Photosynthesis 24544 226051497 2008-07-16T16:43:20Z 72.138.189.157 {{Expert-subject|botany}} [[Image:Leaf 1 web.jpg|thumb|right|250px|The [[leaf]] is the primary site of [[photosynthesis]] in plants.]] '''Photosynthesis''' is a series of [[Enzyme catalysis|enzyme-catalyzed]] steps for the conversion of [[light]] [[energy]] into [[chemical energy]] by living [[organism]]s. Its initial [[Substrate (biochemistry)|substrates]] are [[carbon dioxide]] and [[water]]; the energy source is light ([[electromagnetic radiation]]); and the end-products are [[oxygen]] and (energy-containing) [[carbohydrate]]s, such as [[sucrose]], [[glucose]] or [[starch]]. This process is arguably the most important [[biochemical pathway]],<ref name=bryantfrigaard>{{cite journal |author=D.A. Bryant & N.-U. Frigaard |month=Nov |year=2006 |title=Prokaryotic photosynthesis and phototrophy illuminated |journal=Trends Microbiol. |volume=14 |issue=11 |pages=488 |doi=10.1016/j.tim.2006.09.001 }}</ref> since nearly all life on [[Earth]] either directly or indirectly depends on it. It is a complex process occurring in higher [[plant]]s, [[algae]], as well as [[bacteria]] such as [[cyanobacteria]]. Photosynthetic organisms are also referred to as ''[[photoautotroph]]s''.<ref name=bryantfrigaard/> The word comes from the [[Greek language|Greek]] ''{{polytonic|φώτο-}} (photo-)'', "light," and ''{{polytonic|σύνθεσις}} ([[synthesis]])'', "placing with." ==Overview== [[Image:Simple photosynthesis overview.svg|thumb|Photosynthesis splits water to liberate O<sub>2</sub> and fixes CO<sub>2</sub> into sugar]] Photosynthesis uses light energy and carbon dioxide to make [[Glyceraldehyde 3-phosphate|triose phosphates]] (G3P). G3P is generally considered the first end-product of photosynthesis.{{Fact|date=June 2008}} It can be used as a source of metabolic energy, or combined and rearranged to form [[monosaccharide]] or [[disaccharide]] sugars, such as [[glucose]] or [[sucrose]], respectively, which can be transported to other cells, stored as insoluble [[polysaccharide]]s such as [[starch]], or converted to structural carbohydrates, such as [[cellulose]] or [[glucan]]s. A commonly used slightly simplified [[chemical equation|equation]] for photosynthesis is: :6 CO<sub>2(g)</sub> + 12 H<sub>2</sub>O<sub>(l)</sub> + [[photons]] → [[Glucose|C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>]]<sub>(aq)</sub> + 6 O<sub>2(g)</sub> + 6 H<sub>2</sub>O<sub>(l)</sub> :carbon dioxide + water + light energy → glucose + oxygen + water When written as a word equation, light energy appears above the arrow, as it is required for photosynthesis, but it is not a reactant. Here the monosaccharide, glucose, is shown as a product, although the biosynthetic pathways in plants produce [[disaccharide]]s. {{Fact|date=June 2008}} The equation is often presented in introductory chemistry texts in an even more simplified form as:<ref>Brown, LeMay, Burslen, ''Chemistry The Central Science'', ISBN 0-13-048450-4, p. 958</ref> :6 CO<sub>2(g)</sub> + 6 H<sub>2</sub>O<sub>(l)</sub> + [[photons]] → C<sub>6</sub>H<sub>12</sub>O<sub>6(aq)</sub> + 6 O<sub>2(g)</sub> Photosynthesis occurs in two stages. In the first stage, ''light-dependent reactions'' or ''photosynthetic reactions'' (also called the ''Light Reactions'') capture the energy of light and use it to make high-energy molecules. During the second stage, the ''light-independent reactions'' (also called the [[Calvin cycle|Calvin-Benson Cycle]], and formerly known as the ''Dark Reactions'') use the high-energy molecules to capture and [[Reduction (chemistry)|chemically reduce]] carbon dioxide (CO<sub>2</sub>) (also called [[carbon fixation]]) to make the [[Precursor (chemistry)|precursor]]s of [[carbohydrate]]s. In the [[light reaction]]s, one molecule of the [[pigment]] [[chlorophyll]] absorbs one [[photon]] and loses one [[electron]]. This electron is passed to a modified form of chlorophyll called [[pheophytin]], which passes the electron to a [[quinone]] molecule, allowing the start of a flow of electrons down an [[electron transport chain]] that leads to the ultimate reduction of [[NADP]] to [[NADPH]]. In addition, this creates a [[proton gradient]] across the [[chloroplast membrane]]; its dissipation is used by [[ATP Synthase]] for the concomitant synthesis of [[Adenosine triphosphate|ATP]]. The chlorophyll molecule regains the lost electron from a [[water]] molecule through a process called [[photolysis]], which releases a [[dioxygen]] (O<sub>2</sub>) molecule. In the [[Light-independent reaction|Light-independent]] or dark reactions the [[enzyme]] [[RuBisCO]] captures [[carbon dioxide|CO<sub>2</sub>]] from the [[Earth's atmosphere|atmosphere]] and in a process that requires the newly formed NADPH, called the Calvin-Benson Cycle releases three-carbon sugars, which are later combined to form sucrose and starch. Photosynthesis may simply be defined as the conversion of light energy into [[Chemical_thermodynamics#Chemical_energy|chemical energy]] by living [[organism]]s. It is affected by its surroundings, and the rate of photosynthesis is affected by the concentration of carbon dioxide in the air, the light intensity, and the [[temperature]]. Photosynthesis uses only 1% of the entire electromagnetic spectrum, and 2% of the visible spectrum.