Editing Photosynthesis (section)

==Overview==
{{Main article|Biological carbon fixation}}
[[File:Simple photosynthesis overview.svg|thumb|Photosynthesis changes sunlight into chemical energy, splits water to liberate O<sub>2</sub>, and fixes CO<sub>2</sub> into sugar.]]
Most photosynthetic organisms are [[photoautotroph]]s, which means that they are able to [[Chemical synthesis|synthesize]] food directly from [[carbon dioxide]] and [[water]] using [[energy]] from light. However, not all organisms use carbon dioxide as a source of carbon atoms to carry out photosynthesis; [[photoheterotroph]]s use organic compounds, rather than carbon dioxide, as a source of carbon.<ref name="Bryant-2006"/>

In [[plant]]s, [[algae]], and [[cyanobacteria]], photosynthesis releases oxygen. This '''oxygenic photosynthesis''' is by far the most common type of photosynthesis used by living organisms. Some shade-loving plants (sciophytes) produce such low levels of oxygen during photosynthesis that they use all of it themselves instead of releasing it to the atmosphere.<ref>[https://books.google.com/books?id=L8DHHSO2RFsC&dq=Sciophytes+shade+plants+compensation+uptake+aerobic+respiration&pg=PA282 Plants: Diversity and Evolution]</ref>

Although there are some differences between oxygenic photosynthesis in plants, algae, and cyanobacteria, the overall process is quite similar in these organisms. There are also many varieties of [[anoxygenic photosynthesis]], used mostly by bacteria, which consume carbon dioxide but do not release oxygen or which produce elemental sulfur instead of molecular oxygen.<ref>{{Cite journal |last1= George |first1= Drishya M. |last2= Vincent |first2= Annette S. |last3= Mackey |first3= Hamish R. |date= 2020 |title= An overview of anoxygenic phototrophic bacteria and their applications in environmental biotechnology for sustainable Resource recovery |journal= Biotechnology Reports (Amsterdam, Netherlands) |volume= 28 |pages= e00563 |doi= 10.1016/j.btre.2020.e00563 |issn= 2215-017X |pmc= 7714679 |pmid= 33304839 }}</ref><ref>{{Cite book |last= Fuchs |first= Georg |date= 1987 |chapter= Carbon Dioxide Reduction by Anaerobic Bacteria |editor-last= Aresta |editor-first= M. |editor2-last= Forti |editor2-first= G. |title= Carbon Dioxide as a Source of Carbon: Biochemical and Chemical Uses |place= Dordrecht |publisher= Springer Netherlands |language= en |pages= 263–273 |doi= 10.1007/978-94-009-3923-3_14 |isbn= 978-94-009-3923-3 |chapter-url= https://doi.org/10.1007/978-94-009-3923-3_14 |access-date= 2024-06-10 }}</ref>

Carbon dioxide is converted into sugars in a process called [[carbon fixation]]; photosynthesis captures energy from sunlight to convert carbon dioxide into [[carbohydrate]]s. Carbon fixation is an [[endothermic]] [[redox]] reaction. In general outline, photosynthesis is the opposite of [[cellular respiration]]: while photosynthesis is a process of reduction of carbon dioxide to carbohydrates, cellular respiration is the oxidation of carbohydrates or other [[nutrient]]s to carbon dioxide. Nutrients used in cellular respiration include carbohydrates, amino acids and fatty acids. These nutrients are oxidized to produce carbon dioxide and water, and to release chemical energy to drive the organism's [[metabolism]].

Photosynthesis and cellular respiration are distinct processes, as they take place through different sequences of chemical reactions and in different [[cellular compartment]]s (cellular respiration in [[mitochondria]]).<ref>{{Cite journal |last1= Stefano |first1= George B. |last2= Snyder |first2= Christopher |last3= Kream |first3= Richard M. |date= 2015-07-17 |title= Mitochondria, Chloroplasts in Animal and Plant Cells: Significance of Conformational Matching |journal= Medical Science Monitor: International Medical Journal of Experimental and Clinical Research |volume= 21 |pages= 2073–2078 |doi= 10.12659/MSM.894758 |issn= 1643-3750 |pmc= 4517925 |pmid= 26184462 }}</ref><ref>{{Cite journal |last1= Shimakawa |first1= Ginga |last2= Matsuda |first2= Yusuke |last3= Burlacot |first3= Adrien |date= 2024 |title= Crosstalk between photosynthesis and respiration in microbes |journal= Journal of Biosciences |volume= 49 |issue= 2 |pages=45 |doi= 10.1007/s12038-023-00417-4 |issn= 0973-7138 |pmid= 38516912 |url= https://pubmed.ncbi.nlm.nih.gov/38516912 }}</ref>

