In cell biology, pyrenoids are centers of carbon dioxide fixation. They are not membrane-bound organelles, but specialized areas in algalplastids 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 photosyntheticenzymes.
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 CO fixation.
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.
The fine structure of the pyrenoid in the mature vegetative cell of Tetracystis excentrica Brown and Bold is described.
Zoospore pyrenoids lack the 2 convoluted thylakoids between the starch plates and the ground substance characteristic of those in the mature vegetative cell.
Conclusive evidence is presented for the presence of RNA in the cytoplasm and nucleolus, DNA in the nucleus, and protein in the pyrenoid.
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