{{Fact|date=June 2008}} It has been estimated that the productivity of photosythesis is 115 petagrams (Pg, equals 10<sup>15</sup> grams or 10<sup>9</sup> metric tons).{{Fact|date=June 2008}} ===In plants=== Most plants are [[photoautotroph]]s, which means that they are able to [[Chemical synthesis|synthesize]] food directly from [[inorganic compound]]s using light energy - for example from the sun, instead of eating other organisms or relying on nutrients derived from them. This is distinct from [[chemoautotroph]]s that do ''not'' depend on light energy, but use energy from inorganic compounds. :6 CO<sub>2</sub> + 12 H<sub>2</sub>O → C<sub>6</sub>H<sub>12</sub>O<sub>6</sub> + 6 O<sub>2</sub> The energy for photosynthesis ultimately comes from absorbed [[photon]]s and involves a [[reducing agent]], which is [[water]] in the case of plants, releasing [[oxygen]] as [[Product (chemistry)|product]]. The light energy is converted to chemical energy (known as [[light-dependent reaction]]s), in the form of [[Adenosine triphosphate|ATP]] and [[NADPH]], which are used for synthetic reactions in photoautotrophs. The overall equation for the light-dependent reactions under the conditions of non-cyclic electron flow in green plants is:<ref name="Raven">{{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 = 124-127 | isbn = 0-7167-1007-2}}</ref> :2 H<sub>2</sub>O + 2 NADP<sup>+</sup> + 2 ADP + 2 P<sub>i</sub> + light → 2 NADPH + 2 H<sup>+</sup> + 2 ATP + O<sub>2</sub> Most notably, plants use the chemical energy to fix [[carbon dioxide]] into [[carbohydrate]]s and other organic compounds through [[light-independent reaction]]s. The overall equation for carbon fixation (sometimes referred to as carbon reduction) in green plants is:<ref name="Raven">{{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 = 128 | isbn = 0-7167-1007-2}}</ref> :3 CO<sub>2</sub> + 9 ATP + 6 NADPH + 6 H<sup>+</sup> → C<sub>3</sub>H<sub>6</sub>O<sub>3</sub>-phosphate + 9 ADP + 8 P<sub>i</sub> + 6 NADP<sup>+</sup> + 3 H<sub>2</sub>O To be more specific, carbon fixation produces an intermediate product, which is then converted to the final carbohydrate products. The carbon skeletons produced by photosynthesis are then variously used to form other organic compounds, such as the building material [[cellulose]], as precursors for [[lipid]] and [[amino acid]] biosynthesis, or as a fuel in [[cellular respiration]]. The latter occurs not only in plants but also in [[animal]]s when the energy from plants gets passed through a [[food chain]]. Organisms dependent on photosynthetic and [[chemosynthesis|chemosynthetic]] organisms are called [[heterotroph]]s. In general outline, cellular respiration is the opposite of photosynthesis: Glucose and other compounds are oxidized to produce carbon dioxide, water, and chemical energy. However, the two processes take place through a different sequence of chemical reactions and in different cellular compartments. Plants absorb light primarily using the [[pigment]] [[chlorophyll]], which is the reason that most plants have a green color. The function of chlorophyll is often supported by other [[accessory pigment]]s such as [[carotene]]s and [[xanthophyll]]s. Both chlorophyll and accessory pigments are contained in [[organelle]]s (compartments within the [[cell (biology)|cell]]) called [[chloroplast]]s. Although all cells in the green parts of a plant have chloroplasts, most of the energy is captured in the [[leaf|leaves]]. The cells in the interior tissues of a leaf, called the [[mesophyll]], can contain between 450,000 and 800,000 chloroplasts for every square millimeter of leaf. The surface of the leaf is uniformly coated with a water-resistant [[wax]]y [[Plant cuticle|cuticle]] that protects the leaf from excessive [[evaporation]] of water and decreases the absorption of [[ultraviolet]] or [[blue]] [[light]] to reduce [[heat]]ing. The transparent [[Leaf#Epidermis|epidermis]] layer allows light to pass through to the [[Leaf#Mesophyll|palisade]] mesophyll cells where most of the photosynthesis takes place. [[Plant]]s convert light into [[chemical energy]] with a maximum [[photosynthetic efficiency]] of approximately 6%.<ref name="What is the maximum efficiency with which photosynthesis can convert solar energy into biomass?"/><ref>[http://www.fao.org/docrep/w7241e/w7241e05.htm#1.2.1%20photosynthetic%20efficiency Chapter 1 - Biological energy production<!