Peroxisomes
Peroxisomes are small, spherical organelles found in nearly all eukaryotic cells, characterized by a single lipid bilayer membrane and a granular interior rich in enzymes. These organelles play a crucial role in various metabolic processes, as they contain around fifty different enzymes that facilitate functions such as the oxidation of fatty acids and the detoxification of harmful hydrogen peroxide. Peroxisomes do not have their own genetic material or ribosomes; instead, they import proteins from the cytosol. They reproduce through growth and fission, and interestingly, they are not part of the endomembrane system of the cell.
Typically ranging from 0.2 to 2 micrometers in diameter, peroxisomes are distributed throughout the cytosol, with higher concentrations near the nucleus. Their functions vary significantly across different organisms: in animal cells, they are involved in lipid biosynthesis and the breakdown of fatty acids for energy, while in plant cells, they contribute to processes like photorespiration and the conversion of fatty acids into sugars for seed germination. Additionally, in leguminous plants, peroxisomes assist in nitrogen transport through the formation of ureides. Overall, peroxisomes are essential for maintaining cellular health and energy balance.
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Peroxisomes
Categories: Anatomy; cellular biology; physiology
Peroxisomes are small, vesicle-like organelles that are found in virtually all eukaryotic cells. They are surrounded by a single lipid bilayer membrane and are spherically shaped microbodies with a granular interior containing proteins that usually function as enzymes. Many of these enzymes are important in the process of metabolism. Because peroxisomes contain no genome or ribosomes, their protein is imported from the cytosol. Peroxisomes reproduce by cycles of growth and fission and are not members of the endomembrane system.
Typically, peroxisomes vary in diameter from 0.2 to two micrometers. They are generally distributed throughout the cytosol, usually with more abundance around the nucleus and where the cell walls of two cells abut. One theory about the evolution of peroxisomes suggests that they are of primitive origin, predating the appearance of the mitochondria, and developed in response to increased levels of oxygen in the environment generated by cyanobacteria. However, the peroxisome's relationship to bacteria has been challenged by other theories that suggest a close relationship with the mitochondria.
Functions
Peroxisomes contain approximately fifty different enzymes that perform many important roles in eukaryotic tissues and development. Their various functions are dictated by their enzyme content, which is constantly modified throughout the development of an organism. One function of peroxisomes is to remove hydrogen atoms from organic substrates by using oxygen, resulting in the production of hydrogen peroxide, a strong oxidizing agent. Hydrogen peroxide is harmful to the cell, however, so peroxisomes also contain catalase, an enzyme that decomposes the harmful compound by converting it into water or using it to oxidize another organic compound. Other oxidative reactions in peroxisomes include amino acids, uric acid, and fatty acids. The oxidation of fatty acids is important for the creation of metabolic energy; the process occurs in both peroxisomes and mitochondria in animal cells, but only in peroxisomes in plant and yeast cells.
Lipid biosynthesis is another function of peroxisomes. In animal cells, they synthesize dolichol and cholesterol, a function also performed by the endoplasmic reticulum. Peroxisomes also synthesize bile acids, derived from cholesterol, in the liver of an animal. Peroxisome enzymes synthesize plasmalogens as well, which are important in tissues such as the brain and heart.
In plants, peroxisomes play a vital role in a part of photosynthesis called photorespiration. This process typically occurs when carbon dioxide levels inside a leaf are too low and the enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO) catalyses the addition of oxygen instead of carbon dioxide—the reverse of photosynthesis. This produces a wasteful product, phosphoglycolate. A series of enzyme reactions involving chloroplasts, peroxisomes, and mitochondria then occur in order to convert phosphoglycolate into carbohydrates that the plant can use.
A second function of peroxisomes unique to plants is the conversion of fatty acids to sugars by oxidation. This provides young seeds with a source of food energy for germination and growth until they can carry out photosynthesis. The specialized peroxisomes that carry out this function are called glyoxysomes. In leguminous plants, peroxisomes are vital in the formation of ureides, compounds that transport nitrogen to cellular locations, where it can be used in organic combination. Peroxisomes also function as sites of defense against activated oxygen species that produce oxidative stress in plants.
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