Biological process to convert light into chemical energy
Schematic of photosynthesis in plants. Thecarbohydrates produced are stored in or used by the plant.Composite image showing the global distribution of photosynthesis, including both oceanicphytoplankton and terrestrialvegetation. Dark red and blue-green indicate regions of high photosynthetic activity in the ocean and on land, respectively.
While the details may differ betweenspecies, the process always begins when light energy is absorbed by thereaction centers, proteins that containphotosynthetic pigments orchromophores. In plants, these pigments arechlorophylls (aporphyrin derivative that absorbs the red and bluespectra of light, thus reflecting green) held insidechloroplasts, abundant inleaf cells. In cyanobacteria, they are embedded in theplasma membrane. In these light-dependent reactions, some energy is used to stripelectrons from suitable substances, such as water, producing oxygen gas. Thehydrogen freed by the splitting of water is used in the creation of two important molecules that participate in energetic processes: reducednicotinamide adenine dinucleotide phosphate (NADPH) and ATP.
In plants,algae, andcyanobacteria, sugars are synthesized by a subsequent sequence oflight-independent reactions called theCalvin cycle. In this process, atmospheric carbon dioxide is incorporated into already existing organic compounds, such asribulose bisphosphate (RuBP).[6] Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are thenreduced and removed to form further carbohydrates, such asglucose. In other bacteria, different mechanisms like thereverse Krebs cycle are used to achieve the same end.
The first photosynthetic organisms probablyevolved early in theevolutionary history of life usingreducing agents such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons.[7] Cyanobacteria appeared later; theexcess oxygen they produced contributed directly to theoxygenation of the Earth,[8] which rendered the evolution of complex life possible. The average rate of energy captured by global photosynthesis is approximately 130terawatts,[9][10][11] which is about eight times the totalpower consumption of human civilization.[12] Photosynthetic organisms also convert around 100–115 billiontons (91–104 Pgpetagrams, or billions of metric tons), of carbon intobiomass per year.[13][14] Photosynthesis was discovered in 1779 byJan Ingenhousz who showed that plants need light, not just soil and water.
Photosynthesis changes sunlight into chemical energy, splits water to liberate O2, and fixes CO2 into sugar.
Most photosynthetic organisms arephotoautotrophs, which means that they are able tosynthesize food directly fromcarbon dioxide andwater usingenergy from light. However, not all organisms use carbon dioxide as a source of carbon atoms to carry out photosynthesis;photoheterotrophs use organic compounds, rather than carbon dioxide, as a source of carbon.[2]
Inplants,algae, andcyanobacteria, photosynthesis releases oxygen. Thisoxygenic photosynthesis is by far the most common type of photosynthesis used by living organisms. Some shade-loving plants (sciophytes) produce such low levels of oxygen during photosynthesis that they use all of it themselves instead of releasing it to the atmosphere.[15]
Although there are some differences between oxygenic photosynthesis in plants, algae, and cyanobacteria, the overall process is quite similar in these organisms. There are also many varieties ofanoxygenic photosynthesis, used mostly by bacteria, which consume carbon dioxide but do not release oxygen or which produce elemental sulfur instead of molecular oxygen.[16][17]
Carbon dioxide is converted into sugars in a process calledcarbon fixation; photosynthesis captures energy from sunlight to convert carbon dioxide intocarbohydrates. Carbon fixation is anendothermicredox reaction. In general outline, photosynthesis is the opposite ofcellular respiration: while photosynthesis is a process of reduction of carbon dioxide to carbohydrates, cellular respiration is the oxidation of carbohydrates or othernutrients to carbon dioxide. Nutrients used in cellular respiration include carbohydrates, amino acids and fatty acids. These nutrients are oxidized to produce carbon dioxide and water, and to release chemical energy to drive the organism'smetabolism.
