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| − | Photosynthesis. Green plants exposed to sunlight at a growing temperature are able to manufacture organic food substances, that is, carbohydrates. The term photosynthesis, derived from Greek words signifying "light" and "putting together," is applied to this process of food manufacture. Green plants manufacture not only their own food carbohydrates but also are the sources of practically all of the organic matter which may eventually furnish food for botn plants and
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| − | animals. It may be said, therefore, that life today is dependent upon the green leaf. The first carbon-containing compound made is a relatively simple substance, but the first recognizable material is sugar. The crude materials out of which organic substance is made in the cells of the green tissues are CO2 (carbon dioxide) and water. The leaf green, chlorophyl, and the protoplasm of the cell may be regarded as the important mechanism, while the source of energy for the chemical change induced is radiant energy, light. Air ordinarily contains about .03 per cent of CO2, yet the ordinary green plant obtains all of its carbon for the making of organic matter from this extremely small quantity in the atmosphere. The chlorophyl is important inasmuch as it absorbs the radiant energy which is directly or indirectly responsible for the process. Chlorophyl is distributed within the cells in definite granules, or small bodies, protoplasmic in nature, commonly ovoidal in form. The light absorbed is largely from the red or red-orange portion of the spectrum. It is possible that the energy so derived is first transformed into electrical energy, yet little is known upon this point. It is certain, however, that green plants are unable to utilize energy derived, for example, from the absorption of heat. The process may be briefly pictured in the following manner: The cell-sap absorbs the CO2 which diffuses into the tissues from the air. By means of the energy absorbed by the chlorophyl bodies, within the cells, the CO2 is supposed to be reduced to CO (carbon monoxide), and the same means resolves the water into its constituents. The products of these molecular changes form new substances, perhaps formaldehyde (CH2O) and oxygen (O2). The formation of formaldehyde is still somewhat uncertain; but in any case sugar is soon recognized. In all probability the formaldehyde molecules are immediately condensed to sugar (C6H12O6). It will be noted that the surplus oxygen is in reality a by-product and during active photosynthesis it is produced in such quantity as to be actively eliminated from the plant by diffusion. The usual test for photosynthesis is carried out by counting the bubbles given off from the cut stem of a water plant exposed to sunlight in a well-aerated vessel of spring-water. The content of oxygen in these bubbles is greater than that of normal air, and the rate of bubble-production is a fair estimate of the rate of photo-synthesis.
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| − | As a rule the sugar formed in the leaf does not accumulate to any large extent, but is transformed into starch. Some of the sugar, however, may be immediately diffused to other cells or "transported," supplying the needs of this substance in growth. The starch which is deposited is in the form of insoluble granules, and the formation of these bodies on exposure of the green leaf to sunlight is so rapid as to make it possible in some cases to use starch formation as an index to rate of photosynthesis. During the night, when no photosynthesis occurs, the transformation and removal of the starch usually goes on rapidly, so that within an interval of twelve hours most of that formed during the day seems to have disappeared from the leaf. It is, in fact, changed to sugar prior to transportation but may be removed to other organs of the plant, as, for example, to fleshy roots or tubers, where it may again be converted into starch, accumulating at times to a very considerable extent.
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| − | Photosynthesis is most rapid under those conditions of temperature which are favorable for growth. Under strong light and favorable temperature, however, a slight increase in the amount of CO2 gives a higher rate of starch-production. The presence in the leaf or stem of other color bodies, such as browns and reds, is no indication that chlorophyl is absent. As a matter of fact, chlorophyl is generally present in such cases, but may be veiled by the more prominent color. In showy flowers, however, chlorophyl seldom occurs. Pho
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| − | tosynthesis is inhibited by any condition affecting the general health of the plant, and it is low during cold and dark weather. The larger number of plants are most active in the brightest sunlight, but certain shade-loving species are injured by such exposures, and are adjusted to conditions of half-shade, such as obtain in the shade of trees or bushes. B.M.Duggar
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| | [[Image:Leaf 1 web.jpg|thumb|right|300px|The [[leaf]] is the primary site of photosynthesis in plants.]] | | [[Image:Leaf 1 web.jpg|thumb|right|300px|The [[leaf]] is the primary site of photosynthesis in plants.]] |
| − | '''Photosynthesis''' ''(photo=light, synthesis=putting together)'', generally, is the synthesis of [[glucose]] from [[sunlight]], [[carbon dioxide]] and [[water]], with [[oxygen]] as a waste product. It is arguably the most important biochemical pathway known; nearly all life depends on it. It is an extremely complex process, comprised of many coordinated [[biochemical]] reactions. It occurs in higher [[plant]]s, [[phytoplankton]], [[algae]], some [[bacterium|bacteria]], and some [[protist]]s, organisms collectively referred to as [[photoautotroph]]s.
