Term Paper on Endoplasmic Reticulum | Cell Organelle | Cell Biology

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Term Paper on Endoplasmic Reticulum

Term Paper Contents:

1. Term Paper on the Introduction to Endoplasmic Reticulum
2. Term Paper on the Origin of Endoplasmic Reticulum
3. Term Paper on the Occurrence of Endoplasmic Reticulum
4. Term Paper on the Morphology of Endoplasmic Reticulum
5. Term Paper on the Types of Endoplasmic Reticulum
6. Term Paper on the Modification of Endoplasmic Reticulum
7. Term Paper on the Isolation of Endoplasmic Reticulum
8. Term Paper on the Enzymes of the Endoplasmic Reticulum Membranes
9. Term Paper on the Other Functions of Endoplasmic Reticulum

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Term Paper # 1. Introduction to Endoplasmic Reticulum:

Endoplasmic reticulum was first or all observed in 1945 by Porter, Claude and Fullam. They noted the presence of a network or reticulum of strands associated with vesicle-like bodies in the cytoplasm of the cultural fibroblast or thinly-spread tissue culture cells. Further electron microscopy by Porter and Thompson has revealed that these strands of reticulum are vesicular bodies inter-connected, so as to form a complex network in the inner endoplasmic part of the cytoplasm.

As this network is concentrated in the endoplasm of the cell more than in the ectoplasm, therefore, it is known as endoplasmic reticulum (ER), or ergastoplasm or vacuolar system of the cell. The endoplasmic reticulum is not visible in the cytoplasm of a living cell under the phase-contrast microscope but the observations by electron microscope on the fixed cell have revealed this structure.

In 1952, studies with electron microscope further confirmed the presence of endoplasmic reticulum as reported by Porter and his colleagues. Recent studies have further confirmed and accepted the concept of a structural organisation of cytoplasm. Recently, under the phase-contrast microscope, Fawcett and Ito, and Rose and Pomerat have studied the structure and distribution of endoplasmic reticulum in the living tissue-culture cells.

The endoplasmic reticulum (ER) is a network of disk-like tubules, sacks and vesicles found in eukaryotic cells. Its main function is to operate as a transport system. It consists of lipid bi -layers, which contain embedded proteins. This internal system of membrane is continuous with the double membrane that surrounds the cell’s nucleus. Therefore, the encoded instruc­tions that the nucleus sends out for the synthesis of proteins flow directly into the endoplas­mic reticulum.

Within the cell, the endoplasmic reticulum synthesizes lipids and proteins. The endoplasmic reticulum is responsible for the production of the protein and lipid compo­nents of most of the cell’s organelles. The ER is additionally responsible for moving proteins and other carbohydrates to the Golgi apparatus, to the plasma membrane, to the lysosomes, or wherever else needed. The ER is made up of three types of structures- cisternae (diameter 400-500 Å), tubules (500-1000 Å) and vesicles (250-5000 Å).

All the three structures are bound by a single unit membrane of about 50 Å thicknesses. In most cells the endoplasmic reticulum is thought to consist of only one continuous mem­brane enclosing only a single space. However, in protozoa, some unicellular algae, and pos­sibly some fungi, the endoplasmic reticulum occurs as separate, multiple vesicles. Several morphologically and functionally distinct domains of this continuous membrane system can be distinguished.

The outer nuclear membrane in turn is continuous with the rough endoplas­mic reticulum, which contains specialized regions, termed transitional elements, and is con­tinuous with the smooth endoplasmic reticulum. Endoplasmic reticulum is chiefly made up of a phospholipid membrane. The rough and smooth endoplasmic reticula and the transi­tional element enclose a space called the intra-cistemal space, or lumen. Both intra-cisternal and peri-nuclear spaces form a single compartment.

The amount of smooth and rough endo­plasmic reticulum varies greatly among different cell types. The ER is folded and stacked layer upon layer within the cell and is connected to the cell’s nuclear membrane. Under a microscope, the endoplasmic reticulum is seen as a highly folded structure surrounding the cell nucleus. The endoplasmic reticulum often makes up more than 10 percent of a cell’s to­tal volume. The endoplasmic reticulum is generally divided into two major sections: the rough endoplasmic reticulum and the smooth endoplasmic reticulum.

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Term Paper # 2. Origin of Endoplasmic Reticulum:

Multi Step Mechanism:

Although, the origin of the new endoplasmic reticulum membrane, has not been fully understood. The views are there. In fact, one of the possible functions attributed to the endoplasmic reticulum is that of membrane biosynthesis. The protein components of endoplasmic reticulum and other membranes may be assembly by activity of the endoplasmic reticulum. There is certainly convincing evidence that Golgi membranes and many cytoplasmic vesicles can be derived from endoplasmic reticulum.