Photosynthesis and cellular respiration are distinct processes, as they take place through different sequences of chemical reactions and in differentcellular compartments (cellular respiration inmitochondria).[18][19]
CO2carbon dioxide +2H2Aelectron donor +photonslight energy →[CH2O]carbohydrate +2Aoxidized electron donor +H2Owater
Since water is used as the electron donor in oxygenic photosynthesis, the equation for this process is:
CO2carbon dioxide +2H2Owater +photonslight energy →[CH2O]carbohydrate +O2oxygen +H2Owater
This equation emphasizes that water is both a reactant in thelight-dependent reaction and a product of thelight-independent reaction, but cancelingn water molecules from each side gives the net equation:
CO2carbon dioxide +H2O water +photonslight energy →[CH2O]carbohydrate +O2 oxygen
Other processes substitute other compounds (such asarsenite) for water in the electron-supply role; for example some microbes use sunlight to oxidize arsenite toarsenate:[21] The equation for this reaction is:
CO2carbon dioxide +(AsO3− 3) arsenite +photonslight energy →(AsO3− 4) arsenate +COcarbon monoxide(used to build other compounds in subsequent reactions)[22]
Photosynthesis occurs in two stages. In the first stage,light-dependent reactions orlight reactions capture the energy of light and use it to make the hydrogen carrierNADPH and the energy-storage moleculeATP. During the second stage, thelight-independent reactions use these products to capture and reduce carbon dioxide.
Most organisms that use oxygenic photosynthesis usevisible light for the light-dependent reactions, although at least three use shortwaveinfrared or, more specifically, far-red radiation.[23]
Some organisms employ even more radical variants of photosynthesis. Somearchaea use a simpler method that employs a pigment similar to those used for vision in animals. Thebacteriorhodopsin changes its configuration in response to sunlight, acting as a proton pump. This produces a proton gradient more directly, which is then converted to chemical energy. The process does not involve carbon dioxide fixation and does not release oxygen, and seems to have evolved separately from the more common types of photosynthesis.[24]
In photosynthetic bacteria, the proteins that gather light for photosynthesis are embedded incell membranes. In its simplest form, this involves the membrane surrounding the cell itself.[25] However, the membrane may be tightly folded into cylindrical sheets calledthylakoids,[26] or bunched up into roundvesicles calledintracytoplasmic membranes.[27] These structures can fill most of the interior of a cell, giving the membrane a very large surface area and therefore increasing the amount of light that the bacteria can absorb.[26]
In plants and algae, photosynthesis takes place inorganelles calledchloroplasts. A typicalplant cell contains about 10 to 100 chloroplasts. The chloroplast is enclosed by a membrane. This membrane is composed of a phospholipid inner membrane, a phospholipid outer membrane, and an intermembrane space. Enclosed by the membrane is an aqueous fluid called the stroma. Embedded within the stroma are stacks of thylakoids (grana), which are the site of photosynthesis. The thylakoids appear as flattened disks. The thylakoid itself is enclosed by the thylakoid membrane, and within the enclosed volume is a lumen or thylakoid space. Embedded in the thylakoid membrane are integral andperipheral membrane protein complexes of the photosynthetic system.
These pigments are embedded in plants and algae in complexes called antenna proteins. In such proteins, the pigments are arranged to work together. Such a combination of proteins is also called alight-harvesting complex.[29]
Although all cells in the green parts of a plant have chloroplasts, the majority of those are found in specially adapted structures calledleaves. Certain species adapted to conditions of strong sunlight andaridity, such as manyEuphorbia andcactus species, have their main photosynthetic organs in their stems. The cells in the interior tissues of a leaf, called themesophyll, can contain between 450,000 and 800,000 chloroplasts for every square millimeter of leaf. The surface of the leaf is coated with a water-resistantwaxycuticle that protects the leaf from excessiveevaporation of water and decreases the absorption ofultraviolet orbluelight to minimizeheating. The transparentepidermis layer allows light to pass through to thepalisade mesophyll cells where most of the photosynthesis takes place.
Not allwavelengths oflight can support photosynthesis. The photosyntheticaction spectrum depends on the type ofaccessory pigments present. For example, ingreen plants, the action spectrum resembles theabsorption spectrum forchlorophylls andcarotenoids with absorption peaks in violet-blue and red light. Inred algae, the action spectrum is blue-green light, which allows thesealgae to use the blue end of the spectrum to grow in the deeper waters that filter out the longer wavelengths (red light) used by above-ground green plants. The non-absorbed part of thelight spectrum is what givesphotosynthetic organisms theircolor (e.g., green plants, red algae,purple bacteria) and is the least effective for photosynthesis in the respectiveorganisms.
