In cell biology, pyrenoids are organelles, centers of carbon dioxide fixation within the chloroplasts of algae and hornworts.[1][2] Pyrenoids are not membrane-bound, but specialized areas of the plastid that contain high levels of ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO).
RubisCO fixes carbon dioxide by adding it to the 5-carbon sugar-phosphate, ribulose-1,5-bisphosphate, yielding two molecules of the 3-carbon compound, 3-phosphoglycerate. In a competing reaction, the enzyme uses oxygen to break down ribulose-1,5-bisphosphate to phosphoglycolate and 3-phosphoglycerate, with no net fixation of carbon. In some organisms, the concentration of RubisCO in the pyrenoid is so high that the contents of the organelle assume a crystalline appearance. Complex pyrenoids are highly differentiated areas of chloroplast surrounded by a thick starch sheath. The pyrenoid may serve to aid the concentration of dissolved carbon dioxide by preventing diffusion away from the site of fixation while simultaneously reducing the level of oxygen at the site of CO2 fixation. Excessive CO2 can also inhibit the carbon fixation reaction catalysed by RubisCO.
Pyrenoids are not found in higher plants and it is thought that the slower rate of diffusion of carbon dioxide in water compared to air (1:1000) favors their presence in these small submerged organisms.
Paramylon, a polysaccharide, occurs in the pyrenoids of Euglenoids, such as Euglena gracilis.[3]
Fletcher, R.L. 1987. Seaweeds of the British Isles. Volume 3 Fucophyceae (Phaeophyceae) Part. 1, p.7. British Museum (Natural History). ISBN 0-565-00992-3