Pyrenoid

<p>Pyrenoids are sub-cellular phase-separated micro-compartments found in <a href="/facts/Chloroplast/fo8U3yTx">chloroplasts</a> of many <a href="/facts/Algae/U1ftVKU3">algae</a>, and in a single group of land plants, the <a href="/facts/Hornwort/NLDvbs9G">hornworts</a>. Pyrenoids are associated with the operation of a carbon-concentrating mechanism (CCM). Their main function is to act as centres of carbon dioxide (CO2) fixation, by generating and maintaining a CO2-rich environment around the <a href="/facts/Photosynthesis/TMV7w693">photosynthetic</a> <a href="/facts/Enzyme/DSI1Fh7y">enzyme</a> <a href="/facts/Ribulose-1%2c5-bisphosphate_carboxylase%2foxygenase/C8asuQsc">ribulose-1,5-bisphosphate carboxylase/oxygenase</a> (RuBisCO). Pyrenoids therefore seem to have a role analogous to that of <a href="/facts/Carboxysomes/NY8LTF2f">carboxysomes</a> in <a href="/facts/Cyanobacteria/S8bKxIU9">cyanobacteria</a>.
</p><p>Algae are restricted to aqueous environments, even in aquatic habitats, and this has implications for their ability to access CO2 for photosynthesis. CO2 diffuses 10,000 times slower in water than in air, and is also slow to equilibrate. The result of this is that water, as a medium, is often easily depleted of CO2 and is slow to gain CO2 from the air. Finally, CO2 equilibrates with bicarbonate (HCO−3) when dissolved in water, and does so on a <a href="/facts/PH/RnSv8D7y">pH</a>-dependent basis. In sea water for example, the pH is such that dissolved inorganic carbon (DIC) is mainly found in the form of HCO−3. The net result of this is a low concentration of free CO2 that is barely sufficient for an algal RuBisCO to run at a quarter of its <a href="/facts/Enzyme_kinetics/LJ0lWoMc">maximum velocity</a>, and thus, CO2 availability may sometimes represent a major limitation of algal photosynthesis.
</p>

Pyrenoid open-in-new

Pyrenoid