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<p><ins class="ice-ins ice-cts-1" data-changedata="" data-cid="2" data-last-change-time="1521843186239" data-time="1521843182523" data-userid="162" data-username="Author"><b>Photosynthesis</b>&nbsp;is a process used by plants and other organisms to convert&nbsp;<a href="https://en.wikipedia.org/wiki/Light" title="Light">light</a>&nbsp;energy into&nbsp;<a href="https://en.wikipedia.org/wiki/Chemical_energy" title="Chemical energy">chemical energy</a>&nbsp;that can later be&nbsp;<a href="https://en.wikipedia.org/wiki/Cellular_respiration" title="Cellular respiration">released</a>&nbsp;to fuel the organisms&#39; activities (<a href="https://en.wikipedia.org/wiki/Energy_transformation" title="Energy transformation">energy transformation</a>). This chemical energy is stored in&nbsp;<a href="https://en.wikipedia.org/wiki/Carbohydrates" title="Carbohydrates">carbohydrate molecules</a>, such as&nbsp;<a href="https://en.wikipedia.org/wiki/Sugar" title="Sugar">sugars</a>, which are synthesized from&nbsp;<a href="https://en.wikipedia.org/wiki/Carbon_dioxide" title="Carbon dioxide">carbon dioxide</a>&nbsp;and&nbsp;<a href="https://en.wikipedia.org/wiki/Water" title="Water">water</a>&nbsp;&ndash; hence the name&nbsp;<i>photosynthesis</i>, from the&nbsp;<a href="https://en.wikipedia.org/wiki/Greek_language" title="Greek language">Greek</a>&nbsp;<a href="https://en.wiktionary.org/wiki/%CF%86%E1%BF%B6%CF%82" title="wiktionary:φῶς">&phi;ῶ&sigmaf;</a>,&nbsp;<i>phōs</i>, &quot;light&quot;, and&nbsp;<a href="https://en.wiktionary.org/wiki/%CF%83%CF%8D%CE%BD%CE%B8%CE%B5%CF%83%CE%B9%CF%82" title="wiktionary:σύνθεσις">&sigma;ύ&nu;&theta;&epsilon;&sigma;&iota;&sigmaf;</a>,&nbsp;<i>synthesis</i>, &quot;putting together&quot;.<sup id="cite_ref-OnlineEtDict_1-0"><a href="https://en.wikipedia.org/wiki/Photosynthesis#cite_note-OnlineEtDict-1">[1]</a></sup><sup id="cite_ref-LSJ1_2-0"><a href="https://en.wikipedia.org/wiki/Photosynthesis#cite_note-LSJ1-2">[2]</a></sup><sup id="cite_ref-LSJ2_3-0"><a href="https://en.wikipedia.org/wiki/Photosynthesis#cite_note-LSJ2-3">[3]</a></sup>&nbsp;In most cases,&nbsp;<a href="https://en.wikipedia.org/wiki/Oxygen" title="Oxygen">oxygen</a>&nbsp;is also released as a waste product. Most&nbsp;<a href="https://en.wikipedia.org/wiki/Plant" title="Plant">plants</a>, most&nbsp;<a href="https://en.wikipedia.org/wiki/Algae" title="Algae">algae</a>, and&nbsp;<a href="https://en.wikipedia.org/wiki/Cyanobacteria" title="Cyanobacteria">cyanobacteria</a>&nbsp;perform photosynthesis; such organisms are called&nbsp;<a href="https://en.wikipedia.org/wiki/Photoautotroph" title="Photoautotroph">photoautotrophs</a>. Photosynthesis is largely responsible for producing and maintaining the&nbsp;<a href="https://en.wikipedia.org/wiki/Atmospheric_oxygen" title="Atmospheric oxygen">oxygen content</a>&nbsp;of the Earth&#39;s atmosphere, and supplies all of the organic compounds and most of the energy necessary for&nbsp;<a href="https://en.wikipedia.org/wiki/Life" title="Life">life on Earth</a>.<sup id="cite_ref-bryantfrigaard_4-0"><a href="https://en.wikipedia.org/wiki/Photosynthesis#cite_note-bryantfrigaard-4">[4]</a></sup></ins></p>

<p><ins class="ice-ins ice-cts-1" data-changedata="" data-cid="2" data-last-change-time="1521843186239" data-time="1521843182523" data-userid="162" data-username="Author">Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by&nbsp;<a href="https://en.wikipedia.org/wiki/Protein" title="Protein">proteins</a>&nbsp;called&nbsp;<a href="https://en.wikipedia.org/wiki/Photosynthetic_reaction_centre" title="Photosynthetic reaction centre">reaction centres</a>&nbsp;that contain green&nbsp;<a href="https://en.wikipedia.org/wiki/Chlorophyll" title="Chlorophyll">chlorophyll</a>&nbsp;pigments. In plants, these proteins are held inside&nbsp;<a href="https://en.wikipedia.org/wiki/Organelle" title="Organelle">organelles</a>&nbsp;called&nbsp;<a href="https://en.wikipedia.org/wiki/Chloroplast" title="Chloroplast">chloroplasts</a>, which are most abundant in leaf cells, while in bacteria they are embedded in the&nbsp;<a href="https://en.wikipedia.org/wiki/Plasma_membrane" title="Plasma membrane">plasma membrane</a>. In these light-dependent reactions, some energy is used to strip&nbsp;<a href="https://en.wikipedia.org/wiki/Electron" title="Electron">electrons</a>&nbsp;from suitable substances, such as water, producing oxygen gas. The hydrogen freed by the splitting of water is used in the creation of two further compounds that serve as short-term stores of energy, enabling its transfer to drive other reactions: these compounds are reduced&nbsp;<a href="https://en.wikipedia.org/wiki/Nicotinamide_adenine_dinucleotide_phosphate" title="Nicotinamide adenine dinucleotide phosphate">nicotinamide adenine dinucleotide phosphate</a>&nbsp;(NADPH) and&nbsp;<a href="https://en.wikipedia.org/wiki/Adenosine_triphosphate" title="Adenosine triphosphate">adenosine triphosphate</a>&nbsp;(ATP), the &quot;energy currency&quot; of cells.</ins></p>

