Get complete information on Endoplasmic Reticulum (ER)

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(Ergastoplasm of Gamier, 1897). E.R or EPR was discovered by Porter et al (1945). It was given the present name by Porter (1953). According to Dallmer (1966) E.R. originats from the plasma membrane by invagination. According to De Roberties (1970), ER originates from the nuclear envelope. E.R. is a system of membrane lined channels found in all eucaryotic cells except ova, embryonic cells and mature erythrocytes.

It constitutes more than 50% of total cell membranes. In muscle cells it is called SARCOPLASMIC RETICULUM. Myeloid bodies (granulls at base of retinal pigment cells) and Nissl granules are believed to be formed from E.R. It is less developed in meristematic cells. It is made of a few tubules in adipose tissue, few vesicles in spermatocytes but is best develpped in cells of liver, pancreas and fibroblast. Endoplasmic reticulum has three parts

(i) Cisternae (parallel interconnected flattened sacs)

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(ii) Tubules (often branched network)

(iii) Vesicles (round or oval)

Membranes of E.R. may bear ribosomes when E.R. is called granular/rough endoplasmic reticulum or R.E.R. Attachment between ribosome and endoplasmic reticulum is through glycoprotein called ribophorin. The union is actually between 60s ribosome subunit and E.R. by means of two types of glycoproteins: ribophorin I (mol. wt.-65000) and ribophorin II (mol. wt. 64000). R.E.R. often contains minute pores below ribosomes to pass synthesied polypetide into its lumen for transport.

E.R. without attached ribosomes is called agranular/smooth endoplasmic reticulum or S.E.R. It takes part in synthesis of glycogen, fats and sterols, and detoxification. S.E.R., therefore, occurs in adipose cells, muscle cells, liver cells, steroid synthesising cells, etc. R.E.R. is abundant in cells engaged in production and excretion of protein, e.g. plasma cells, goblet cells, pancreatic cells, certain liver cells.

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Broken pieces of endoplasmic reticulum appear as microsomes (Claude, 1941). Transitional E.R. is R.E.R. without ribosomes. Glycosylation of newly synthesized proteins occurs within the cisternae through activity of glycosyl transferase located in the E.R. membrane.

Function of ER:

(i) The endoplasmic reticulum divides the cell into separate compartments, making it possible for the cells different chemical products and activities to be segregated from each other.

(ii) Many of the enzymes that carry out their activity from a part of the lipoprotien structure of the membrane forming the endoplasmic reticulum.

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(iii) According to Sjostrand (1964), the tubules and vesicles have surfaces which may play role in enzymic reactions.

(iv) In addition to site of protein synthesis, it is also thought to function as a transport and storage system.

(v) E.R. also plays important role in lipid metabolism and glycogen synthesis.

Ribosomes (Protein Factories, Palade Granules): They are submicroscopic polypeptide manufacturing naked nucleoprotein organelles which were discovered by Robinson and Brown (1953) in plant cells and palade (1955) in animal cells. Palade (1955) also gave them the present name, ribosomes. Ribosomes are of two types, cytoplasmic & organelle.

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Organelle ribosomes are 70 s (Svedberg or sedimentation units). They occur inside mitochondria and plastids. Organelle ribosomes do not occur in procaryotes. Instead, their cytoplasmic ribosomes are 70 s. Cytoplasmic ribosomes of eucaryotes are 80 s. They may occur freely or attached to E.R. Ribosomes are subspherical in outline with a diameter of 150- 250 A° and are not bounded by any membrane. Each ribosome has two parts, smaller avoid or cap like and larger dome-shaped. The smaller subunit has a platform, cleft, head and base.

The larger subunit has a protuberance, a ridge and a stalk. On the basis of sedimentation coefficient, the larger and smaller subunits are respectively 60 s and 40 s (50 s & 30 s in 70 s ribosomes). The larger subunit has 28 s, 5.8 s and 5 s r RNA (23 s and 5 s in 70 s ribosomes). While the smaller subunit has 18 s r RNA (16 s r RNA is 70 s ribosome’s smaller unit).

Besides r RNA, ribosomes has a number of protein molecules like core and split proteins, structural and enzymatic proteins. As compared to r RNA, protein content is higher in 80 s ribosomes (40 : 60) while reverse is true is 70 s ribosomes (60 : 40). A groove is present in between the two ribosomes subunits for m RNA.

The larger subunit also has a channel for the passage of synthesized polypeptide. Magnesium ion is essential for binding the ribosome subunits. Smaller subunit has peptidyl transferase, binding site for t RNA and A-site. P-site is jointly formed by the two subunits. Ribosomes may occurs in helical or rosette group linked by m RNA. Such a group is called polyribo- some (Rich, 1963) or polysome.

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It is useful in producing a number of copies of the same polypeptide. The ribosomes remain attached with the membrane of endoplasmic reticulum by 60 s subunits. The 40 s subunit occurs on the larger subunit and forms cap-like structure. Peterman (1964) reported several kinds of RNA from ribosomes on the basis of analysis of purine and pyrimidine bases.