-- Bot generated title -->]</ref><ref name="Photosynthesis Outline"/> By comparison [[photovoltaic module|solar panel]]s convert light into [[electric energy]] at a photosynthetic efficiency of approximately 10-20%. Actual plant's photosynthetic efficiency varies with the frequency of the light being converted, light intensity, temperature and proportion of [[carbon dioxide|{{chem|CO|2}}]] in atmosphere. === In algae and bacteria === Algae come in multiple forms from multicellular organisms like [[kelp]], to [[microscopy|microscopic]], [[microorganism|single-cell organisms]]. Although they are not as complex as land plants, the biochemical process of photosynthesis is the same. Very much like plants, algae have chloroplasts and chlorophyll, but various [[accessory pigment]]s are present in some algae such as [[phycocyanin]], [[carotene]]s, and [[xanthophyll]]s in [[green algae]] and [[phycoerythrin]] in [[red algae]] (rhodophytes), resulting in a wide variety of colors. [[Brown algae]] and [[diatoms]] contain fucoxanthol as their primary pigment. All algae produce oxygen, and many are [[autotroph]]ic. However, some are heterotrophic, relying on materials produced by other organisms. For example, in [[coral reef]]s, there is a [[mutualism|mutualistic]] relationship between [[zooxanthella]]e and the coral [[polyp]]s.<ref>{{cite web | last = Natural History Museum | first = | title = ''AlgaeVision'' | url=http://www.nhm.ac.uk/research-curation/projects/algaevision/about-algae/about-algae.html | accessdate = 2007-06-13 }}</ref> Photosynthetic bacteria do not have chloroplasts (or any membrane-bound [[organelle]]s). Instead, photosynthesis takes place directly within the cell. [[Cyanobacteria]] contain [[thylakoid membrane]]s very similar to those in chloroplasts and are the only prokaryotes that perform oxygen-generating photosynthesis. In fact, chloroplasts are now considered to have [[evolution|evolved]] from an [[endosymbiosis|endosymbiotic]] bacterium, which was also an ancestor of cyanobacterium. The other photosynthetic bacteria have a variety of different pigments, called [[bacteriochlorophyll]]s, and do not produce oxygen. Some bacteria, such as ''[[Chromatium]]'', oxidize [[hydrogen sulfide]] instead of water for photosynthesis, producing [[sulfur]] as waste. All photosynthesizing organisms must be in the photic (light-receiving) zone, except for those near hydrothermal vents which give faint light. ==Evolution== [[Image:Chloroplasten.jpg|frame|Plant cells with visible chloroplasts.]] The ability to convert light energy to chemical energy confers a significant [[Natural selection|evolutionary advantage]] to living organisms. Early photosynthetic systems, such as those from [[Green sulfur bacteria|green]] and [[Purple sulfur bacteria|purple sulfur]] and [[Chloroflexi|green]] and [[purple bacteria|purple non-sulfur bacteria]], are thought to have been anoxygenic, using various molecules as [[electron donor]]s. Green and purple sulfur bacteria are thought to have used [[hydrogen]] and [[sulfur]] as an electron donor. Green nonsulfur bacteria used various [[amino acid|amino]] and other [[organic acid]]s. Purple nonsulfur bacteria used a variety of non-specific organic molecules. The use of these molecules is consistent with the geological evidence that the atmosphere was highly [[Reducing environment|reduced]] at [[History of Earth#The Hadean eon|that time]]. {{Fact|date=February 2007}} Fossils of what are thought to be [[Protein filament|filamentous]] photosynthetic organisms have been dated at 3.4 billion years old.<ref>[http://www.newscientist.com/article/mg19125654.200.html ''New Scientist'', 19 Aug., 2006]</ref> [[Oxygen]] in the [[Earth's atmosphere|atmosphere]] exists due to the evolution of [[Oxygen evolution|oxygenic photosynthesis]], sometimes referred to as the [[oxygen Catastrophe|oxygen catastrophe]]. Geological evidence suggests that oxygenic photosynthesis, such as that in [[cyanobacteria]], became important during the [[Paleoproterozoic]] era around 2 billion years ago. Modern photosynthesis in plants and most photosynthetic prokaryotes is oxygenic. Oxygenic photosynthesis uses water as an electron donor which is [[Redox|oxidized]] into molecular oxygen by the absorption of a [[photon]] by the [[photosynthetic reaction center]]. === Origin of chloroplasts === In plants the process of photosynthesis occurs in [[organelle]]s called [[chloroplast]]s. Chloroplasts have many similarities with [[cyanobacteria|photosynthetic bacteria]] including a circular [[chromosome]], prokaryotic-type [[ribosome]]s, and similar proteins in the photosynthetic reaction center. The [[endosymbiotic theory]] suggests that photosynthetic bacteria were acquired (by [[endocytosis]] or [[gene fusion]]) by early [[Eukaryote|eukaryotic]] cells to form the first [[plant]] cells. In other words, chloroplasts may simply be primitive photosynthetic bacteria adapted to life inside plant cells, whereas plants themselves have not actually evolved photosynthetic processes on their own. Another example of this can be found in complex plants and animals, including humans, whose cells depend upon [[mitochondria]] as their energy source; mitochondria are thought to have evolved from endosymbiotic bacteria, related to modern ''[[Rickettsia]]'' bacteria. Both chloroplasts and mitochondria actually have their own DNA, separate from the nuclear DNA of their animal or plant host cells. This contention is supported by the finding that the marine [[Mollusca|mollusc]]s [[Elysia viridis]] and [[Elysia chlorotica]] seem to maintain a [[symbiosis|symbiotic]] relationship with chloroplasts from algae with similar