Editing Photorespiration (section)

===Biophysical carbon-concentrating mechanisms===
This type of carbon-concentrating mechanism (CCM) relies on a contained compartment within the cell into which {{CO2}} is shuttled, and where RuBisCO is highly expressed. In many species, biophysical CCMs are only induced under low carbon dioxide concentrations. Biophysical CCMs are more evolutionary ancient than biochemical CCMs. There is some debate as to when biophysical CCMs first evolved, but it is likely to have been during a period of low carbon dioxide, after the [[Great Oxygenation Event]] (2.4 billion years ago). Low {{chem|CO|2}} periods occurred around 750, 650, and 320–270 million years ago.<ref>{{cite journal | vauthors = Raven JA, Giordano M, Beardall J, Maberly SC | title = Algal evolution in relation to atmospheric {{CO2}}: carboxylases, carbon-concentrating mechanisms and carbon oxidation cycles | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 367 | issue = 1588 | pages = 493–507 | date = February 2012 | pmid = 22232762 | pmc = 3248706 | doi = 10.1098/rstb.2011.0212 }}</ref>

====Eukaryotic algae====
In nearly all species of [[Algae|eukaryotic algae]] (''[[Chloromonas]]'' being one notable exception), upon induction of the CCM, ~95% of RuBisCO is densely packed into a single subcellular compartment: the [[pyrenoid]]. Carbon dioxide is concentrated in this compartment using a combination of CO<sub>2</sub> pumps, [[Carbonic acid|bicarbonate]] pumps, and [[carbonic anhydrase]]s. The pyrenoid is not a membrane-bound compartment but is found within the chloroplast, often surrounded by a starch sheath (which is not thought to serve a function in the CCM).<ref>{{Cite journal | vauthors = Villarejo A, Martinez F, Pino Plumed M, Ramazanov Z | title = The induction of the CO<sub>2</sub> concentrating mechanism in a starch-less mutant of Chlamydomonas reinhardtii | doi = 10.1111/j.1399-3054.1996.tb06687.x | journal = Physiologia Plantarum | volume = 98 | issue = 4 | pages = 798–802 | year = 1996 | bibcode = 1996PPlan..98..798V }}</ref>

====Hornworts====
Certain species of [[hornwort]] are the only land plants that are known to have a biophysical CCM involving concentration of carbon dioxide within [[pyrenoid]]s in their chloroplasts.<ref>{{cite journal | vauthors=((Robison, T. A.)), ((Oh, Z. G.)), ((Lafferty, D.)), ((Xu, X.)), ((Villarreal, J. C. A.)), ((Gunn, L. H.)), ((Li, F.-W.)) | journal=Nature Plants | title=Hornworts reveal a spatial model for pyrenoid-based CO2-concentrating mechanisms in land plants | pages=63–73 | publisher=Nature Publishing Group | date=3 January 2025 | volume=11 | issue=1 | issn=2055-0278 | doi=10.1038/s41477-024-01871-0| pmid=39753956 }}
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====Cyanobacteria====
[[Cyanobacteria]]l CCMs are similar in principle to those found in eukaryotic algae and hornworts, but the compartment into which carbon dioxide is concentrated has several structural differences. Instead of the pyrenoid, cyanobacteria contain [[carboxysome]]s, which have a protein shell, and linker proteins packing RuBisCO inside with a very regular structure.
Cyanobacterial CCMs are much better understood than those found in [[eukaryote]]s, partly due to the ease of genetic manipulation of [[prokaryote]]s.