Pyrenoid
Encyclopedia
In cell biology
, pyrenoids are organelles, centers of carbon dioxide fixation
within the chloroplast
s of algae
and hornwort
s. 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 fixation. Excessive 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.
Cell biology
Cell biology is a scientific discipline that studies cells – their physiological properties, their structure, the organelles they contain, interactions with their environment, their life cycle, division and death. This is done both on a microscopic and molecular level...
, pyrenoids are organelles, centers of carbon dioxide fixation
Carbon fixation
In biology, carbon fixation is the reduction of carbon dioxide to organic compounds by living organisms. The obvious example is photosynthesis. Carbon fixation requires both a source of energy such as sunlight, and an electron donor such as water. All life depends on fixed carbon. Organisms that...
within the chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
s of algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
and hornwort
Hornwort
Hornworts are a group of bryophytes, or non-vascular plants, comprising the division Anthocerotophyta. The common name refers to the elongated horn-like structure, which is the sporophyte. The flattened, green plant body of a hornwort is the gametophyte plant.Hornworts may be found worldwide,...
s. Pyrenoids are not membrane
Biological membrane
A biological membrane or biomembrane is an enclosing or separatingmembrane that acts as a selective barrier, within or around a cell. It consists of a lipid bilayer with embedded proteins that may constitute close to 50% of membrane content...
-bound, but specialized areas of the plastid
Plastid
Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...
that contain high levels of ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO
RuBisCO
Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly known by the shorter name RuBisCO, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is converted by plants to energy-rich molecules such as glucose. RuBisCo is an abbreviation...
).
RubisCO fixes carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
by adding it to the 5-carbon sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...
-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
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....
assume a crystalline appearance. Complex pyrenoids are highly differentiated areas of chloroplast surrounded by a thick starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...
sheath. The pyrenoid may serve to aid the concentration of dissolved carbon dioxide by preventing diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
away from the site of fixation while simultaneously reducing the level of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
at the site of fixation. Excessive 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
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
of carbon dioxide in water compared to air (1:1000) favors their presence in these small submerged organisms.
Paramylon
Paramylon
Paramylon is a carbohydrate similar to starch. The chloroplasts found in Euglena contain chlorophyll which aids in the synthesis of carbohydrates to be stored as starch granules and paramylon. Paramylon is made in the pyrenoids of Euglena. The eugenoids have chlorophylls a and b and they store...
, a polysaccharide
Polysaccharide
Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...
, occurs in the pyrenoids of Euglenoids, such as Euglena gracilis.