurs 4 times. Both O will be released from the agrénat and will form O2 (oxygen which one breathes). 7 - Let us return in Phéo which has just gained an electron. This electron will be taken again by another molecule which will give it to the plastoquinione (PQ). It will take a proton coming from the stroma to remain stable. Another electron will arrive and the PQ will take another proton. 8 - The molecule will move towards the complexes of the cytochromes b6/f. It will release its protons in the lumen and will give its electrons to the complexes of the cytochromes. 9 - Consequently there are much more protons in the lumen than in the stroma. The membrane is far from permeable with the protons then those must cross by the channel of the ATP synthase. The fact that a proton passes in this channel produced energy necessary to the production of ATP by this enzyme.
The photosystème I (PSI) is responsible for the release of NADPH in Stroma.
1 - the plastocyanine (PC) takes an electron of the complexes of the cytochromes b6/f and brings it to P700 2 - P700 is a chlorophyl molecule has which absorbs well the photons a wavelength being located at the neighborhoods of 700 Nm. The operation of the PS I is similar to the operation of the PS II: the LHC direct their energy towards the P 700 which loses an electron with the detriment of an other molecule. This electron is replaced by an electron of the PC. 3 - The ejected electron of P700 is collected by a chlorophyl molecule has which yields it to another molecule which yields it to another molecule, which yields it to another molecule (the passage of the electron from one molecule to another makes him lose energy), which yields it to another molecule. This molecule yields the electron to the ferrédoxine. 4 - The ferrédoxine is a molecule made up of 2 atoms of iron and from 2 atoms of suffers. It is located close to the stroma between the complexes of the cytochromes and the PS I. It can provide electrons to several other metabolisms such that of nitrogen. In the case of photosynthesis it gives its electron to a molecule called ferrédoxine NADP réductase. This molecule will link two protons coming from the stroma to a molecule of NADP using the electron which it has just received. It is located in the stroma.
The noncyclic photophosphorylation is the process explained Ci-high bus the electrons never return to the same molecule.
The cyclic photophosphorylation intervenes when the rate of NADPH becomes too high, because one needs more ATP than of NADPH. 1 - P700 of the PS I becomes excited, an electron is ejected, it follows the chain of electrons until the ferrédoxine. The ferrédoxine moves until the plastoquinione gives him an electron. 2 it PQ takes a proton of the stroma and the last stage reproduces. The PC having 2 protons moves towards the complexes of the cytochromes b6/f. 3 them electrons turn over towards P700 by the plastocyanine. 4 them protons induced in the lumen by PQ and the complexes of the cytochromes are used to produce ATP thanks to the ATP synthase.
A carotenoid is close to P680 and P700. When tyr Z or the PC cannot provide electron in P680 or carotenoid P700 it yields an electron to prevent that the P… destroys all CR while removing an electron with a neighbouring molecule. The carotenoids can spread their energy in the form of heat so too much of energy moves towards CR.
The cycle of Calvin or “phase sinks”
See also: Cycle of Calvin
In the second time, the chemical energy contained in the ATP and NADPH+H+ make it possible to fix the carbon contained in the atmospheric Carbonic gas by binding it to the atoms of Hydrogène molecules of Eau. It is the Cycle of Calvin or phase of fixing of carbon. This stage bears also sometimes the name of " phase somber" , although being able to be carried out with the light: this name reflects only the fact that the light is not directly necessary to this stage, contrary to the photochemical phase (or " phase claire").
Fixed carbon is then made reduce in Glucide by the addition of electron S and protons H+. The reducing potential is provided by NADPH+H+ which acquired electrons thanks to the photochemical phase. Lastly, the cycle of Calvin has energy requirement in the form of ATP to convert carbon into glucid.
However, at the majority of the plants, the cycle of Calvin proceeds day because it is during the day that the photochemical phase can regenerate NADPH+H+ and the ATP essential to the transformation of carbon into glucid. Because without the presence of the light and the products which result from the photochemical phase, the phase " somber" place would not have. The photochemical phase and the phase " somber" are complementary, one does not go without the other.
Various types of fixing of carbon
The plants present various mechanisms at the time of the stage of fixing of carbon dioxide during photosynthesis. These three mechanisms differ by the effectiveness from this stage. The type of photosynthesis of the plant is determined by the number of carbon atoms of the organic molecule formed in first during fixing of CO2.