RDA structures that they encounter. However, they do not transfer these chloroplasts to the next generations. ===Cyanobacteria and the evolution of photosynthesis=== The biochemical capacity to use water as the source for electrons in photosynthesis evolved once, in a common ancestor of extant [[cyanobacteria]]. The geological record indicates that this transforming event took place early in our planet's history, at least 2450-2320 million years ago (Ma), and possibly much earlier. Geobiological interpretation of [[Archean]] (>2500 Ma) sedimentary rocks remains a challenge; available evidence indicates that life existed 3500 Ma, but the question of when oxygenic photosynthesis evolved continues to engender debate and research. A clear paleontological window on cyanobacterial [[evolution]] opened about 2000 Ma, revealing an already-diverse biota of blue-greens. [[Cyanobacteria]] remained principal primary producers throughout the [[Proterozoic Eon]] (2500-543 Ma), in part because the redox structure of the oceans favored photautotrophs capable of [[nitrogen fixation]]. [[Green algae]] joined blue-greens as major primary producers on continental shelves near the end of the [[Proterozoic]], but only with the [[Mesozoic]] (251-65 Ma) radiations of dinoflagellates, coccolithophorids, and diatoms did primary production in marine shelf waters take modern form. Cyanobacteria remain critical to marine ecosystems as primary producers in oceanic gyres, as agents of biological nitrogen fixation, and, in modified form, as the plastids of marine algae.<ref name=Herrero>{{cite book | author = Herrero A and Flores E (editor). | title = The Cyanobacteria: Molecular Biology, Genomics and Evolution | edition = 1st ed. | publisher = Caister Academic Press | year = 2008 | id = ISBN 978-1-904455-15-8 }}</ref> == Molecular production == [[Image:Thylakoid membrane.png|thumb|250px|left|Light-dependent reactions of photosynthesis at the thylakoid membrane]] ====Temporal Order==== The overall process of photosynthesis takes place in four stages. The first, energy transfer in antenna chlorophyll takes place in the femtosecond [1 femtosecond (fs) = 10,<sup>−15</sup> s] to picosecond [1 picosecond (ps) = 10<sup>−12</sup> s] time scale. The next phase, the transfer of electrons in photochemical reactions, takes place in the picosecond to nanosecond time scale [1 nanosecond (ns) = 10<sup>−9</sup> s]. The third phase, the electron transport chain and ATP synthesis, takes place on the microsecond [1 microsecond (μs) = 10<sup>−6</sup> s] to millisecond [1 millisecond (ms) = 10<sup>−3</sup> s) time scale. The final phase is carbon fixation and export of stable products and takes place in the millisecond to second time scale. The first three stages occur in the thylakoid membranes. ====Light to chemical energy==== {{main|Light-dependent reaction}} The light energy is converted to chemical energy using the [[light-dependent reaction]]s. This chemical energy production is about 5-6% efficient, with the majority of the light that strikes a plant reflected and not absorbed.<ref name="What is the maximum efficiency with which photosynthesis can convert solar energy into biomass?">{{cite journal |author=Zhu XG, Long SP, Ort DR |title=What is the maximum efficiency with which photosynthesis can convert solar energy into biomass? |journal=Curr. Opin. Biotechnol. |volume= 19|issue= | pages = 153|year=2008 |pmid=18374559 |doi=10.1016/j.copbio.2008.02.004}}</ref> However, of the energy that is absorbed, approximately 30-50% is captured as chemical energy.<ref>{{cite journal |author=Skillman JB |title=Quantum yield variation across the three pathways of photosynthesis: not yet out of the dark |journal=J. Exp. Bot. |volume= 59|issue= |pages= 1647|year=2008 |pmid=18359752 |doi=10.1093/jxb/ern029 |url=http://jexbot.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=18359752}}</ref> The products of the [[light-dependent reaction]]s are [[adenosine triphosphate|ATP]] from [[photophosphorylation]] and [[NADPH]] from photoreduction. Both are then utilized as an energy source for the [[light-independent reaction]]s. Not all [[wavelength]]s of light can support photosynthesis. The photosynthetic action spectrum depends on the type of [[accessory pigment]]s present. For example, in green plants, the [[action spectrum]] resembles the [[absorption spectrum]] for [[chlorophyll]]s and [[carotenoid]]s with peaks for violet-blue and red light. In red algae, the action spectrum overlaps with the absorption spectrum of [[phycobilin]]s for blue-green light, which allows these algae to grow in deeper waters that filter out the longer wavelengths used by green plants. The non-absorbed part of the light spectrum is what gives photosynthetic organisms their color (e.g., green plants, red algae, purple bacteria) and is the least effective for photosynthesis in the respective organisms. =====Z scheme===== [[Image:Photosystems.png|thumb|right|250px|A Photosystem: A '''light-harvesting''' cluster of photosynthetic pigments present in the thylakoid membrane of chloroplasts.]] [[Image:Z-scheme.png|thumb|250px|right|The "Z scheme"]] In plants, [[light-dependent reaction]]s occur in the [[thylakoid membrane]]s of the [[chloroplast]]s and use light energy to synthesize ATP and NADPH. The light-dependent reaction has two forms; cyclic and non-cyclic reaction. In the non-cyclic reaction, the [[photon]]s are captured in the light-harvesting [[antenna complex]]es of [[Photosystem|photosystem II]] by [[chlorophyll]] and other [[accessory pigments]] (see diagram at right). When a chlorophyll molecule at the core of the photosystem II reaction center obtains sufficient excitation energy from the adjacent antenna pigments, an electron is transferred to the primary electron-acceptor molecule, Pheophytin, through a process called [[Photoinduced charge separation]]. These electrons are shuttled through an [[Electron transfer chain|electron transport chain]], the so called '''''Z-scheme''''' shown in the diagram, that initially functions to generate a [[chemiosmotic potential]] across the membrane. An [[ATP synthase]] enzyme uses the chemiosmotic potential to make ATP during photophosphorylation, whereas [[NADPH]] is a product of the terminal [[redox]] reaction in the ''Z-scheme''. The electron enters the Photosystem I molecule. The electron is excited due to the light absorbed by the [[photosystem]]. A second electron carrier accepts the electron, which again is passed down lowering energies of [[electron acceptor]]s. The energy created by the electron acceptors is used to move hydrogen ions across the thylakoid membrane into the lumen. The electron is used to reduce the co-enzyme NADP, which has functions in the light-independent reaction. The cyclic reaction is similar to that of the non-cyclic, but differs in the form 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 back to photosystem I, from where it was emitted, hence the name ''cyclic reaction''. =====Water photolysis===== {{main|Photodissociation|Oxygen evolution}} The NADPH is the main [[reducing agent]] in chloroplasts, providing a source of energetic electrons to other reactions. Its production leaves chlorophyll with a deficit of electrons (oxidized), which must be obtained from some other reducing agent. The excited electrons lost from chlorophyll in [[Photosystem|photosystem I]] are replaced from the electron transport chain by [[plastocyanin]]. However, since [[Photosystem|photosystem II]] includes the first steps of the ''Z-scheme'', an external source of electrons is required to reduce its oxidized '''chlorophyll ''a''''' molecules. The source of electrons in green-plant and cyanobacterial photosynthesis is water. Two water molecules are oxidized by four successive charge-separation reactions by photosystem II to yield a molecule of diatomic [[oxygen]] and four [[hydrogen]] ions; the electron yielded in each step is transferred to a redox-active [[tyrosine]] residue that then reduces the photoxidized paired-chlorophyll ''a'' species called P680 that serves as the primary (light-driven) electron donor in the photosystem II reaction center. The oxidation of water is [[catalysis|catalyzed]] in photosystem II by a redox-active structure that contains four [[manganese]] ions; this [[Oxygen evolution|oxygen-evolving complex]] binds two water molecules and stores the four oxidizing equivalents that are required to drive the water-oxidizing reaction. Photosystem II is the only known biological [[enzyme]] that carries out this oxidation of water. The hydrogen ions contribute to the transmembrane chemiosmotic potential that leads to ATP synthesis. Oxygen is a waste product of light-independent reactions, but the majority of organisms on Earth use oxygen for [[cellular respiration]], including photosynthetic organisms. =====Quantum mechanical effects===== Through photosynthesis, sunlight energy is transferred to molecular reaction centers for conversion into chemical energy with nearly 100-percent efficiency. The transfer of the solar energy takes place almost instantaneously, so little energy is wasted as heat. However, only 43% of the total solar incident radiation can be used (only light in the range 400-700 nm), 20% of light is blocked by canopy, and plant respiration requires about 33% of the stored energy, which brings down the actual efficiency of photosynthesis to about 6.6%.<ref name="Photosynthesis Outline">University of Prince Edwards Island, Canada. [http://www.upei.ca/~physics/p261/Content/Sources_Conversion/Photo-_synthesis/photo-_synthesis.htm "Photosynthesis Outline"]. Accessed 2007-Nov-25.</ref> A study led by researchers with the [[U.S. Department of Energy]]’s [[Lawrence Berkeley National Laboratory]] (Berkeley Lab) and the [[University of California at Berkeley]] suggests that long-lived wavelike electronic [[quantum coherence]] plays an important part in this instantaneous transfer of energy by allowing the photosynthetic system to simultaneously try each potential energy pathway and choose the most efficient option. Results of the study are presented in the April 12, 2007 issue of the journal ''[[Nature (journal)|Nature]]''.