<p><ins class="ice-ins ice-cts-1" data-changedata="" data-cid="2" data-last-change-time="1521843186239" data-time="1521843182523" data-userid="162" data-username="Author">In plants, algae and cyanobacteria, long-term energy storage in the form of sugars is produced by a subsequent sequence of&nbsp;light-independent&nbsp;reactions called the&nbsp;<a href="https://en.wikipedia.org/wiki/Calvin_cycle" title="Calvin cycle">Calvin cycle</a>; some bacteria use different mechanisms, such as the&nbsp;<a href="https://en.wikipedia.org/wiki/Reverse_Krebs_cycle" title="Reverse Krebs cycle">reverse Krebs cycle</a>, to achieve the same end. In the Calvin cycle, atmospheric carbon dioxide is&nbsp;<a href="https://en.wikipedia.org/wiki/Carbon_fixation" title="Carbon fixation">incorporated</a>&nbsp;into already existing organic carbon compounds, such as&nbsp;<a href="https://en.wikipedia.org/wiki/Ribulose_bisphosphate" title="Ribulose bisphosphate">ribulose bisphosphate</a>&nbsp;(RuBP).<sup id="cite_ref-isbn0-321-73975-2_5-0"><a href="https://en.wikipedia.org/wiki/Photosynthesis#cite_note-isbn0-321-73975-2-5">[5]</a></sup>&nbsp;Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then&nbsp;<a href="https://en.wikipedia.org/wiki/Biological_reductant" title="Biological reductant">reduced</a>&nbsp;and removed to form further carbohydrates, such as&nbsp;<a href="https://en.wikipedia.org/wiki/Glucose" title="Glucose">glucose</a>.</ins></p>

<p><ins class="ice-ins ice-cts-1" data-changedata="" data-cid="2" data-last-change-time="1521843186239" data-time="1521843182523" data-userid="162" data-username="Author">The first photosynthetic organisms probably&nbsp;<a href="https://en.wikipedia.org/wiki/Evolution" title="Evolution">evolved</a>&nbsp;early in the&nbsp;<a href="https://en.wikipedia.org/wiki/Evolutionary_history_of_life" title="Evolutionary history of life">evolutionary history of life</a>&nbsp;and most likely used&nbsp;<a href="https://en.wikipedia.org/wiki/Reducing_agent" title="Reducing agent">reducing agents</a>&nbsp;such as&nbsp;<a href="https://en.wikipedia.org/wiki/Hydrogen" title="Hydrogen">hydrogen</a>&nbsp;or&nbsp;<a href="https://en.wikipedia.org/wiki/Hydrogen_sulfide" title="Hydrogen sulfide">hydrogen sulfide</a>, rather than water, as sources of electrons.<sup id="cite_ref-pmid16453059_6-0"><a href="https://en.wikipedia.org/wiki/Photosynthesis#cite_note-pmid16453059-6">[6]</a></sup>&nbsp;Cyanobacteria appeared later; the excess oxygen they produced contributed directly to the&nbsp;<a href="https://en.wikipedia.org/wiki/Oxygen_catastrophe" title="Oxygen catastrophe">oxygenation of the Earth</a>,<sup id="cite_ref-pmid18468984_7-0"><a href="https://en.wikipedia.org/wiki/Photosynthesis#cite_note-pmid18468984-7">[7]</a></sup>&nbsp;which rendered the&nbsp;<a href="https://en.wikipedia.org/wiki/Evolution_of_multicellularity" title="Evolution of multicellularity">evolution of complex life</a>&nbsp;possible. Today, the average rate of energy capture by photosynthesis globally is approximately 130&nbsp;<a href="https://en.wikipedia.org/wiki/Terawatts" title="Terawatts">terawatts</a>,<sup id="cite_ref-pmid10670014_8-0"><a href="https://en.wikipedia.org/wiki/Photosynthesis#cite_note-pmid10670014-8">[8]</a></sup><sup id="cite_ref-Whitmarsh_Govindjee_1999_9-0"><a href="https://en.wikipedia.org/wiki/Photosynthesis#cite_note-Whitmarsh_Govindjee_1999-9">[9]</a></sup><sup id="cite_ref-Steger_2005_10-0"><a href="https://en.wikipedia.org/wiki/Photosynthesis#cite_note-Steger_2005-10">[10]</a></sup>&nbsp;which is about three times the current&nbsp;<a href="https://en.wikipedia.org/wiki/World_energy_resources_and_consumption" title="World energy resources and consumption">power consumption of human civilization</a>.<sup id="cite_ref-EIA_11-0"><a href="https://en.wikipedia.org/wiki/Photosynthesis#cite_note-EIA-11">[11]</a></sup>&nbsp;Photosynthetic organisms also convert around 100&ndash;115 thousand million metric tonnes of carbon into&nbsp;<a href="https://en.wikipedia.org/wiki/Biomass" title="Biomass">biomass</a>&nbsp;per year.</ins></p>