The cyclic reaction is similar to that of the non-cyclic but differs in that it generates only ATP, and no reduced NADP (NADPH) is created. The cyclic reaction takes place only at photosystem I. Once the electron is displaced from the photosystem, the electron is passed down the electron acceptor molecules and returns to photosystem I, from where it was emitted, hence the namecyclic reaction.
Linear electron transport through a photosystem will leave thereaction center of that photosystemoxidized. Elevating another electron will first require re-reduction of the reaction center. The excited electrons lost from the reaction center (P700) ofphotosystem I are replaced by transfer fromplastocyanin, whose electrons come from electron transport throughphotosystem II. Photosystem II, as the first step of theZ-scheme, requires an external source of electrons to reduce its oxidizedchlorophylla reaction center. The source of electrons for photosynthesis in green plants andcyanobacteria is water. Two water molecules are oxidized by the energy of four successive charge-separation reactions of photosystem II to yield a molecule ofdiatomic oxygen and fourhydrogen ions. The electrons yielded are transferred to a redox-activetyrosine residue that is oxidized by the energy ofP680+. This resets the ability of P680 to absorb another photon and release anotherphoto-dissociated electron. The oxidation of water iscatalyzed in photosystem II by a redox-active structure that contains fourmanganese ions and acalcium ion; thisoxygen-evolving complex binds twowater molecules and contains the four oxidizing equivalents that are used to drive the water-oxidizing reaction (Kok's S-state diagrams). The hydrogen ions are released in thethylakoid lumen and therefore contribute to the transmembrane chemiosmotic potential that leads toATP synthesis. Oxygen is awaste product of light-dependent reactions, but the majority of organisms onEarth use oxygen and its energy forcellular respiration, includingphotosynthetic organisms.[31][32]
Plants that use theC4 carbon fixation process chemically fix carbon dioxide in thecells of themesophyll by adding it to the three-carbon moleculephosphoenolpyruvate (PEP), a reactioncatalyzed by anenzyme calledPEP carboxylase, creating the four-carbon organic acidoxaloacetic acid. Oxaloacetic acid ormalate synthesized by this process is thentranslocated to specializedbundle sheath cells where the enzymeRuBisCO and other Calvin cycle enzymes are located, and where CO2 released bydecarboxylation of the four-carbon acids is then fixed by RuBisCO activity to the three-carbon3-phosphoglyceric acids. The physical separation of RuBisCO from the oxygen-generating light reactions reduces photorespiration and increases CO2 fixation and, thus, thephotosynthetic capacity of theleaf.[34]C4 plants can produce more sugar thanC3 plants in conditions of high light andtemperature. Many importantcrop plants are C4 plants, includingmaize,sorghum,sugarcane, andmillet. Plants that do not use PEP-carboxylase in carbon fixation are called C3 plants because the primarycarboxylation reaction, catalyzed by RuBisCO, produces the three-carbon 3-phosphoglyceric acids directly in theCalvin-Benson cycle. Over 90% of plants use C3 carbon fixation, compared to 3% that use C4 carbon fixation;[35] however, the evolution of C4 in over sixty plant lineages makes it a striking example ofconvergent evolution.[33]C2 photosynthesis, which involves carbon-concentration by selective breakdown of photorespiratory glycine, is both an evolutionary precursor to C4 and a usefulcarbon-concentrating mechanism in its own right.[36]
Xerophytes, such ascacti and mostsucculents, also use PEP carboxylase to capture carbon dioxide in a process calledCrassulacean acid metabolism (CAM). In contrast to C4 metabolism, whichspatially separates the CO2 fixation to PEP from the Calvin cycle, CAMtemporally separates these two processes. CAM plants have a differentleaf anatomy from C3 plants, and fix the CO2 at night, when their stomata are open. CAM plants store the CO2 mostly in the form ofmalic acid via carboxylation ofphosphoenolpyruvate tooxaloacetate, which is then reduced to malate. Decarboxylation of malate during the day releases CO2 inside the leaves, thus allowing carbon fixation to 3-phosphoglycerate by RuBisCO. CAM is used by 16,000species of plants.[37]