<ref>Lawrence Berkeley National Lab. [http://www.physorg.com/news95605211.html "Quantum secrets of photosynthesis revealed"], ''physorg.com'', April 12, 2007. Accessed April 13, 2007.</ref> =====Oxygen and photosynthesis===== With respect to oxygen and photosynthesis, there are two important concepts. * Plant and [[cyanobacteria]]l (blue-green algae) cells ''also use oxygen'' for cellular respiration, although they have a net output of oxygen since much more is produced during photosynthesis. * Oxygen is a ''product of the light-driven water-oxidation reaction catalyzed by [[Photosystem|photosystem II]]''; it is not generated by the fixation of carbon dioxide. Consequently, the source of oxygen during photosynthesis is water, not carbon dioxide. =====Bacterial variation===== The concept that oxygen production is not directly associated with the fixation of carbon dioxide was first proposed by [[Cornelis Van Niel]] in the 1930s, who studied photosynthetic bacteria. Aside from the [[cyanobacteria]], bacteria only have one [[photosystem]] and use reducing agents other than water. They get electrons from a variety of different inorganic chemicals including [[sulfide]] or [[hydrogen]], so for most of these bacteria oxygen is not produced. Others, such as the [[halophile]]s (an [[Archaea]]), produced so-called purple membranes where the [[bacteriorhodopsin]] could harvest light and produce energy. The purple membranes was one of the first to be used to demonstrate the [[chemiosmosis|chemiosmotic]] theory: light hit the membranes and the pH of the solution that contained the purple membranes dropped as protons were pumping out of the membrane. [[Image:calvin-cycle3.png|thumb|left|250px|Overview of the Calvin cycle and carbon fixation]] ====Carbon fixation ==== {{main|Carbon fixation|Light-independent reaction}} The fixation or reduction of carbon dioxide is a light-independent process in which [[carbon dioxide]] combines with a five-carbon sugar, [[ribulose 1,5-bisphosphate]] (RuBP), to yield two molecules of a three-carbon compound, [[glycerate 3-phosphate]] (GP), also known as 3-phosphoglycerate (PGA). GP, in the presence of [[Adenosine triphosphate|ATP]] and [[NADPH]] from the light-dependent stages, is reduced to [[glyceraldehyde 3-phosphate]] (G3P). This product is also referred to as 3-phosphoglyceraldehyde ([[PGAL]]) or even as triose phosphate. [[Triose]] is a 3-carbon sugar (see [[carbohydrate]]s). Most (5 out of 6 molecules) of the G3P produced is used to regenerate RuBP so the process can continue (see [[Calvin-Benson cycle]]). The 1 out of 6 molecules of the triose phosphates not "recycled" often condense to form [[hexose]] phosphates, which ultimately yield [[sucrose]], [[starch]] and [[cellulose]]. The sugars produced during carbon [[metabolism]] yield carbon skeletons that can be used for other metabolic reactions like the production of [[amino acids]] and [[lipids]]. ====C<sub>4</sub>, C<sub>3</sub> and CAM ==== [[Image:HatchSlackpathway.png|thumb|right|250px|Overview of [[C4 carbon fixation]] ]] In hot and dry conditions, plants will close their [[stomata]] to prevent loss of water. Under these conditions, oxygen gas, produced by the light reactions of photosynthesis, will concentrate in the leaves causing [[photorespiration]] to occur. Some plants have [[evolution|evolved]] mechanisms to increase the CO<sub>2</sub> concentration in the leaves under these conditions. [[C4 carbon fixation|'''C<sub>4</sub> plants''']] capture carbon dioxide using an enzyme called [[PEP Carboxylase]] that adds carbon dioxide to the three carbon molecule [[Phosphoenolpyruvate|Phosphoenolpyruvate (PEP)]] creating the 4-carbon molecule [[oxaloacetic acid]]. Plants without this enzyme are called [[C3 carbon fixation|'''C<sub>3</sub> plants''']] because the primary carboxylation reaction produces the three-carbon sugar [[3-phosphoglycerate]] directly in the Calvin-Benson Cycle. When oxygen levels rise in the leaf, C4 plants reverse the reaction to release carbon dioxide thus preventing photorespiration. By preventing photorespiration, C<sub>4 </sub>plants can produce more sugar than C<sub>3</sub> plants in conditions of strong light and high temperature. Many important crop plants are C<sub>4</sub> plants including maize, sorghum, sugarcane, and millet. [[Xerophytes]] such as [[cacti]] and most [[succulents]] also can use PEP Carboxylase to capture carbon dioxide in a process called [[CAM photosynthesis|Crassulacean acid metabolism (CAM)]]. They store the CO<sub>2</sub> in different molecules than the C<sub>4</sub> plants (mostly they store it in the form of [[malic acid]] via carboxylation of [[phosphoenolpyruvate]] to oxaloacetate, which is then reduced to malate). Nevertheless, C<sub>4</sub> plants capture the CO<sub>2</sub> in one type of cell tissue ([[mesophyll]]) and then transfer it to another type of tissue (bundle sheath cells) so that carbon fixation may occur via the Calvin cycle. They also have a different leaf anatomy than C<sub>4</sub> plants. They grab the CO<sub>2</sub> at night, when their stomata are open, and they release it into the leaves during the day to increase their photosynthetic rate. C4 metabolism ''physically'' separates CO<sub>2</sub> fixation from the Calvin cycle, while CAM metabolism ''temporally'' separates CO<sub>2</sub> fixation from the Calvin cycle. == Discovery == Although some of the steps in photosynthesis are still not completely understood, the overall photosynthetic equation has been known since the 1800s. [[Jan van Helmont]] began the research of the process in the mid-1600s when he carefully measured the [[mass]] of the soil used by a plant and the mass of the plant as it grew. After noticing that the soil mass changed very little, he hypothesized that the mass of the growing plant must come from the water, the only substance he added to the potted plant. His hypothesis was partially accurate - much of the gained mass also comes from carbon dioxide as well as water. However, this was a signaling point to the idea that the bulk of a plant's [[Biomass (ecology)|biomass]] comes from the inputs of photosynthesis, not the soil itself. [[Joseph Priestley]], a chemist and minister, discovered that when he isolated a volume of air under an inverted jar, and burned a candle in it, the candle would burn out very quickly, much before it ran out of wax. He further discovered that a mouse could similarly "injure" air. He then showed that the air that had been "injured" by the candle and the mouse could be restored by a plant. In 1778, [[Jan Ingenhousz]], court physician to the [[Austria]]n Empress, repeated Priestley's experiments. He discovered that it was the influence of sunlight on the plant that could cause it to rescue a mouse in a matter of hours. In 1796, [[Jean Senebier]], a Swiss pastor, botanist, and naturalist, demonstrated that green plants consume carbon dioxide and release oxygen under the influence of light. Soon afterwards, [[Nicolas-Théodore de Saussure]] showed that the increase in mass of the plant as it grows could not be due only to uptake of CO<sub>2</sub>, but also to the incorporation of water. Thus the basic reaction by which photosynthesis is used to produce food (such as glucose) was outlined. [[Cornelis Van Niel]] made key discoveries explaining the chemistry of photosynthesis. By studying [[purple sulfur bacteria]] and green bacteria he was the first scientist to demonstrate that photosynthesis is a light-dependent [[redox]] reaction, in which hydrogen reduces carbon dioxide. Robert Emerson discovered two light reactions by testing plant productivity using different wavelgnths 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 aborbing up to 600 nm wavelengths, the other up to 700. 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.<ref> "Photosynthesis" McGraw Hill Encyclopedia of Science and Technology, 2007 vol. 13, p. 470 </ref> Further experiments to prove that the oxygen developed during the photosynthesis of green plants came from water, were performed by [[Robert Hill (plant biochemist)|Robert Hill]] in 1937 and 1939. He showed that isolated [[chloroplast]]s give off oxygen in the presence of unnatural reducing agents like [[iron]] [[oxalate]], [[ferricyanide]] or [[benzoquinone]] after exposure to light. The Hill reaction is as follows: :2 H<sub>2</sub>O + 2 A + (light, chloroplasts) → 2 AH<sub>2</sub> + O<sub>2</sub> where A is the electron acceptor. Therefore, in light the electron acceptor is reduced and oxygen is evolved. Cyt b<sub>6</sub>, now known as a plastoquinone, is one electron acceptor. [[Sam Ruben|Samuel Ruben]] and [[Martin Kamen]] used radioactive isotopes to determine that the oxygen liberated in photosynthesis came from the water. [[Melvin Calvin]] and [[Andrew Benson]], along with [[James Bassham]], elucidated the path of carbon assimilation (the photosynthetic carbon reduction cycle) in plants. The carbon reduction cycle is known as the [[Calvin cycle]], which inappropriately ignores the contribution of Bassham and Benson. Many scientists refer to the cycle as the Calvin-Benson Cycle, Benson-Calvin, and some even call it the Calvin-Benson-Bassham (or CBB) Cycle. A [[Nobel Prize]] winning scientist, [[Rudolph A. Marcus]], was able to discover the function and significance of the electron transport chain. ==Factors== There are three main factors affecting photosynthesis and several corollary factors. The three main are: * Light [[irradiance]] and [[wavelength]] * [[Carbon dioxide]] [[concentration]] * [[Temperature]]. === Light intensity (Irradiance), wavelength and temperature === In the early 1900s [[Frederick Blackman|Frederick Frost Blackman]] along with [[Gabrielle Matthaei]] investigated the effects of light intensity ([[irradiance]]) and temperature on the rate of carbon assimilation. * At constant temperature, the rate of carbon assimilation varies with irradiance, initially increasing as the irradiance increases. However at higher irradiance this relationship no longer holds and the rate of carbon assimilation reaches a plateau. * At constant irradiance, the rate of carbon assimilation increases as the temperature is increased over a limited range. This effect is only seen at high irradiance levels. At low irradiance, increasing the temperature has little influence on the rate of carbon assimilation. These two experiments illustrate vital points: firstly, from [[research]] it is known that [[photochemical]] reactions are not generally affected by [[temperature]]. 