Ribosomes are not attached to membranes in prokaryotes, chloroplasts and mitrochondria but are commonly membrane bound is eukaryotic cells (specially actively secreting ones), forming rough endoplasmic reticulum, as well as being attached to outer surface of outer nuclear membrane. The origin of r RNA is the nucleolus.

Golgi Apparatus (Golgi Complex): It is complex organelle made of membrane lined stack of cisternae, network of tubules, vesicles and vacuoles which was first seen by George (1867) but studied by Camillo Golgi in 1898 in nerve cells of Barn Wol and cat through metallic impregnation technique (osmium chloride + silver salts).

Due to metallic impregnation artefacts, the apparatus was once called internal reticular apparatus/canalicular system/apparato reticolare. The concept changed after observation of the apparatus under electron microscope by Dalton and Felix (1954). Golgi apparatus is presented in all eucaryotic cells except RBC and sieve tube elements. It is also absent in procaryotes and sperm cells of seedless embroyphytes. A unit of Golgi apparatus is called Golgisome.

In paint cells, Golgi apparatus consists of a number of isolated units called dictyosomes while in animal cells it occurs as single compact or loose complex. The number of Golgi bodies/dictyosomes in generally 9-10 in a plant cell but is very high in cells engaged in secretory activity (e.g. root cap cells) and rapid division.

Golgi apparatus has a central stack of 3-10 curved but parallel membrane-lined narrow sacs called cisternae, cisternae are interconnected. Unicisternal dictyo­somes occur in fungi.

Golgi apparatus has two faces, maturing and forming (Mollenhauer and Whaley, 1963). The convex forming face (cis-face) receives materials from endoplasmic reticulum (GER = Golgi associated E.R.) and cytosol while concave maturing face (trans-face, normally towards plasma membrane) gives out large Golgian vacuoles and small vesicles having transformed materials.

Vesicles develop from the tubules as well. They are of two types, smooth and coated. Golgi apparatus produces materials for secretion, takes parts in transformation of membranes, formation of number of products like glycoproteins (e.g. mucin from goblet cells), complex hetero- polysaccharides (e.g. mucilage from root cap cells), hormones, melanin, matrix of connective tissue, middle lamella, acrosome and lysosome.

Animal cells usualy contain one. In granular cells, the golgi bodies are usually situated close to the nucleus and has according to Grasse (1957), definite polarity. There are also known as Lyphochondria. The membrane of G.C. or apparatus are of lipoproteins and are supposed to be originated from the membranes of endoplasmic reticulums.

In plant cells, scales, both organic and inorganic, are formed in the G.C. before being transported to the periphery. In diatoms, the golgi body gives rise to translucent vesicles which collect beneath the plasmalemma where they fuse to form a silicalemma in which the siliceous cell wall forms. Two types of vesicles are budded from the golgi body:

Coated vesicles (about-50 nm diameter) and the larger secretory vesicles (about 100 nm diameter), which tend to leave from the trans-surfaces. It has a system of outer flattened cisternae which appear as roughly parallel membranes enclosing a space 60-90 A° with a distance of about 200 A° between them. Cisternae and vesciles are associated with the inner cisternae particularly in glandular cells. It is also considered as the source of primary lysosomes membrane bounded particles containing hydrolytic enzymes and also functions as packing centre of the cell.

Lysosomes (Suicide bags, Disposal Units): These are small vesicles (0.2-0.8jim) containing hydrolysing enzymes and surrounded by single membrane. They were discovered by Christian de Duve in 1955 but were named and observed under electron microscopes by Novikoff (1956).

They are formed by golgi apparatus and contain some 40 types of acid hydrolases for digestion of various materials (e.g. nucleases, proteases, phosphatases, sulphatases). Therefore, they can hydrolyse almost all types of organic substances except cellulose. Lysosomes are most abundant in phagocytic cells (WBC) and osteoclasts.

Lysosomes show polymorphism: primary lysosomes, secondary lysosomes (digestive vacuoles = heterophagosomes, primary lysosomes + phagosome); residual (lysosomes having undigested materials, undergo ephagy) and autophagic vacuoles (performing autophagy or autolysis of degenerate and useless parts). In many plant cells the function of lysosome is performed by spherosomes and vacuoles. Lysosome membrane is strengthened by cortisome, Cortisol, antihistamise, heparin, chloroquine and a type of cholesterol.

It becomes fragile in the absence of oxygen or presence of excess vitamin A, vitamin E, progesterone, testosterone, bile salts and high energy radiations. Failure of exocytosis or absence of some hydrolytic enzymes causes accumulation of residual bodies in the cells producing various diseases like hapatitis, polynephritis, Hurler’s disease (deformed bones due to accumulation of mucopoly saccharide glucosaminoglycan as its hydrolytic enzyme is absent). Fruit rotting can be however, checked by slowing down the action of rotting enzyme polygalactouronase.

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