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| − | ==Overview==
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| − | Photosynthesis uses energy of light to make the [[sugar]] called [[glucose]]. A simple general [[chemical equation|equation]] for photosynthesis follows.<ref>Brown, LeMay, Burslen, ''Chemistry The Central Science'', ISBN 0-13-048450-4, p. 958</ref>
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| − | :<center>''<b>6 CO<sub>2(gas)</sub> + 12 H<sub>2</sub>O<sub>(liquid)</sub> + [[photons]] → C<sub>6</sub>H<sub>12</sub>O<sub>6(aqueous)</sub> + 6 O<sub>2(gas)</sub> + 6 H<sub>2</sub>O<sub>(liquid)</sub>''</center>
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| − | :<center>carbon dioxide + water + light energy → glucose + oxygen + water</center>
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| − | Photosynthesis occurs in two stages. In the first phase '''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 phase, 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 [[carbon dioxide]] (CO<sub>2</sub>) and make the precursors of glucose.
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| − | In the [[light-dependent reaction]]s one molecule of the pigment [[chlorophyll]] absorbs one [[photon]] and loses one [[electron]]. This electron excites [[pheophytin]] allowing the start of a flow of electrons down an [[electron transport chain]] that leads to the ultimate reduction of [[NADP]] into [[NADPH]]. In addition, it serves to create 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 by taking one from a [[water]] molecule through a process called [[photolysis]], that releases [[oxygen]] gas as a waste product.
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| − | 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 [[glucose]].
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| − | Photosynthesis may simply be defined as the conversion of light energy into chemical energy by living organisms. It is affected by its surroundings and the rate of photosynthesis is affected by the concentration of carbon dioxide, the intensity of light, and the temperature.
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| − | ===In plants===
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| − | Most plants are [[photoautotroph]]s, which means that they are able to synthesize food directly from inorganic compounds 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.
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| − | 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 a waste 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. 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{{Fact|date=March 2007}}
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| − | :''n'' CO<sub>2</sub> + ''2n'' H<sub>2</sub>O + ATP + NADPH → (CH<sub>2</sub>O)''<sub>n</sub>'' + ''n'' H<sub>2</sub>O + ''n'' O<sub>2</sub>,
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| − | :<small>Where '''''n''''' is defined according to the structure of the resulting carbohydrate. </small>
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| − | More specifically, carbon fixation produces an intermediate product, which is then converted to the final hexose carbohydrate products. These carbohydrate products 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 not only occurs in plants, but also in [[animal]]s when the energy from plants get passed through a [[food chain]]. Organisms dependent on photosynthetic and [[chemosynthesis|chemosynthetic]] organisms are called heterotrophs. In general outline, cellular respiration is the opposite of photosynthesis: glucose and other compounds are oxidised to produce carbon dioxide, water, and chemical energy. However, both processes actually take place through a different sequence of reactions and in different cellular compartments.
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| − | 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]], contain about half a million 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.
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| − | === In algae and bacteria ===
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| − | Algae is a range from multicellular forms like [[kelp]], to [[microscope|microscopic]], single-celled organisms. Although they are not as complex as land plants, photosynthesis takes place biochemically the same way. Very much like plants, algae have chloroplasts and chlorophyll, but various accessory pigments are present in some algae such as phycoerythrin in red algae (rhodophytes), resulting in a wide variety of colors. All algae produce oxygen, and many are autotrophic. However, some are [[heterotroph]]ic, relying on materials produced by other organisms. For example, in coral reefs, there is a symbiotic relationship between [[zooxanthella]]e and the coral polyps.
