Editing Photorespiration (section)

{{Short description|Process in plant metabolism}}
{{too detailed|lead|reason= It starts (and stays) too complex for the everyman encyclopedia user (without them ever giving them a chance to grasp the concept). Someone who understands the subject well needs to craft a plain English introductory sentence or two to explain the basic concepts in basic terms before it goes full scientific. After that, every man is on their own. But it needs to establish the topic in plain language before going on (as this is not an extremely rarefied or abstruse scientific matter for scientists only to approach and understand, though its chemistry is complex).|date=December 2023}}
[[File:Photorespiration allgemein.svg|class=skin-invert-image|thumb|upright=1.5| {{center|Simplified photorespiration cycle}}]]
[[File:Simplified photorespiration diagram.svg|class=skin-invert-image|thumb|upright=1.5|{{center|Simplified photorespiration and Calvin cycle}}]]

'''Photorespiration''' (also known as the '''oxidative photosynthetic carbon cycle''' or '''C<sub>2</sub> cycle''') refers to a process in [[plant physiology|plant metabolism]] where the [[enzyme]] [[RuBisCO]] oxygenates [[RuBP]], wasting some of the energy produced by photosynthesis. The desired reaction is the addition of [[carbon dioxide]] to RuBP ([[carboxylation]]), a key step in the [[Calvin–Benson cycle]], but approximately 25% of reactions by RuBisCO instead add [[oxygen]] to RuBP ([[Oxygenase|oxygenation]]), creating a product that cannot be used within the Calvin–Benson cycle. This process lowers the efficiency of photosynthesis, potentially lowering photosynthetic output by 25% in [[C3 carbon fixation|{{C3}} plants]].<ref name="Sharkey">{{cite journal | vauthors = Sharkey T |title=Estimating the rate of photorespiration in leaves |journal=Physiologia Plantarum |volume=73 |issue=1 |pages=147–152 |year=1988 |doi=10.1111/j.1399-3054.1988.tb09205.x|bibcode=1988PPlan..73..147S }}</ref> Photorespiration involves a complex network of enzyme reactions that exchange metabolites between [[chloroplast]]s, leaf [[peroxisome]]s and [[mitochondria]].

The oxygenation reaction of RuBisCO is a wasteful process because [[3-Phosphoglyceric acid|3-phosphoglycerate]]  is created at a lower rate and higher metabolic cost compared with [[Calvin cycle|RuBP carboxylase activity]]. While photorespiratory carbon cycling results in the formation of [[Glyceraldehyde 3-phosphate|G3P]] eventually, around 25% of carbon fixed by photorespiration is re-released as {{CO2}}<ref>{{cite journal | vauthors = Leegood RC | title = A welcome diversion from photorespiration | journal = Nature Biotechnology | volume = 25 | issue = 5 | pages = 539–40 | date = May 2007 | pmid = 17483837 | doi = 10.1038/nbt0507-539 | s2cid = 5015366 }}</ref> and nitrogen, as [[ammonia]]. [[Ammonia]] must then be detoxified at a substantial cost to the cell. Photorespiration also incurs a direct cost of one [[Adenosine triphosphate|ATP]] and one [[NAD(P)H]].

While it is common to refer to the entire process as photorespiration, technically the term refers only to the metabolic network which acts to rescue the products of the oxygenation reaction (phosphoglycolate).