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 of course the [[Light-dependent reaction|light-dependent 'photochemical']] stage and the [[Light-independent reaction|light-independent, temperature-dependent]] stage. Second, Blackman's experiments illustrate the concept of [[limiting factor]]s. 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, a series of proteins with different pigments surround the reaction center. This unit is called a [[phycobilisome]]. === Carbon dioxide levels and photorespiration=== As carbon dioxide concentrations rise, the rate at which sugars are made by the [[light-independent reaction]]s 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 [[Carbon fixation|fix carbon dioxide]]. However, if the oxygen concentration is high, RuBisCO will bind oxygen instead of carbon dioxide. This process, called [[photorespiration]], uses energy, but does not make sugar. RuBisCO oxygenase activity is disadvantageous to plants for several reasons: # One product of oxygenase activity is [[phosphoglycolate]] (2 carbon) instead of [[3-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 the [[Calvin-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. ::A highly-simplified summary is: :::2 glycolate + ATP → 3-phophoglycerate + carbon dioxide + ADP +NH<sub>3</sub> 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. ==See also== <div style="-moz-column-count:2; column-count:2;"> * [[Artificial photosynthesis]] * [[Calvin-Benson cycle]] * [[Carbon fixation]] * [[Cellular respiration]] * [[Chemosynthesis]] * [[Light-dependent reaction]] * [[Photoinhibition]] * [[Photosynthetic reaction center]] * [[Photosynthetically active radiation]] * [[Quantum biology]] * [[Red edge]] </div> ==Notes== {{Refimprove|date=August 2007}} {{reflist}} ==References== * Blankenship, R.E., 2002. ''Molecular Mechanisms of Photosynthesis''. Blackwell Science. * Campbell, N., & Reece, J., 2005. ''Biology'' 7th ed. San Francisco: Benjamin Cummings. * Gregory, R.P.F., 1971. ''Biochemistry of Photosynthesis''. Belfast: Universities Press. * Govindjee, 1975. ''Bioenergetics of Photosynthesis''. New York: Academic Press. * Govindjee; Beatty, J.T., Gest, H. and Allen, J.F. (Eds.), 2005. Discoveries in Photosynthesis. ''Advances in Photosynthesis and Respiration'', Volume 20, Springer. * Rabinowitch, E. and Govindjee., 1969. ''Photosynthesis''. New York: John Wiley & Sons, Inc. * Stern, Kingsley R., Shelley Jansky, James E Bidlack, 2003. ''Introductory Plant Biology''. McGraw Hill. ISBN 0-07-290941-2 ==External links== {{Commonscat|Photosynthesis}} * [http://www.life.uiuc.edu/govindjee/linksPSed.htm A collection of photosynthesis pages for all levels from a renowned expert (Govindjee)] * [http://www.life.uiuc.edu/govindjee/paper/gov.html In depth, advanced treatment of photosynthesis, also from Govindjee] * [http://scienceaid.co.uk/biology/physiology/photosynthesis.html Science Aid: Photosynthesis] Article appropriate for high school science * [http://www.ljmu.ac.uk/NewsCentre/63012.htm Liverpool John Moores University, Dr.David Wilkinson] * [http://www.biochemweb.org/metabolism.shtml Metabolism, Cellular Respiration and Photosynthesis - The Virtual Library of Biochemistry and Cell Biology] * [http://www.chemsoc.org/networks/learnnet/cfb/Photosynthesis.htm Overall examination of Photosynthesis at an intermediate level] * [http://www.life.uiuc.edu/govindjee/photosynBook.html Overall Energetics of Photosynthesis] {{-}} {{Botany}} [[Category:Cellular respiration]] [[Category:Botany]] [[Category:Photosynthesis| ]] [[Category:Plant physiology]] [[Category:Metabolism]] [[Category:Agronomy]] {{Link FA|af}} {{Link FA|cs}} [[af:Fotosintese]] [[ar:تمثيل ضوئى]] [[zh-min-nan:Kng-ha̍p-sêng chok-iōng]] [[bs:Fotosinteza]] [[bg:Фотосинтеза]] [[ca:Fotosíntesi]] [[cs:Fotosyntéza]] [[cy:Ffotosynthesis]] [[da:Fotosyntese]] [[de:Photosynthese]] [[et:Fotosüntees]] [[el:Φωτοσύνθεση]] [[es:Fotosíntesis]] [[eo:Fotosintezo]] [[eu:Fotosintesi]] [[fa:نورساخت]] [[fr:Photosynthèse]] [[gl:Fotosíntese]] [[ko:광합성]] [[hi:प्रकाश संश्लेषण]] [[hr:Fotosinteza]] [[id:Fotosintesis]] [[ia:Photosynthesis]] [[is:Ljóstillífun]] [[it:Fotosintesi clorofilliana]] [[he:פוטוסינתזה]] [[pam:Photosynthesis]] [[kn:ದ್ಯುತಿಸಂಶ್ಲೇಷಣೆ]] [[la:Photosynthesis]] [[lv:Fotosintēze]] [[lt:Fotosintezė]] [[hu:Fotoszintézis]] [[mk:Фотосинтеза]] [[mr:प्रकाशसंश्लेषण]] [[ms:Fotosintesis]] [[mn:Фотосинтез]] [[nl:Fotosynthese]] [[ja:光合成]] [[no:Fotosyntese]] [[nn:Fotosyntese]] [[oc:Fotosintèsi]] [[nds:Photosynthees]] [[pl:Fotosynteza]] [[pt:Fotossíntese]] [[ro:Fotosinteză]] [[qu:Inti wayllay]] [[ru:Фотосинтез]] [[sq:Fotosinteza]] [[simple:Photosynthesis]] [[sk:Fotosyntéza]] [[sl:Fotosinteza]] [[sr:Фотосинтеза]] [[sh:Fotosinteza]] [[su:Potosintésis]] [[fi:Yhteyttäminen]] [[sv:Fotosyntes]] [[ta:ஒளிச்சேர்க்கை]] [[th:การสังเคราะห์ด้วยแสง]] [[vi:Quang hợp]] [[tr:Fotosentez]] [[uk:Фотосинтез]] [[ur:ضیائی تالیف]] [[zh-yue:光合作用]] [[zh:光合作用]]