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| − | Photosynthetic bacteria do not have chloroplasts (or any membrane-bound organelles). Instead, photosynthesis takes place directly within the cell. [[Cyanobacteria]] contain thylakoid membranes 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 and later gave rise to 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.
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| − | ==The evolution of photosynthesis==
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| − | 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|that time]]. {{Fact|date=February 2007}}
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| − | Fossils have been found of what are thought to be filamentous photosynthetic organisms dating from 3.4 billion years ago ([http://www.newscientist.com/article/mg19125654.200.html ''New Scientist'', 19 Aug., 2006]).
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| − | The [[oxygen]] in the [[Earth's atmosphere|atmosphere]] today 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 centre]].
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| − | === Origin of chloroplasts ===
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| − | In plants the process of photosynthesis is compartmentalized in [[organelle]]s called [[chloroplasts]]. Chloroplasts have many similarities with [[cyanobacteria|photosynthetic bacteria]] including a circular [[chromosome]], prokaryotic-type [[ribosomes]], and similar proteins in the photosynthetic reaction centre.
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| − | The [[endosymbiotic theory]] suggests that photosynthetic bacteria were acquired (by endocytosis or fusion) by early [[eukaryotic]] cells to form the first [[plant]] cells. In other words, chloroplasts may simply be primitive photosynthetic bacteria adapted to life inside plant cells, while plants themselves have not actually evolved photosynthetic processes on their own. Another example of this can be found in complex 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.
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| − | This contention is supported by the finding that the marine molluscs [[Elysia viridis]] and [[Elysia chlorotica]] seem to maintain a biological relationship with chloroplasts from algae that they ingest. However, they do not transfer these chloroplasts to the next generations.
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| − | == Molecular production ==
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| − | ====Light to chemical energy====
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| − | {{main|Light-dependent reaction}}
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| | [[Image:Photosystems.png|thumb|right|250px|A Photosystem: A '''light-harvesting''' cluster of photosynthetic pigments present in the thylakoid membrane of chloroplasts.]] | | [[Image:Photosystems.png|thumb|right|250px|A Photosystem: A '''light-harvesting''' cluster of photosynthetic pigments present in the thylakoid membrane of chloroplasts.]] |
| − | <!-- Unsourced image removed: [[Image:NewZScheme.jpg|thumb|right|250px|The Z scheme]] -->
| + | Photosynthesis. Green plants exposed to sunlight at a growing temperature are able to manufacture organic food substances, that is, carbohydrates. The term photosynthesis, derived from Greek words signifying "light" and "putting together," is applied to this process of food manufacture. Green plants manufacture not only their own food carbohydrates but also are the sources of practically all of the organic matter which may eventually furnish food for botn plants and animals. It may be said, therefore, that life today is dependent upon the green leaf. The first carbon-containing compound made is a relatively simple substance, but the first recognizable material is sugar. The crude materials out of which organic substance is made in the cells of the green tissues are CO2 (carbon dioxide) and water. The leaf green, chlorophyl, and the protoplasm of the cell may be regarded as the important mechanism, while the source of energy for the chemical change induced is radiant energy, light. Air ordinarily contains about .03 per cent of CO2, yet the ordinary green plant obtains all of its carbon for the making of organic matter from this extremely small quantity in the atmosphere. The chlorophyl is important inasmuch as it absorbs the radiant energy which is directly or indirectly responsible for the process. Chlorophyl is distributed within the cells in definite granules, or small bodies, protoplasmic in nature, commonly ovoidal in form. The light absorbed is largely from the red or red-orange portion of the spectrum. It is possible that the energy so derived is first transformed into electrical energy, yet little is known upon this point. It is certain, however, that green plants are unable to utilize energy derived, for example, from the absorption of heat. The process may be briefly pictured in the following manner: The cell-sap absorbs the CO2 which diffuses into the tissues from the air. By means of the energy absorbed by the chlorophyl bodies, within the cells, the CO2 is supposed to be reduced to CO (carbon monoxide), and the same means resolves the water into its constituents. The products of these molecular changes form new substances, perhaps formaldehyde (CH2O) and oxygen (O2). The formation of formaldehyde is still somewhat uncertain; but in any case sugar is soon recognized. In all probability the formaldehyde molecules are immediately condensed to sugar (C6H12O6). It will be noted that the surplus oxygen is in reality a by-product and during active photosynthesis it is produced in such quantity as to be actively eliminated from the plant by diffusion. The usual test for photosynthesis is carried out by counting the bubbles given off from the cut stem of a water plant exposed to sunlight in a well-aerated vessel of spring-water. The content of oxygen in these bubbles is greater than that of normal air, and the rate of bubble-production is a fair estimate of the rate of photo-synthesis. |
