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Photosynthesis is a process used by plants and other organisms to convert light energy, normally from the Sun, into chemical energy that can be later released to fuel the organisms' activities. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water – hence the name photosynthesis, from the Greek φῶς, ''phōs'', "light", and σύνθεσις, ''synthesis'', "putting together".〔(【引用サイトリンク】url=http://www.etymonline.com/index.php?term=photosynthesis&allowed_in_frame=0 )〕 In most cases, oxygen is also released as a waste product. Most plants, most algae, and cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. Photosynthesis maintains atmospheric oxygen levels and supplies all of the organic compounds and most of the energy necessary for life on Earth. Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins called reaction centres that contain green chlorophyll pigments. In plants, these proteins are held inside organelles called chloroplasts, which are most abundant in leaf cells, while in bacteria they are embedded in the plasma membrane. In these light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas. Furthermore, two further compounds are generated: reduced nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), the "energy currency" of cells. In plants, algae and cyanobacteria, sugars are produced by a subsequent sequence of light-independent reactions called the Calvin cycle, but some bacteria use different mechanisms, such as the reverse Krebs cycle. In the Calvin cycle, atmospheric carbon dioxide is incorporated into already existing organic carbon compounds, such as ribulose bisphosphate (RuBP). Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then reduced and removed to form further carbohydrates, such as glucose. The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents, such as hydrogen or hydrogen sulfide, as sources of electrons, rather than water. Cyanobacteria appeared later; the excess oxygen they produced contributed to the oxygen catastrophe, which rendered the evolution of complex life possible. Today, the average rate of energy capture by photosynthesis globally is approximately 130 terawatts, which is about three times the current power consumption of human civilization. Photosynthetic organisms also convert around 100–115 thousand million metric tonnes of carbon into biomass per year. ==Overview== Photosynthetic organisms are photoautotrophs, which means that they are able to synthesize food directly from carbon dioxide and water using energy from light. However, not all organisms that use light as a source of energy carry out photosynthesis, since ''photoheterotrophs'' use organic compounds, rather than carbon dioxide, as a source of carbon.〔 In plants, algae and cyanobacteria, photosynthesis releases oxygen. This is called ''oxygenic photosynthesis''. Although there are some differences between oxygenic photosynthesis in plants, algae, and cyanobacteria, the overall process is quite similar in these organisms. However, there are some types of bacteria that carry out anoxygenic photosynthesis. These consume carbon dioxide but do not release oxygen. Carbon dioxide is converted into sugars in a process called carbon fixation. Carbon fixation is an endothermic redox reaction, so photosynthesis needs to supply both a source of energy to drive this process, and the electrons needed to convert carbon dioxide into a carbohydrate. This addition of the electrons is a reduction reaction. In general outline and in effect, photosynthesis is the opposite of cellular respiration, in which glucose and other compounds are oxidized to produce carbon dioxide and water, and to release exothermic chemical energy to drive the organism's metabolism. However, the two processes take place through a different sequence of chemical reactions and in different cellular compartments. The general equation for photosynthesis as first proposed by Cornelius van Niel is therefore: :CO2 + 2H2A + photons → () + 2A + H2O :carbon dioxide + electron donor + light energy → carbohydrate + oxidized electron donor + water Since water is used as the electron donor in oxygenic photosynthesis, the equation for this process is: :''n'' CO2 + 2''n'' H2O + photons → (CH2O)''n'' + ''n'' O2 + ''n'' H2O :carbon dioxide + water + light energy → carbohydrate + oxygen + water This equation emphasizes that water is both a reactant (in the light-dependent reaction) and a product (in the light-independent reaction), but canceling ''n'' water molecules from each side gives the net equation: : CO2 + 2 H2O + photons → CH2O + O2 :carbon dioxide + water + light energy → carbohydrate + oxygen Other processes substitute other compounds (such as arsenite) for water in the electron-supply role; for example some microbes use sunlight to oxidize arsenite to arsenate:〔''Anaerobic Photosynthesis'', Chemical & Engineering News, 86, 33, August 18, 2008, p. 36〕 The equation for this reaction is: :CO2 + (AsO33−) + photons → (AsO43−) + CO :carbon dioxide + arsenite + light energy → arsenate + carbon monoxide (used to build other compounds in subsequent reactions) Photosynthesis occurs in two stages. In the first stage, ''light-dependent reactions'' or ''light reactions'' capture the energy of light and use it to make the energy-storage molecules ATP and NADPH. During the second stage, the ''light-independent reactions'' use these products to capture and reduce carbon dioxide. Most organisms that utilize photosynthesis to produce oxygen use visible light to do so, although at least three use shortwave infrared or, more specifically, far-red radiation.〔(【引用サイトリンク】 url = http://www.bio-medicine.org/biology-news/Scientists-discover-unique-microbe-in-Californias-largest-lake-203-1/ )〕 Archaeobacteria use a simpler method using a pigment similar to the pigments used for vision. The archaearhodopsin 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. It seems to have evolved separately.〔(Plants: Diversity and Evolution ), page 14, Martin Ingrouille, Bill Eddie〕〔(Evolution of Photosynthesis )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「photosynthesis」の詳細全文を読む スポンサード リンク
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