Editing Pyrenoid (section)

==Role of pyrenoids in the CCM==

[[File:Carbon Concentrating Model for Chlamydomonas.png|thumb|300px| The currently hypothesised composition of the CCM found in ''Chlamydomonas reinhardtii''. 1= Extracellular environment. 2= Plasma membrane.
3= Cytoplasm. 4= Chloroplast membrane. 
5= Stroma. 6= Thylakoid membrane.
7= Thylakoid lumen. 8= Pyrenoid.]]

The confinement of the CO<sub>2</sub>-fixing enzyme into a subcellular micro-compartment, in association with a mechanism to deliver CO<sub>2</sub> to that site, is believed to enhance the efficacy of photosynthesis in an aqueous environment. Having a CCM favours carboxylation over [[photorespiration|wasteful oxygenation]] by RuBisCO. The molecular basis of the pyrenoid and the CCM have been characterised to some detail in the model green alga ''Chlamydomonas reinhardtii''.

The current model of the biophysical CCM reliant upon a pyrenoid<ref>Moroney, J. V., & Ynalvez, R. A. (2007). Proposed carbon dioxide concentrating mechanism in ''Chlamydomonas reinhardtii''. Eukaryotic cell, 6(8), 1251-1259. {{PMID|17557885}}</ref><ref>Grossman, A. R., Croft, M., Gladyshev, V. N., Merchant, S. S., Posewitz, M. C., Prochnik, S., & Spalding, M. H. (2007). Novel metabolism in ''Chlamydomonas'' through the lens of genomics. Current Opinion in Plant Biology, 10(2), 190-198 {{PMID|17291820}}</ref> considers active transport of bicarbonate from the extracellular environment to the vicinity of RuBisCO, via transporters at the [[cell membrane|plasma membrane]], the [[chloroplast membrane]], and [[thylakoid|thylakoid membranes]]. [[Carbonic anhydrases]] in the periplasm and also in the [[cytoplasm]] and [[stroma (fluid)|chloroplast stroma]] are thought to contribute to maintaining an intracellular pool of dissolved inorganic carbon, mainly in the form of bicarbonate. This bicarbonate is then thought to be pumped into the lumen of transpyrenoidal thylakoids, where a resident carbonic anhydrase is hypothesised to convert bicarbonate to CO<sub>2</sub>, and saturate RuBisCO with carboxylating substrate. It is likely that different algal groups evolved different types of CCMs, but it is generally taken that the algal CCM is articulated around a combination of carbonic anhydrases, inorganic carbon transporters, and some compartment to package RuBisCO.

Pyrenoids are highly plastic structures and the degree of RuBisCO packaging correlates with the state of induction of the CCM. In ''Chlamydomonas'', when the CCM is repressed, for example when cells are maintained in a CO<sub>2</sub>-rich environment, the pyrenoid is small and the matrix is unstructured.<ref>Rawat, M., Henk, M. C., Lavigne, L. L., & Moroney, J. V. (1996). ''Chlamydomonas reinhardtii'' mutants without ribulose-1, 5-bisphosphate carboxylase-oxygenase lack a detectable pyrenoid. ''Planta'', 198(2), 263-270.</ref> In the dinoflagellate ''Gonyaulax'', the localisation of RuBisCO to the pyrenoid is under circadian control: when cells are photosynthetically active during the day, RuBisCO assembles into multiple chloroplasts at the centre of the cells; at night, these structures disappear.<ref>Nassoury, N., Wang, Y., & Morse, D. (2005). Brefeldin a inhibits circadian remodeling of chloroplast structure in the dinoflagellate Gonyaulax. ''Traffic'', 6(7), 548-561. {{PMID|15941407}}</ref>