Calcium-oxalate-accumulating plants, such asAmaranthus hybridus andColobanthus quitensis, show a variation of photosynthesis where calcium oxalatecrystals function as dynamiccarbon pools, supplying carbon dioxide (CO2) to photosynthetic cells when stomata are partially or totally closed. This process was namedalarm photosynthesis. Understress conditions (e.g.,water deficit),oxalate released from calcium oxalate crystals is converted to CO2 by anoxalate oxidase enzyme, and the produced CO2 can support theCalvin cycle reactions. Reactivehydrogen peroxide (H2O2), thebyproduct of oxalate oxidase reaction, can beneutralized bycatalase. Alarm photosynthesis represents a photosynthetic variant to be added to the well-known C4 and CAM pathways. However, alarm photosynthesis, in contrast to these pathways, operates as a biochemical pump that collects carbon from the organ interior (or from thesoil) and not from the atmosphere.[38][39]
In water
Cyanobacteria possesscarboxysomes, which increase the concentration of CO2 around RuBisCO to increase the rate of photosynthesis. An enzyme,carbonic anhydrase, located within the carboxysome, releases CO2 from dissolvedhydrocarbonate ions (HCO− 3). Before the CO2 can diffuse out, RuBisCO concentrated within the carboxysome quickly sponges it up. HCO− 3 ions are made from CO2 outside the cell by another carbonic anhydrase and are actively pumped into the cell by a membrane protein. They cannot cross the membrane as they are charged, and within the cytosol they turn back into CO2 very slowly without the help of carbonic anhydrase. This causes the HCO− 3 ions to accumulate within the cell from where they diffuse into the carboxysomes.[40]Pyrenoids inalgae andhornworts also act to concentrate CO2 around RuBisCO.[41][42]
Order and kinetics
The overallprocess of photosynthesis takes place in four stages:[14]
Gas exchange systems that offer control of CO2 levels, above and belowambient, allow the common practice of measurement of A/Ci curves, at different CO2 levels, to characterize a plant's photosynthetic response.[56]
Integrated chlorophyll fluorometer – gas exchange systems allow a moreprecise measure of photosynthetic response and mechanisms.[54][55] While standard gas exchange photosynthesis systems can measure Ci, or substomatal CO2 levels, the addition of integrated chlorophyll fluorescence measurements allows a more precise measurement of CC, the estimation of CO2 concentration at the site ofcarboxylation in the chloroplast, to replace Ci.[55][54] CO2 concentration in the chloroplast becomes possible to estimate with the measurement of mesophyll conductance or gm using an integrated system.[54][55][57]
Photosynthesis measurement systems are not designed to directly measure the amount of light the leaf absorbs, but analysis ofchlorophyll fluorescence,P700- and P515-absorbance, andgas exchange measurements reveal detailed information about, e.g., thephotosystems,quantum efficiency and the CO2 assimilation rates. With some instruments, even wavelength dependency of the photosynthetic efficiency can beanalyzed.[58]
Aphenomenon known asquantum walk increases the efficiency of the energy transport of light significantly. In the photosynthetic cell of analga,bacterium, or plant, there are light-sensitive molecules calledchromophores arranged in an antenna-shaped structure called a photocomplex. When aphoton is absorbed by a chromophore, it is converted into aquasiparticle referred to as anexciton, which jumps from chromophore to chromophore towards the reaction center of the photocomplex, a collection of molecules that traps its energy in a chemical form accessible to the cell's metabolism. The exciton's wave properties enable it to cover a wider area and try out several possible paths simultaneously, allowing it to instantaneously "choose" the most efficient route, where it will have the highest probability of arriving at its destination in the minimum possible time.
