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 paracrystalline 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. Since RubisCO doesn't work very efficiently in the presence of oxygen, the pyrenoids are presumably used to reduce oxygen influence by locally increasing the amount of carbon dioxide. Plants use different mechanisms to circumvent too high amounts of oxygenation reactions of their RubisCO.
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.