Editing Photorespiration (section)

===Biochemical carbon concentrating mechanisms===
Biochemical CCMs concentrate carbon dioxide in one temporal or spatial region, through [[metabolite]] exchange. C<sub>4</sub> and CAM photosynthesis both use the enzyme [[Phosphoenolpyruvate carboxylase]] (PEPC) to add {{chem|CO|2}} to a 4-carbon sugar. PEPC is faster than RuBisCO, and more selective for {{chem|CO|2}}.

====C<sub>4</sub>====
[[C4 carbon fixation|C<sub>4</sub>]] plants capture carbon dioxide in their mesophyll cells (using an enzyme called [[phosphoenolpyruvate carboxylase]] which catalyzes the combination of carbon dioxide with a compound called phosphoenolpyruvate (PEP)), forming oxaloacetate. This oxaloacetate is then converted to malate and is transported into the bundle sheath cells (site of carbon dioxide fixation by RuBisCO) where [[oxygen]] concentration is low to avoid photorespiration. Here, carbon dioxide is removed from the malate and combined with RuBP by RuBisCO in the usual way, and the [[Calvin cycle]] proceeds as normal. The {{chem|CO|2}} concentrations in the Bundle Sheath are approximately 10–20 fold higher than the concentration in the mesophyll cells.<ref name="Ehleringer 1991" />

This ability to avoid photorespiration makes these plants more hardy than other plants in dry and hot environments, wherein stomata are closed and internal carbon dioxide levels are low. Under these conditions, photorespiration does occur in C<sub>4</sub> plants, but at a much lower level compared with C<sub>3</sub> plants in the same conditions. C<sub>4</sub> plants include [[sugar cane]], [[maize|corn (maize)]], and [[sorghum]].

====CAM (Crassulacean acid metabolism)====
[[File:CAMplantgraph.jpg|class=skin-invert-image|thumb|300px|left|Overnight graph of {{CO2}} absorbed by a CAM plant]]
CAM plants, such as [[cacti]] and [[succulent plant]]s, also use the enzyme PEP carboxylase to capture carbon dioxide, but only at night. [[Crassulacean acid metabolism]] allows plants to conduct most of their gas exchange in the cooler night-time air, [[Carbon sequestration|sequestering carbon]] in 4-carbon sugars which can be released to the photosynthesizing cells during the day. This allows CAM plants to minimize water loss ([[transpiration]]) by maintaining closed stomata during the day. CAM plants usually display other water-saving characteristics, such as thick cuticles, stomata with small apertures, and typically lose around 1/3 of the amount of water per {{chem|CO|2}} fixed.<ref>{{cite book | title = Plant Physiology | edition = Fifth | publisher = Sinauer Associates, Inc. | vauthors = Taiz L, Zeiger E | date = 2010 | chapter = Chapter 8: Photosynthesis: The Carbon Reactions: Inorganic Carbon–Concentrating Mechanisms: Crassulacean Acid Metabolism (CAM) | page = 222 }}</ref>


====C<sub>2</sub>====
[[File:C2 Photosynthesis.svg|thumb|right|In C<sub>2</sub> plants, the mitochondria of mesophyll cells have no glycine decarboxylase (GDC).]]
'''C<sub>2</sub> photosynthesis''' (also called '''glycine shuttle''' and '''photorespiratory CO<sub>2</sub> pump''') is a CCM that works by making use of &ndash; as opposed to avoiding &ndash; photorespiration. It performs ''carbon refixation'' by delaying the breakdown of photorespired glycine, so that the molecule is shuttled from the [[mesophyll]] into the [[bundle sheath]]. Once there, the glycine is decarboxylated in [[mitochondria]] as usual, releasing CO<sub>2</sub> and concentrating it to triple the usual concentration.<ref name=Cee2>{{cite journal |last1=Lundgren |first1=Marjorie R. |title=C 2 photosynthesis: a promising route towards crop improvement? |journal=New Phytologist |date=December 2020 |volume=228 |issue=6 |pages=1734–1740 |doi=10.1111/nph.16494 |pmid=32080851 |doi-access=free}}</ref>

Although C<sub>2</sub> photosynthesis is traditionally understood as an intermediate step between C<sub>3</sub> and C<sub>4</sub>, a wide variety of plant lineages do end up in the C<sub>2</sub> stage without further evolving, showing that it is an evolutionary steady state of its own. C<sub>2</sub> may be easier to engineer into crops, as the phenotype requires fewer anatomical changes to produce.<ref name=Cee2/>

====Algae====
There have been some reports of algae operating a biochemical CCM: shuttling metabolites within single cells to concentrate {{CO2}} in one area. This process is not fully understood.<ref>{{cite journal | vauthors = Giordano M, Beardall J, Raven JA | title = {{CO2}} concentrating mechanisms in algae: mechanisms, environmental modulation, and evolution | journal = Annual Review of Plant Biology | volume = 56 | issue = 1 | pages = 99–131 | date = June 2005 | pmid = 15862091 | doi = 10.1146/annurev.arplant.56.032604.144052 | bibcode = 2005AnRPB..56...99G }}</ref>