Because that quantum walking takes place at temperatures far higher than quantum phenomena usually occur, it is only possible over very short distances. Obstacles in the form of destructive interference cause the particle to lose its wave properties for an instant before it regains them once again after it is freed from its locked position through a classic "hop". The movement of the electron towards the photo center is therefore covered in a series of conventional hops and quantum walks.[59][60][61]
Fossils of what are thought to befilamentous photosyntheticorganisms have been dated at 3.4 billion years old.[62][63] More recentstudies also suggest that photosynthesis may have begun about 3.4 billion years ago,[64][65] though the first directevidence of photosynthesis comes fromthylakoid membranes preserved in 1.75-billion-year-oldcherts.[66]
An even closer form of symbiosis may explain the origin of chloroplasts. Chloroplasts have many similarities with photosyntheticbacteria, including a circularchromosome, prokaryotic-typeribosome, and similarproteins in the photosynthetic reaction center.[71][72] Theendosymbiotic theory suggests that photosynthetic bacteria were acquired (byendocytosis) by earlyeukaryotic cells to form the first plant cells. Therefore, chloroplasts may be photosynthetic bacteria that adapted to life inside plant cells. Likemitochondria, chloroplasts possess their ownDNA, separate from thenuclear DNA of their plant host cells and the genes in this chloroplast DNA resemble those found incyanobacteria.[73] DNA in chloroplasts codes forredox proteins such as those found in the photosynthetic reaction centers. TheCoRR Hypothesis proposes that this co-location of genes with their gene products is required for redox regulation ofgene expression, and accounts for the persistence of DNA in bioenergeticorganelles.[74]
Except for the euglenids, which are found within theExcavata, all of these belong to theDiaphoretickes. Archaeplastida and the photosynthetic Paulinella got their plastids, which are surrounded by two membranes, through primaryendosymbiosis in two separate events, by engulfing a cyanobacterium. The plastids in all the other groups have either a red or green algal origin, and are referred to as the "red lineages" and the "green lineages". The only known exception is the ciliatePseudoblepharisma tenue, which in addition to its plastids that originated from green algae also has apurple sulfur bacterium as symbiont. In dinoflagellates and euglenids the plastids are surrounded by three membranes, and in the remaining lines by four. Anucleomorph, remnants of the original algal nucleus located between the inner and outer membranes of the plastid, is present in the cryptophytes (from a red alga) and chlorarachniophytes (from a green alga).[75]Some dinoflagellates that lost their photosynthetic ability later regained it again through new endosymbiotic events with different algae.While able to perform photosynthesis, many of these eukaryotic groups aremixotrophs and practiceheterotrophy to various degrees.
Photosynthetic prokaryotic lineages
Early photosynthetic systems, such as those ingreen andpurple sulfur andgreen andpurple nonsulfur bacteria, are thought to have beenanoxygenic, and used various other molecules than water aselectron donors. Green and purple sulfur bacteria are thought to have usedhydrogen andsulfur as electron donors. Green nonsulfur bacteria used variousamino and otherorganic acids as electron donors. Purple nonsulfur bacteria used a variety of nonspecific organic molecules. The use of these molecules is consistent with the geological evidence that Earth's early atmosphere was highlyreducing atthat time.[76]
With a possible exception ofHeimdallarchaeota, photosynthesis is not found inarchaea.[77]Haloarchaea arephotoheterotrophic; they can absorb energy from the sun, but do not harvest carbon from the atmosphere and are therefore not photosynthetic.[78] Instead of chlorophyll they use rhodopsins, which convert light-energy to ion gradients but cannot mediate electron transfer reactions.[79][80]
Cyanobacteria, the only prokaryotes performing oxygenic photosynthesis and the only prokaryotes that contain two types of photosystems (type I (RCI), also known as Fe-S type, and type II (RCII), also known as quinone type). The seven remaining prokaryotes haveanoxygenic photosynthesis and use versions of either type I or type II.
