Short Essay on Peroxisomes of Structural Cell

Peroxisomes are a kind of micro bodies present in almost all the eukaryotic cells. Normally they co-sediment with lysosome in density gradient centrifugation.

Their existence as a separate intracellular organelle became generally recognized only in 1960s by De Duve et.al. Beaufaytt and Berther (1963) gave the name peroxisome to it. It is a single membrane bound vesicle with a diameter of about 0.5 mm having a concentrated source of at least three oxidative enzymes in liver cells: D-amino acid oxidize, urate oxidize and catalase.

Small peroxisome or microperoxisomes w a diameter of about 0.15 – 0.25 jam are ubiquitous in mammalian cells. Photosynthetic plant cells may have about 70 – 100 peroxisomes where it performs photorespiration being associated with chloroplasts and mitochondria.

Like mitochondria peroxisome is a major site of Oxygen utilization. In fact, peroxisomes are thought to be the vestige of some ancient organelle involved in carrying out all kind of the oxygen metabolism.

Later on Mitochondria evolved with a mechanism of coupling oxygen metabolism with ATP synthesis. The oxidative reactions carried out by peroxisomes are still useful to cell despite the presence of the mitochondria.

It is thought that certain integral membrane proteins unique to the peroxisomes are synthesized in the ER membrane to from a pre-organelle. This pre-organelle then forms a bud from the region of smooth ER.

Several major peroxisomal enzymes including catalase and urate oxidize are synthesized in the cytosol and transported into peroxisome as it is forming. Most of the mature peroxisomes remain attached to smooth ER by a thin sleeve like projection.

Oxidative enzymes contained in peroxisomes remove hydrogen atoms from specific substrates using molecular oxygen and forming hydrogen peroxide. Hydrogen peroxide is toxic and is thus broken down to water and molecular oxygen by the catalase enzyme present in peroxisomes.

Other functions of peroxisomes include:

1. In photorespiration in C₃ plants where glycolate from chloroplasts enter peroxisome get oxidized with molecular 0₂ to form glyoxylate. Hydrogen peroxide is the by-product, which is split by catalase. Glyoxlate is then converted to amino acid glycine that condenses to form serine and C0₂.

2. Long chain and branched fatty acids are broken down in peroxisome.

3. A special kind of peroxisomes called glyoxysomes is found in plant tissues like germinating seeds. Here it serves to convert the fatty acids stored in the seed into sugars needed for the young plants.

This is accomplished through a series of reactions called glyoxylate cycle. Two molecules of acetyl COA produced by fatty acid breakdown in peroxisome, are used to make succinic acid which is converted to glucose.

4. In animal cells peroxisome detoxify a number of substances like phenols, methanol, ethanol etc. The alcohol consumed by a person is partly detoxified in the liver cell peroxisomes.

Sphaerosomes: Sphaerosomes were previously considered as lysosomes of plant cell as they contain enzymes most of which are identical to those of lysosomes. But, now it is known that sphaerosomes contain some additional enzymes.

These micro bodies are small single unit membrane bound vesicles of o.2-o.8^m in diameter. Their main function is synthesis and storage of fats and oils.

Lomasomes: Lomasomes are small vesicles present between the cell wall and plasma membrane in plant cells. They are associated with the synthesis of cell wall materials.