Editing Photorespiration (section)

==Photorespiratory reactions==
[[File:Photorespiration.svg|class=skin-invert-image|thumb|350px|PhotorespirationFrom left to right: [[chloroplast]], [[peroxisome]], and [[mitochondrion]]]]
Addition of molecular oxygen to ribulose-1,5-bisphosphate produces [[3-phosphoglycerate]] (PGA) and [[2-phosphoglycolate]] (2PG, or PG). PGA is the normal product of carboxylation, and productively enters the [[Calvin cycle]]. Phosphoglycolate, however, inhibits certain enzymes involved in photosynthetic carbon fixation (hence is often said to be an 'inhibitor of photosynthesis').<ref>{{cite journal | vauthors = Peterhansel C, Krause K, Braun HP, Espie GS, Fernie AR, Hanson DT, Keech O, Maurino VG, Mielewczik M, Sage RF | title = Engineering photorespiration: current state and future possibilities | journal = Plant Biology | volume = 15 | issue = 4 | pages = 754–8 | date = July 2013 | pmid =  23121076| doi = 10.1111/j.1438-8677.2012.00681.x | bibcode = 2013PlBio..15..754P }}</ref> It is also relatively difficult to recycle: in higher plants it is salvaged by a series of reactions in the [[peroxisome]], [[mitochondria]], and again in the [[peroxisome]] where it is converted into [[glycerate]]. Glycerate reenters the [[chloroplast]] and by the same transporter that exports [[Glycolic acid|glycolate]]. A cost of 1 [[Adenosine triphosphate|ATP]] is associated with conversion to 3-phosphoglycerate (PGA) ([[Phosphorylation]]), within the [[chloroplast]], which is then free to re-enter the Calvin cycle.

Several costs are associated with this metabolic pathway; the production of [[hydrogen peroxide]] in the peroxisome (associated with the conversion of glycolate to glyoxylate). Hydrogen peroxide is a dangerously strong oxidant which must be immediately split into water and oxygen by the enzyme [[catalase]]. The conversion of 2× 2Carbon [[glycine]] to 1× {{C3}} [[serine]] in the mitochondria by the enzyme glycine-decarboxylase is a key step, which releases {{CO2}}, NH<sub>3</sub>, and reduces NAD to NADH. Thus, one {{chem|CO|2}} molecule is produced for every two molecules of {{chem|O|2}} (two deriving from RuBisCO and one from peroxisomal oxidations). The assimilation of NH<sub>3</sub> occurs via the [[Glutamine synthetase|GS]]-[[Glutamine oxoglutarate aminotransferase|GOGAT]] cycle, at a cost of one ATP and one NADPH.

[[Cyanobacteria]] have three possible pathways through which they can metabolise 2-phosphoglycolate. They are unable to grow if all three pathways are knocked out, despite having a carbon concentrating mechanism that should dramatically lower the rate of photorespiration [[#Biological adaptation to minimize photorespiration|(see below)]].<ref>{{cite journal | vauthors = Eisenhut M, Ruth W, Haimovich M, Bauwe H, Kaplan A, Hagemann M | title = The photorespiratory glycolate metabolism is essential for cyanobacteria and might have been conveyed endosymbiontically to plants | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 105 | issue = 44 | pages = 17199–204 | date = November 2008 | pmid = 18957552 | pmc = 2579401 | doi = 10.1073/pnas.0807043105 | bibcode = 2008PNAS..10517199E | doi-access = free }}</ref>