The biochemical capacity to use water as the source for electrons in photosynthesis evolved once, in acommon ancestor of extantcyanobacteria (formerly called blue-green algae). The geological record indicates that this transforming event took place early in Earth's history, at least 2450–2320 million years ago (Ma), and, it is speculated, much earlier.[85][86] Because the Earth's atmosphere contained almost no oxygen during the estimated development of photosynthesis, it is believed that the first photosynthetic cyanobacteria did not generate oxygen.[87] Available evidence from geobiological studies ofArchean (>2500 Ma)sedimentary rocks indicates that life existed 3500 Ma, but the question of when oxygenic photosynthesis evolved is still unanswered. A clear paleontological window on cyanobacterialevolution opened about 2000 Ma, revealing an already-diverse biota of cyanobacteria. Cyanobacteria remained the principalprimary producers of oxygen throughout theProterozoic Eon (2500–543 Ma), in part because the redox structure of the oceans favored photoautotrophs capable ofnitrogen fixation.[88][89]Green algae joined cyanobacteria as the major primary producers of oxygen oncontinental shelves near the end of theProterozoic, but only with theMesozoic (251–66 Ma) radiations of dinoflagellates, coccolithophorids, and diatoms did theprimary production of oxygen in marine shelf waters take modern form. Cyanobacteria remain critical tomarine ecosystems asprimary producers of oxygen in oceanic gyres, as agents of biological nitrogen fixation, and, in modified form, as theplastids of marine algae.[90]
Experimental history
Discovery
Although some of the steps in photosynthesis are still not completely understood, the overall photosynthetic equation has been known since the 19th century.
Jan van Helmont began theresearch of theprocess in the mid-17th century when he carefully measured themass of thesoil aplant was using and the mass of the plant as it grew. After noticing that the soil mass changed very little, hehypothesized that the mass of thegrowing plant must come from thewater, the onlysubstance he added to the potted plant. His hypothesis was partiallyaccurate – much of the gained mass comes fromcarbon dioxide as well as water. However, this was a signaling point to the idea that the bulk of a plant'sbiomass comes from the inputs of photosynthesis, not the soil itself.
Joseph Priestley, achemist andminister, discovered that when he isolated avolume of air under an invertedjar and burned acandle in it (which gave offCO2), the candle would burn out very quickly, much before it ran out ofwax. He further discovered that amouse could similarly"injure" air. He then showed that a plant could restore the air the candle and the mouse had "injured".[91]
In 1779,Jan Ingenhousz repeated Priestley'sexperiments. He discovered that it was the influence ofsunlight on the plant that could cause it to revive a mouse in a matter of hours.[91][92]
Robert Emerson discovered two light reactions by testing plant productivity using different wavelengths of light. With the red alone, the light reactions were suppressed. When blue and red were combined, the output was much more substantial. Thus, there were two photosystems, one absorbing up to 600 nm wavelengths, the other up to 700 nm. The former is known as PSII, the latter is PSI. PSI contains only chlorophyll "a", PSII contains primarily chlorophyll "a" with most of the available chlorophyll "b", among other pigments. These include phycobilins, which are the red and blue pigments of red and blue algae, respectively, and fucoxanthol for brown algae and diatoms. The process is most productive when the absorption of quanta is equal in both PSII and PSI, assuring that input energy from the antenna complex is divided between the PSI and PSII systems, which in turn powers the photochemistry.[14]
Robert Hill thought that a complex of reactions consisted of an intermediate to cytochrome b6 (now a plastoquinone), and that another was from cytochrome f to a step in the carbohydrate-generating mechanisms. These are linked by plastoquinone, which does require energy to reduce cytochrome f. Further experiments to prove that the oxygen developed during the photosynthesis of green plants came from water were performed by Hill in 1937 and 1939. He showed that isolatedchloroplasts give off oxygen in the presence of unnatural reducing agents likeironoxalate,ferricyanide orbenzoquinone after exposure to light. In the Hill reaction:[94]
2 H2O + 2 A + (light, chloroplasts) → 2 AH2 + O2
A is the electron acceptor. Therefore, in light, the electron acceptor is reduced and oxygen is evolved.Samuel Ruben andMartin Kamen usedradioactive isotopes to determine that the oxygen liberated in photosynthesis came from the water.
Melvin Calvin works in his photosynthesis laboratory.