| − | The light energy is converted to chemical energy using the [[light-dependent reaction]]s. The products of the light dependent reactions are [[adenosine triphosphate|ATP]] from [[photophosphorylation]] and [[NADPH]] from photoreduction. Both are then utilized as an energy source for the light-independent reactions. | |
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| − | =====Z scheme=====
| + | As a rule the sugar formed in the leaf does not accumulate to any large extent, but is transformed into starch. Some of the sugar, however, may be immediately diffused to other cells or "transported," supplying the needs of this substance in growth. The starch which is deposited is in the form of insoluble granules, and the formation of these bodies on exposure of the green leaf to sunlight is so rapid as to make it possible in some cases to use starch formation as an index to rate of photosynthesis. During the night, when no photosynthesis occurs, the transformation and removal of the starch usually goes on rapidly, so that within an interval of twelve hours most of that formed during the day seems to have disappeared from the leaf. It is, in fact, changed to sugar prior to transportation but may be removed to other organs of the plant, as, for example, to fleshy roots or tubers, where it may again be converted into starch, accumulating at times to a very considerable extent. |
| − | 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 excitat
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| − | ion 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 while [[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 acceptors. 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 only generates 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.
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| − | =====Water photolysis=====
| + | Photosynthesis is most rapid under those conditions of temperature which are favorable for growth. Under strong light and favorable temperature, however, a slight increase in the amount of CO2 gives a higher rate of starch-production. The presence in the leaf or stem of other color bodies, such as browns and reds, is no indication that chlorophyl is absent. As a matter of fact, chlorophyl is generally present in such cases, but may be veiled by the more prominent color. In showy flowers, however, chlorophyl seldom occurs. Photosynthesis is inhibited by any condition affecting the general health of the plant, and it is low during cold and dark weather. The larger number of plants are most active in the brightest sunlight, but certain shade-loving species are injured by such exposures, and are adjusted to conditions of half-shade, such as obtain in the shade of trees or bushes. B.M.Duggar |
| − | 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 I are replaced from the electron transport chain by [[plastocyanin]]. However, since 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 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.
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| − | =====Oxygen and photosynthesis=====
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| − | With respect to oxygen and photosynthesis, there are two important concepts.
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| − | * Plant and cyanobacterial (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.
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| − | * Oxygen is a ''product of the light-driven water-oxidation reaction catalyzed by photosystem II''; it is not generated by the fixation of carbon dioxide. Consequently, the source of oxygen during photosynthesis is water, not carbon dioxide.
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| − | =====Bacterial variation=====
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| − | 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 r | |
| − | educing 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.
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| − | Others, such as the halophiles (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 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.
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| − | ===Carbon fixation ===
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| − | {{main|Carbon fixation|Light-independent reaction}}
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| − | 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]].
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| − | ====C<sub>4</sub> and CAM ====
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| − | In hot and dry conditions, plants will close their [[stomata]] (pores used for gas exchange) 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 devised mechanisms to increase the CO<sub>2</sub> concentration in the leaves under these conditions.
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| − | [[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 '''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.
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| − | [[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.
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| − | '''Stomata''
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| − | ' are small openings on the undersides of leaves that allow carbon dioxide to enter.
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| − | == Discovery ==
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| − | Although some of the steps in photosynthesis are still not completely understood, the overall photosynthetic equation has been known since the [[1800s]].
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| − | [[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]] comes from the inputs of photosynthesis, not the soil itself.
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| − | [[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.
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| − | In [[1778]], [[Jan Ingenhousz]], court physician to the [[Austria]]n Empress, repeated Priestley's experiments. He discovered that it was the influence of sun and light on the plant that could cause it to rescue a mouse in a matter of hours.
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| − | In [[1796]], [[Jean Senebier]], a Swiss pastor, botanist, and naturalist who 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.
