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In this article we will discuss about Endoplasmic Reticulum:- 1. Subject Matter of Endoplasmic Reticulum 2. Morphology of Endoplasmic Reticulum 3. Types 4. Origin 5. Enzymes 6. Functions.
Contents:
- Subject Matter of Endoplasmic Reticulum
- Morphology of Endoplasmic Reticulum
- Types of Endoplasmic Reticulum
- Origin of Endoplasmic Reticulum
- Enzymes of Endoplasmic Reticulum
- Functions of Endoplasmic Reticulum
1. Subject Matter of Endoplasmic Reticulum:
The cytoplasmic matrix of eukaryotic cell contains a complex network of membrane bound structures called endoplasmic reticulum. The term endoplasmic reticulum was introduced by Porter and Kallman (1952).
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By the use of ultrathin sections and improved fixation techniques developed by Palade and Porter (1954) it was finally recognised that the endoplasmic reticulum presented cavities I of a great varieties of shapes and dimensions surrounded by membranes. The endoplasmic reticulum is found almost universally in eukaryotic cells. It is lacking in the bacterial and myxophycean cells.
It is a system or network of inter-connected membrane-bound fine tubules called canaliculae (figs. 5.1 and 5.2). Obviously, endoplasmic reticulum (ER) is a hollow system. Sometimes, it appears as a continuous system, connected on one side to plasma membrane and on the other side to the clear envelope. This continuity is not recognizable in the thin sections of the cell.
2. Morphology of Endoplasmic Reticulum:
The endoplasmic reticulum is composed of the following three kinds of structures:
1. Cisternae.
2. Vesicles.
3. Tubules.
a. Cisternae:
The cisternae are long flattened sac like cavities of variable length and width having a diameter of 40-50 mµ. The cisternae remain arranged parallely in bundles or stacks (Fig. 5.3).
b. Vesicles:
The vesicles are oval membrane-bound vacuolar structures having a diameter of 25-50 mµ. They often remain free in the cytoplasm and occur in most of the cells.
c. Tubules:
Tubules or canaliculae are branched structure which along with cisternae and vesicles form a network. The tubules are about 50-190 mµ in diameter and found almost in all the cells. Ultrastructural studies have revealed that the cavities of cisternae, vesicles and tubules of endoplasmic reticulum are bounded by a thin unit membrane of 50-60 Å thickness (Fig. 5.4).
Thus, they are thinner than the plasma membrane. It may show a trilaminar structure in some regions, a micellar or fluid mosaic configurations in other regions.
The tubules extend in whole cytoplasm except in the golgi body area. The membranes of endoplasmic reticulum are studded with single layer of opaque particles of ribonucleo-protein called the ribosomes (Fig. 5.5). The endoplasmic reticulum in certain region of the cell may be devoid of ribosome particles.
The ribosome particles are firmly attached to the membranes and they can be separated from the membranes when drastic forces and techniques are applied on them. In one such technique the cell is treated with sodium deoxycholate; this chemical dissolves the membranes and releases the ribosomes.
It has been shown that the ribosomes attached to endoplasmic reticulum are the chief sites for the protein synthesis in the cytoplasm. Usually pores or annuli are lacking in the membranes of canaliculae in the majority of cells but in certain cases the pores or annuli have been noticed, as for example, in the E.R. of invertebrate cells, and oocytes and spermatocytes of the vertebrates.
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According to Merriam (1959) and Kessel (1963), the annulate lamellae of endoplasmic reticulum arise by the evagination of nuclear envelope.
Several other membranous systems, such as infoldings of plasma membrane, golgi bodies and vacuoles, when properly analysed have been found in direct continuity with the ribosome dotted endoplasmic reticulum. On this ground it has been suggested that all the intracellular membranes should be recognised in one single endoplasmic membranous system. This is endomembrane concept.
3. Types of Endoplasmic Reticulum:
The endoplasmic membranous system has been classified into two different sub-systems:
1. Rough surfaced endoplasmic reticulum (RER) or granular endoplasmic reticulum.
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2. Agranular or Smooth surfaced endoplasmic reticulum (SER).
i. Granular or rough surfaced endoplasmic reticulum (RER):
The rough surfaced endoplasmic reticulum includes ribosome dotted cisternae and tubules of endoplasmic reticulum and microsomes. The limiting membranes of these tubules are studded with single layer of ribosomes particles which act as the sites of protein synthesis. The region of cytoplasmic matrix containing granular endoplasmic reticulum takes basophilic stains and is named as ergastoplasm.
The ribosome granules are called basophilic or chromophilic substances or nissl bodies. The reticulum takes basic stains due to the presence of RNA in ribosomes.
RER occurs predominantly in the cells which are actively involved in protein and enzyme synthesis. Depending upon the metabolic requirements of the cells, RER may be converted into SER or vice-versa.
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The microsomes are special type of organelles in the cytoplasm which are described as small vesicles bounded by thin surface membranes of lipoproteins impregnated with small ribosome particles. These are supposed to have developed when canaliculae of the ergastoplasm are broken into small spheres. The microsomes lie scattered in the cytoplasm.