The general [[chemical equation|equation]] for photosynthesis as first proposed by [[C. B. van Niel|Cornelis van Niel]] is:{{sfn|Whitmarsh|Govindjee|1999|p=13}}
: {{underset|carbon<br/>dioxide|CO<sub>2</sub>}} + {{underset|electron donor|2H<sub>2</sub>A}} + {{underset|light energy|[[photons]]}} → {{underset|[[carbohydrate]]|[CH<sub>2</sub>O]}} + {{underset|oxidized<br/>electron<br/>donor|2A}} + {{underset|water|H<sub>2</sub>O}}

Since water is used as the electron donor in oxygenic photosynthesis, the equation for this process is:
: {{underset|carbon<br/>dioxide|CO<sub>2</sub>}} + {{underset|water|2H<sub>2</sub>O}} + {{underset|light energy|photons}} → {{underset|carbohydrate|[CH<sub>2</sub>O]}} + {{underset|oxygen|O<sub>2</sub>}} + {{underset|water|H<sub>2</sub>O}}

This equation emphasizes that water is both a reactant in the [[#Light-dependent reactions|light-dependent reaction]] and a product of the [[#Light-independent reactions|light-independent reaction]], but canceling ''n'' water molecules from each side gives the net equation:

: {{underset|carbon<br/>dioxide|CO<sub>2</sub>}} + {{underset| water |H<sub>2</sub>O}} + {{underset|light energy|photons}} → {{underset|carbohydrate|[CH<sub>2</sub>O]}} + {{underset| oxygen |O<sub>2</sub>}}

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]]:<ref>''Anaerobic Photosynthesis'', [[Chemical & Engineering News]], '''86''', 33, August 18, 2008, p. 36</ref> The equation for this reaction is:
: {{underset|carbon<br/>dioxide|CO<sub>2</sub>}} + {{underset|<br/>arsenite|(AsO{{su|b=3|p=3−}})}} + {{underset|light energy|photons}} → {{underset|<br/>arsenate|(AsO{{su|b=4|p=3−}})}} + {{underset|carbon<br/>monoxide|CO}}(used to build other compounds in subsequent reactions)<ref>{{cite journal |vauthors= Kulp TR, Hoeft SE, Asao M, Madigan MT, Hollibaugh JT, Fisher JC, Stolz JF, Culbertson CW, Miller LG, Oremland RS | author10-link= Ronald Oremland |date= Aug 2008 |title= Arsenic(III) fuels anoxygenic photosynthesis in hot spring biofilms from Mono Lake, California |journal= Science |volume= 321 |issue= 5891 |pages= 967–970 |bibcode= 2008Sci...321..967K |doi= 10.1126/science.1160799 |pmid= 18703741 |s2cid= 39479754 |url= https://semanticscholar.org/paper/b193d8bd3632fb917e5d3a7fc9cb9d11fb817669 |access-date= 2020-01-17 |archive-date= 2020-07-28 |archive-url= https://web.archive.org/web/20200728092205/https://www.semanticscholar.org/paper/Arsenic(III)-Fuels-Anoxygenic-Photosynthesis-in-Hot-Kulp-Hoeft/b193d8bd3632fb917e5d3a7fc9cb9d11fb817669 |url-status= live }}</ref>

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 hydrogen carrier [[NADPH]] and the energy-storage molecule [[Adenosine triphosphate|ATP]]. During the second stage, the ''light-independent reactions'' use these products to capture and reduce carbon dioxide.

Most organisms that use oxygenic photosynthesis use [[Visible spectrum|visible light]] for the light-dependent reactions, although at least three use shortwave [[infrared]] or, more specifically, far-red radiation.<ref>{{cite web |title= Scientists discover unique microbe in California's largest lake |website= bio-medicine.org |date= January 2005 |url= http://www.bio-medicine.org/biology-news/Scientists-discover-unique-microbe-in-Californias-largest-lake-203-1/ |access-date= 2009-07-20 |archive-url= https://web.archive.org/web/20090712152053/http://www.bio-medicine.org/biology-news/Scientists-discover-unique-microbe-in-Californias-largest-lake-203-1/ |archive-date= 2009-07-12 |url-status= dead }}</ref>

Some organisms employ even more radical variants of photosynthesis. Some [[archaea]] use a simpler method that employs a pigment similar to those used for vision in animals. The [[bacteriorhodopsin]] changes its configuration in response to sunlight, acting as a proton pump. This produces a proton gradient more directly, which is then converted to chemical energy. The process does not involve carbon dioxide fixation and does not release oxygen, and seems to have evolved separately from the more common types of photosynthesis.<ref>{{Cite book |vauthors= Ingrouille M, Eddie B |date= 2006-08-17 |title= Plants: Diversity and Evolution |publisher= Cambridge University Press |pages= 13–14 |isbn= 978-1-139-45546-6 |url= https://books.google.com/books?id=L8DHHSO2RFsC&dq=bacteriorhodopsin+photosynthesis+evolved+separately&pg=PA14 }}</ref>