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| − | Modern scientists built on the foundation of knowledge from those scientists centuries ago and were able to discover many things.
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| − | [[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.
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| − | 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:
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| − | :2 H<sub>2</sub>O + 2 A + (light, chloroplasts) → 2 AH<sub>2</sub> + O<sub>2</sub>
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| − | where A is the electron acceptor. Therefore, in light the electron acceptor is reduced and oxygen is evolved.
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| − | [[Sam Ruben|Samuel Ruben]] and [[Martin Kamen]] used radioactive isotopes to determine that the oxygen liberated in photosynthesis came from the water.
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| − | [[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.
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| − | A [[Nobel Prize]] winning scientist, [[Rudolph A. Marcus]], was able to discover the function and significance of the electron transport chain.
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| − | ==Factors affecting photosynthesis==
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| − | There are three main factors affecting photosynthesis and several corollary factors. The three main are:
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| − | * Light [[irradiance]] and [[wavelength]]
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| − | * [[Carbon dioxide]] [[concentration]]
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| − | * [[Temperature]]
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| − | === Light intensity (Irradiance), wavelength and temperature ===
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| − | 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.
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| − | * 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.
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| − | * 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.
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| − | 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. Secondly, 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]].
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| − | === Carbon dioxide levels and Photorespiration===
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| − | 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
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| − | RuBisCO oxygenase activity is disadvantageous to plants for several reasons:
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| − | # 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]].
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| − | # Phosphoglycolate is quickly metabolized to glycolate that is toxic to a plant at a high concentration; it inhibits photosynthesis.
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| − | # 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.
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| − | ::A highly simplified summary is:
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| − | :::2 glycolate + ATP → 3-phophoglycerate + carbon dioxide + ADP +NH<sub>3</sub>
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| − | 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.
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| − | === Corollary factors ===
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| − | {{section-stub}}
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| − | ==References==
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| − | * Blankenship, R.E., 2002. ''Molecular Mechanisms of Photosynthesis''. Blackwell Science.
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| − | * Campbell, N., & Reece, J., 2005. ''Biology'' 7th ed. San Francisco: Benjamin Cummings.
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| − | * Gregory, R.P.F., 1971. ''Biochemistry of Photosynthesis''. Belfast: Universities Press.
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| − | * Govindjee, 1975. ''Bioenergetics of Photosynthesis''. New York: Academic Press.
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| − | * Govindjee; Beatty, J.T., Gest, H. and Allen, J.F. (Eds.), 2005. Discoveries in Photosynthesis. ''Advances in Photosynthesis and Respiration'', Volume 20, Springer.
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| − | * Rabinowitch, E. and Govindjee., 1969. ''Photosynthesis''.New York: John Wiley & Sons, Inc.
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| − | * Stern, Kingsley R., Shelley Jansky, James E Bidlack, 2003. ''Introductory Plant Biology''. McGraw Hill. ISBN 0-07-290941-2
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| − | ==Notes==
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| − | <div class="references-small">
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| − | <references />
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| − | | |
| − | </div>
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| − | ==See also==
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| − | * [[Artificial photosynthesis]]
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| − | * [[Calvin-Benson cycle]]
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| − | * [[Cellular respiration]]
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| − | * [[Photosynthetic reaction center]]
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| − | * [[Photoinhibition]]
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| − | ==External links==
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| − | * [http://www.ljmu.ac.uk/NewsCentre/63012.htm Liverpool John Moores University, Dr.David Wilkinson]
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| − | * [http://www.biochemweb.org/metabolism.shtml Metabolism, Cellular Respiration and Photosynthesis - The Virtual Library of Biochemistry and Cell Biology]
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| − | * [http://www.chemsoc.org/networks/learnnet/cfb/Photosynthesis.htm Overall examination of Photosynthesis at an intermediate level]
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| − | * [http://www.life.uiuc.edu/govindjee/photosynBook.html Overall Energetics of Photosynthesis]
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| − | [[Category:Biochemistry]]
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| − | [[Category:Botany]]
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| − | [[Category:Photosynthesis|Photosynthesis]]
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| − | [[Category:Plant physiology]]
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| − | [[Category:Metabolism]]
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| − | [[Category:Agronomy]]
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