Pyrenoid
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In cell biology, pyrenoids are centers of carbon dioxide fixation. They are not membrane-bound organelles, but specialized areas in algal plastids and contain high amounts of ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO). RubisCO takes carbon dioxide and adds it to the sugar ribulose-1,5-bisphosphate. It needs six molecules of carbon dioxide and six molecules of ribulose-1,5-bisphosphate = six cycles of the Calvin cycle to make one new molecule of glucose. 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. This is assumed to aid the concentration of dissolved carbon dioxide by preventing diffusion away from the site of fixation while simultaneously reducing the level of oxygen in the pyrenoid which can 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) necessitates their use by small aquatic organisms.
Pyrenoid in Botany
Differentiated region of the chloroplast that may be the center of starch formation and depositions, or may be the site of certain photosynthetic enzymes.