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Endoplasmic Reticulum

All nucleated cells contain a network of membrane in the cytoplasm. These membranes form boundaries of a series of interconnected regions of the cells. The membrane-bound vacuolar system thus developed is called endoplasmic reticulum, which is of great importance in cellular synthesis and intracellular transport of materials.

The presence of a cytoplasmic network of strands and associated vesicle-like bodies was first noted by Porter, Claudo, and Fullam (1945) in the electron micrographs of culture cells. Further electron microscopic observations revealed that the cytoplasmic strands and vesicular bodies formed a complex network. This vacuolar network was found in most cases to be in the inner endoplasmic portion of the cytoplasm. This network on the basis of its reticular form and endoplasmic location was named endoplasmic reticulum.

Endoplasmic reticulum is generally absent in eggs and embryonic cells but found in increased amounts as the cell differentiates. In spermatocytes it is in the form of only a few vacuoles. In cells engaged in lipid metabolism it appears as a simple structure in the form of a few tubules. However, extensively developed endoplasmic reticulum can be observed in cells actively engaged in synthesis, especially, of proteins (pancreatic cells and liver cells). In striated muscles, endoplasmic reticulum appears in a special form known as sarcoplasmic reticulum.

Structurally the endoplasmic reticulum exists as A network of flattened sacs arranged in parallel arrays or cisternae. 

In cross-section the sacs appear to be bounded by two membranes. Endoplasmic reticulum varies considerably in amount as well as in appearance from cell to cell. Its form and appearance also change according to the metabolic activity of the cell. Endoplasmic reticulum is often continuous with the nuclear membrane of the cell.


Endoplasmic reticulum

Generally there are two types of endoplasmic reticulum in cells. (i) smooth and (ii) rough. In many cells, especially those engaged in protein synthesis, small electron-dense granules are found associated with the outer surface of the membranes surrounding the cavities of endoplasmic reticulum. These granules are the ribosomes and the endoplasmic reticulum with associated ribosomes is called rough endoplasmic reticulum. Smooth endoplasmic reticulum is devoid of ribosomes and is a characteristic feature of cells engaged in the synthesis of lipid and steroid hormones (cells of the testes and adrenal glands). 

Functionally, the endoplasmic reticulum with the associated ribosomes is the site of synthesis and storage of proteins in the cell. It is in these membranes that the polypeptides are packed into proteins, and lipids. The proteins are complexed to form lipoproteins. The enzymes of glycogen metabolism, lipid synthesis and hormone synthesis are all associated with certain types of smooth endoplasmic reticulum. Apart from these synthetic functions, endoplasmic reticulum divides the cytoplasm into compartments so as to facilitate the separation of metabolic products. It functions as a channel through which micromolecules are transported within and outside the cell.


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