If the mechanism in C3 corresponds to the “basic” mechanism, the types in C4 and CAMWOOD are adapations in dry mediums.
The mechanism of the plants in C3
The first of the stages of the cycle of Calvin-Benson consists of a carboxylation (fixing of a molecule of CO2) on the ribulose 1,5 bisphosphate, catalyzed by the RubisCO, to give two molecules of a compound to 3 carbon atoms (Acid 3-phosphoglyceric, APG). A great majority of the plants, of which all the trees, function according to this mechanism. CO2 fixed by the RubisCO comes from the diffusion of atmospheric CO2 through the Stomate S initially then, in dissolved form, through the cells of the sheet until the stroma of the chloroplasts. The RubisCO is able to catalyze a reaction by using oxygen instead of CO2, it is the phenomenon of Photorespiration, seemingly prejudicial with the plant because of reduction in the rate of clear photosynthesis.
The mechanism of the plants in C4
The RubisCO enzyme which fixes CO2 has for second characteristic to fix of O2, which causes a loss of organic molecules. It is the Photorespiration whose utility remains rather badly included/understood (one thinks that it makes it possible to plug the concentration in O2 in the cell, to avoid oxidations, or that it allows the synthesis of amino-acids such as serine in mitochondrion of the plants concerned; but that by decreasing the output of photosynthesis). One of the adaptations of the plant to avoid photorespiration is to increase the partial pressure in CO2 around RubisCO. For that purpose, the plants presenting a metabolism in C4 have another enzyme fixing CO2, the phospho-énol-pyruvate-carboxylase. Atmospheric CO2 is quickly integrated by the PEP-carboxylase in a compound into four carbon atoms (oxaloacétate, then malate or aspartate). These reactions take place in the Mésophylle (cellular base between the vein S). This compound with 4 carbon atoms, an acid dicarboxylic, is then transported towards the cells of the sheath périvasculaire where an enzyme is given the responsability to release trapped CO2 and to recycle the conveyer. CO2 is thus concentrated in these cells and is fixed by the RubisCO, according to the mechanism of the C3 plants but with a better output. This type of photosynthesis exists in particular at Graminées of tropical and arid origin, as the Canne with sugar or the Sorgho. The Photorespiration is zero or very low at these plants, because of enrichment in CO2.
The C4 metabolism dissociates in space , the phases photochemical and nonphotochemical.
Report/ratio of the plants in the hot and dry medium
These plants are " obligées" to close their Stoma S longer to avoid the water losses. That quickly lowers the partial pressure in CO2 in the sheet, because it is taken by photosynthesis. The metabolism in C4 will have the advantage in dry medium of remaining effective, for partial pressures in CO2 however low.Many species in different S are in C4. Opuntia ficus-indica is an example at the Cactaceae. The Maïs is a C4 plant but is not adapted to a dry medium (it is one of the greediest water cultures). This character was inherited the plant ancestor: the Teosinte, which is adapted to a hot and dry medium. This character was maintained by the artificial selection of the farmers because it ensures of good outputs.
The mechanism of the plants CAMWOOD ( Crassulacean Acid Metabolism )
See also: acid Metabolism crassulacéen
These plants have the two types of enzymes carboxylantes like the plants of the C4 type. They differ from these last owing to the fact that the fixing of carbon is not separate in space (mésophylle/sheath périvasculaire) but in time (night/day). During the night, when the Stomates are opened, a stock of malate is produced, then stored in the vacuole of the photosynthetic cells. During the day, these malates is retransformés out of carbon dioxide and the cycle of Calvin can be carried out, the Stomates remaining closed. This mechanism is observed at the Crassulaceae (“fatty plants like the cactus”) and makes it possible to reduce the water losses by perspiration, the closed Stomate S remaining the day without the contribution in CO2 being faded.
Artificial photosynthesis
The artificial propagation of the reactions of photosynthesis is the subject of many research.
In 2007, a team directed by Dr. Hideki Koyanaka at the university of Kyoto announced a process containing manganese dioxide, which could collect the atmospheric CO2 300 times more effectively than the plants (result to confirm after publication in an scientific magazine).
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