These RNA rich bodies are the chief sites for protein synthesis and their membranes are involved in the steroid synthesis (Fig. 5.10B).
ii. Agranular or smooth surfaced endoplasmic reticulum (SER):
The smooth surfaced endoplasmic reticulum occurs generally in those cells which perform a variety of functions other than protein synthesis. SER is characteristic of cells synthesising phospholipids, glycolipids and steroids. SER consists usually of smooth walled tubules which are not studded with ribosomes and includes vesicles, vacuoles, infoldings of plasma membranes, golgi body etc.
Seikevitz and Palade (1966) have reported that the granular endoplasmic reticulum originated first and later the agranular or smooth surfaced reticulum developed from the former.
Buckley demonstrated that endoplasmic reticulum might be both mobile and elastic. The cisternae merge into the network and endoplasmic reticulum continuously sweeps through the cytoplasmic matrix resulting in proper distribution of the endoplasmic reticulum components and functional exchange between endoplasmic reticulum components and the matrix.
4. Origin of Endoplasmic Reticulum:
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The origin of endoplasmic reticulum is not exactly known.
But it is normally assumed that the endoplasmic reticulum originated in one of the following ways:
(i) Origin from nuclear envelope:
Since the membranes of endoplasmic reticulum resemble nuclear membrane and also at telophase stage the ER membranes are found to form nuclear envelope, it is normally assumed that ER originated by evagination of the nuclear membranes.
(ii) Origin from plasma membrane:
Since the tubules of endoplasmic reticulum are connected at places with the plasma membrane it is presumed to have originated by invagination of the plasma membrane.
(iii) De novo origin:
Some workers believe that endoplasmic reticulum has a de novo origin from the matrix of the cytoplasm.
Recently some workers like Leskes et al. (1971) and Eytan and Ohad (1972) have suggested that daughter cells receive full set of membranes from their parental cell and, therefore, the question of de novo synthesis of membranes does not arise and endoplasmic reticulum arises by the expansion of pre-existing membranes.
5. Enzymes of Endoplasmic Reticulum:
The membranes of the canaliculae of endoplasmic reticulum are reported to contain many kinds of enzymes which perform various important functions. The important enzymes occurring in the membranes are the stearases, NADH-cytochrome c reductase, NADH diaphorase, glucose 6 phosphatase and Mg++ activated ATPase.
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Certain enzymes of the endoplasmic reticulum are involved in the biosynthesis of phospholipids, ascorbic acid, glycerides, steroids and hexose, etc.
6. Functions of Endoplasmic Reticulum:
The endoplasmic reticulum is concerned with several important activities of the cells such as secretion, storage, circulation and so on.
The important functions of endoplasmic reticulum are listed under the following heads:
(A) Functions common for RER and SER:
1. The endoplasmic reticulum acts as ultra-structural skeletal framework in the cell and provides mechanical support.
2. The endoplasmic reticulum acts as transporting system and transports various products to different organelles.
RER——> SER ——> Cisternae of golgi body ——>Vacuoles or Secretary granules
3. The exchange of molecules through osmosis, diffusion and active transport occurs through the membranes of endoplasmic reticulum.
4. The endoplasmic membrane contains many kinds of enzyme which perform various synthetic and metabolic activities. Further the membranes of ER provide increased surface for different enzymatic reactions.
5. ER membranes are reported to form new nuclear envelope at telophase stage.
6. ER membranes are found to conduct intra-cellular impulses.
(B) Specific functions of SER:
1. Synthesis and storage of lipids and lipoproteins.
2. Glycogenolysis. Break down of glycogen through the action of glucose 6 phosphatase occurs in SER.
3. Detoxification. Harmful materials such as drugs, pollutants as well as metabolic wastes are converted by SER into the substances suitable for excretion by the cell. The detoxification may take place through oxidation, reduction, hydrolysis or covalent linking which make the poisonous substances either inactive or more soluble so that they may be easily eliminated.
4. Synthetic functions. SER is involved in the synthesis of a variety of substances such as steroids, pigments and cell wall materials.
(C) Specific functions of RER:
The major function of RER is the synthesis of proteins. In fact, the array of ribosomes of RER provides the surface for association of active enzymes, amino acids during the synthesis of proteins. The newly synthesized protein instead of passing into the cytoplasm first passes into the cisternae of RER and is thus protected from protease enzymes of cytoplasm.
The growing protein chain folds into its secondary and tertiary structures and is thus trapped in the cisternae of RER (Figs. 5.6 and 5.10).
Glycosylation of Secretory Proteins:
Nearly all the proteins for secretion are glycoprotein excepting albumin. Addition of carbohydrate to polypeptide chain is called glycosylation. While the polypeptide chain is growing, sugar molecules appear to be added one at a time usually from the nucleotide UDP in the cisternae of RER (Fig. 5.6).
However, if the glycoprotein is to contain a terminal glucose, Fucose or Sialic acid, the sugars are added in the golgi complex and canaliculae of RER where appropriate sugar transferases are localized.