Melvin Calvin andAndrew Benson, along withJames Bassham, elucidated the path of carbon assimilation (the photosynthetic carbon reduction cycle) in plants. The carbon reduction cycle is known as theCalvin cycle, but many scientists refer to it as the Calvin-Benson, Benson-Calvin, or even Calvin-Benson-Bassham (or CBB) Cycle.
Nobel Prize–winning scientistRudolph A. Marcus was later able to discover the function and significance of the electron transport chain.
In 1950, first experimental evidence for the existence ofphotophosphorylationin vivo was presented byOtto Kandler using intactChlorella cells and interpreting his findings as light-dependentATP formation.[96]In 1954,Daniel I. Arnon et al. discovered photophosphorylationin vitro in isolatedchloroplasts with the help of P32.[97][98]
Louis N. M. Duysens andJan Amesz discovered that chlorophyll "a" will absorb one light, oxidize cytochrome f, while chlorophyll "a" (and other pigments) will absorb another light but will reduce this same oxidized cytochrome, stating the two light reactions are in series.
Development of the concept
In 1893, the American botanistCharles Reid Barnes proposed two terms,photosyntax andphotosynthesis, for the biological process ofsynthesis of complex carbon compounds out of carbonic acid, in the presence of chlorophyll, under the influence of light. The termphotosynthesis is derived from theGreekphōs (φῶς, gleam) andsýnthesis (σύνθεσις, arranging together),[99][100][101] while another word that he designated wasphotosyntax, fromsýntaxis (σύνταξις, configuration). Over time, the termphotosynthesis came into common usage. Later discovery of anoxygenic photosynthetic bacteria and photophosphorylation necessitated redefinition of the term.[102]
C3 : C4 photosynthesis research
In the late 1940s at theUniversity of California, Berkeley, the details of photosynthetic carbon metabolism were sorted out by the chemistsMelvin Calvin, Andrew Benson, James Bassham and a score of students and researchers utilizing the carbon-14 isotope and paper chromatography techniques.[103] The pathway of CO2 fixation by the algaeChlorella in a fraction of a second in light resulted in a three carbon molecule called phosphoglyceric acid (PGA). For that original and ground-breaking work, aNobel Prize in Chemistry was awarded to Melvin Calvin in 1961. In parallel, plant physiologists studied leaf gas exchanges using the new method of infrared gas analysis and a leaf chamber where the net photosynthetic rates ranged from 10 to 13 μmol CO2·m−2·s−1, with the conclusion that all terrestrial plants have the same photosynthetic capacities, that are light saturated at less than 50% of sunlight.[104][105]
Later in 1958–1963 atCornell University, field grownmaize was reported to have much greater leaf photosynthetic rates of 40 μmol CO2·m−2·s−1 and not be saturated at near full sunlight.[106][107] This higher rate in maize was almost double of those observed in other species such as wheat and soybean, indicating that large differences in photosynthesis exist among higher plants. At the University of Arizona, detailed gas exchange research on more than 15 species ofmonocots anddicots uncovered for the first time that differences in leaf anatomy are crucial factors in differentiating photosynthetic capacities among species.[108][109] In tropical grasses, including maize, sorghum, sugarcane, Bermuda grass and in the dicot amaranthus, leaf photosynthetic rates were around 38−40 μmol CO2·m−2·s−1, and the leaves have two types of green cells, i.e. outer layer of mesophyll cells surrounding a tightly packed cholorophyllous vascular bundle sheath cells. This type of anatomy was termed Kranz anatomy in the 19th century by the botanistGottlieb Haberlandt while studying leaf anatomy of sugarcane.[110] Plant species with the greatest photosynthetic rates and Kranz anatomy showed no apparent photorespiration, very low CO2 compensation point, high optimum temperature, high stomatal resistances and lower mesophyll resistances for gas diffusion and rates never saturated at full sun light.[111] The research at Arizona was designated a Citation Classic in 1986.[109] These species were later termed C4 plants as the first stable compound of CO2 fixation in light has four carbons as malate and aspartate.[112][113][114] Other species that lack Kranz anatomy were termed C3 type such as cotton and sunflower, as the first stable carbon compound is the three-carbon PGA. At 1000 ppm CO2 in measuring air, both the C3 and C4 plants had similar leaf photosynthetic rates around 60 μmol CO2·m−2·s−1 indicating the suppression of photorespiration in C3 plants.[108][109]
Factors
Theleaf is the primary site of photosynthesis in plants.
