Pyrenoids are centers of carbon dioxide fixation. They are no membrane-bound organelles but specialized areas in algal plastids and contain high amounts of paracrystalline ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO), possibly the most important enzyme in the world. 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 oxygene, the pyrenoids are presumably used to reduce oxygene influence by increasing locally the amount of carbon dioxide. Plants use different mechanisms to circumvent too high amounts of oxygenation reactions of their RubisCO.