There are four main factors influencing photosynthesis and several corollary factors. The four main are:[115]
Total photosynthesis is limited by a range of environmental factors. These include the amount of light available, the amount ofleaf area a plant has to capture light (shading by other plants is a major limitation of photosynthesis), the rate at which carbon dioxide can be supplied to thechloroplasts to support photosynthesis, the availability of water, and the availability of suitable temperatures for carrying out photosynthesis.[116]
Light intensity (irradiance), wavelength and temperature
Absorbance spectra of free chlorophylla (blue) andb (red) in a solvent. The action spectra of chlorophyll molecules are slightly modifiedin vivo depending on specific pigment–protein interactions.
The process of photosynthesis provides the main input of free energy into the biosphere, and is one of four main ways in which radiation is important for plant life.[117]
The radiation climate within plant communities is extremely variable, in both time and space.
At constant temperature, the rate of carbon assimilation varies with irradiance, increasing as the irradiance increases, but reaching a plateau at higher irradiance.
At low irradiance, increasing the temperature has little influence on the rate of carbon assimilation. At constant high irradiance, the rate of carbon assimilation increases as the temperature is increased.
These two experiments illustrate several important points: First, it is known that, in general,photochemical reactions are not affected bytemperature. However, these experiments clearly show that temperature affects the rate of carbon assimilation, so there must be two sets of reactions in the full process of carbon assimilation. These are the light-dependent 'photochemical' temperature-independent stage, and the light-independent, temperature-dependent stage. Second, Blackman's experiments illustrate the concept oflimiting factors. Another limiting factor is the wavelength of light. Cyanobacteria, which reside several meters underwater, cannot receive the correct wavelengths required to cause photoinduced charge separation in conventional photosynthetic pigments. To combat this problem, Cyanobacteria have a light-harvesting complex calledPhycobilisome.[118] This complex is made up of a series of proteins with different pigments which surround the reaction center.
Carbon dioxide levels and photorespiration
Photorespiration
As carbon dioxide concentrations rise, the rate at which sugars are made by the light-independent reactions increases until limited by other factors.RuBisCO, the enzyme that captures carbon dioxide in the light-independent reactions, has a binding affinity for both carbon dioxide and oxygen. When the concentration of carbon dioxide is high, RuBisCO will fix carbon dioxide. However, if the carbon dioxide concentration is low, RuBisCO will bind oxygen instead of carbon dioxide. This process, calledphotorespiration, uses energy, but does not produce sugars.
RuBisCO oxygenase activity is disadvantageous to plants for several reasons:
One product of oxygenase activity is phosphoglycolate (2 carbon) instead of3-phosphoglycerate (3 carbon). Phosphoglycolate cannot be metabolized by the Calvin-Benson cycle and represents carbon lost from the cycle. A high oxygenase activity, therefore, drains the sugars that are required to recycle ribulose 5-bisphosphate and for the continuation of theCalvin-Benson cycle.
Phosphoglycolate is quickly metabolized to glycolate that is toxic to a plant at a high concentration; it inhibits photosynthesis.
Salvaging glycolate is an energetically expensive process that uses the glycolate pathway, and only 75% of the carbon is returned to the Calvin-Benson cycle as 3-phosphoglycerate. The reactions also produceammonia (NH3), which is able todiffuse out of the plant, leading to a loss of nitrogen.
The salvaging pathway for the products of RuBisCO oxygenase activity is more commonly known as photorespiration, since it is characterized by light-dependent oxygen consumption and the release of carbon dioxide.
^Reece, Jane B.; Urry, Lisa A.; Cain, Michael L.; Wasserman, Steven A.; Minorsky, Peter V.; Jackson, Robert B.; Campbel, Neil A. (2011).Biology (International ed.). Upper Saddle River, NJ:Pearson Education. pp. 235, 244.ISBN978-0-321-73975-9.This initial incorporation of carbon into organic compounds is known as carbon fixation
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