Moreover, endoplasmic reticular membranes appear to be continuously synthesized, having a relatively high rate of turn over. At the same time, the several elements of endoplasmic reticulum in the cell are asynchronous in this respect, they are not all replaced at the same time or with the same rate. It has also been suggested that membranes of the endoplasmic reticulum are formed not from pre­existing elements but from the ground substance of the cytoplasm. Thus the process by which a membrane is modified chemically and structurally is called membrane differentiation.

From Nuclear Membrane:

The vacuoles derived from the invagination of the outer membrane of the nuclear envelope, which separates from its inner partner, leaving cavities between. Shortly after the separation, small vesicles appear near the nuclear envelope, suggesting that parts of the envelope give rise to elements of the endoplasmic reticulum. Thus the endoplasmic reticulum seems to have its origin in the nuclear envelop in undifferentiated cells.

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Term Paper # 3. Occurrence of Endoplasmic Reticulum:

The endoplasmic reticulum occurs in all the eukaryotic cells except erythrocytes (R.B.Cs) of mammals. It is absent in prokaryotes. Its deve­lopment varies considerably in various cell types. It is small and undifferentiated in eggs and in undifferentiated embryonic cells. Only a few vacuoles are present in the spermatocytes and muscle cells.

However, it is highly organised in cells synthesizing proteins or in cells that are engaged in lipid metabolism.

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Term Paper # 4. Morphology of Endoplasmic Reticulum:

The endoplasmic reticulum has been found in all kinds of mature cells except the mature mammalian erythrocyte, which is also devoid of a nucleus. Actually the first description of these structures seemed with electron microscope by Porter, Claude, and Fullam in 1945 in cultured cells.

These are membrane bounded sacs in the form of double membranes (cisternae) by Sjostrand or the name cisternae was given by Sjostrand and the name tubules was given by Kurosumi. Rounded and irregular sacs or vesicles were observed by Weiss 1953.

Morphologically the endoplasmic reticulum is composed of following three kinds of structures, viz.:

1. Cisternae,

2. Vesicles and

3. Tubules.

1. Cisternae or Lamellae:

They are long, flattened and usually un-branched tubules, which are arranged in parallel arrays. They are of uniform width throughout and their thickness varies from 40-50 mμ. This pattern of reticulum is characteristic of basophilic regions of the cytoplasm and of those cells, which are active in protein synthesis. Lamellae or cisternae occur in the liver-cells, plasma-cells, pancreatic cells, brain-cells and in notochord cells etc.

2. Vesicles:

The vesicles range in diameter from 25 to 500 μm and are for the most part rounded in shape. These are abundant in the cells engaged in the protein synthesis as in hepatic and pancreatic cells.

All these three patterns of endoplasmic reticulum may occur in the same cell or in different cells. Their arrangement also differs in different cells viz., in parallel rows in the liver-cells of mammals. In notochordal cells of Ambyostoma larva, the pattern of cisternae is of still another type.

3. Tubules:

The tubules are small, smooth-walled, branched tubular spaces having a diameter of about 50-190 mμ. These occur in cells that are busy in the synthesis of steroids like cholesterol, glycerides hormones. These are haphazardly arranged in the cytoplasm of developing spermatids of guinea pig, muscles cells and other non-secretory cells.

Ultrastructure of Endoplasmic Reticulum:

All the three structures of the endoplasmic reticulum are bounded by a thin membrane of 50 to 60 A° thick. Like the plasma membrane, nucleus, etc., its membrane is also formed of three layers—the outer and inner dense layers are composed of protein molecules, and the two middle thin and transparent layers are of phospholipids. The endoplasmic reticular membrane is continuous with the plasma membrane, nuclear membrane and membrane of Golgi complex. The lumen of the endoplasmic reticulum acts as a passage for the secretory products and Palade has observed the secretory granules in it.

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Term Paper # 5. Types of Endoplasmic Reticulum:

On the basis of presence or absence of ribosomes they are of two types:

(i) Granular or Rough Walled Endoplasmic Reticulum:

When the particles or ribosomes are present on the wall of E.R., it is called rough walled E.R. These particles are always present at the outer surface of the ER. i.e. on the surface of the limiting membrane facing the continuous phase the matrix of the cytoplasm. The elements (E.R.) with rough surfaces are high in ribonucleic acid and are intensely basophilic. The membranes themselves are not rough, but associated with their outer surfaces are tiny particulate components 100 or 150A° in diameter.

These are called ribonucleoprotein (RNP) particles or ribosomes and contain as the average 40% RNA and 60% protein. The elements having ribosomes are usually of the cisternal type and are found in cells active in protein synthesis. Biochemical studies have indicated that the ribosomes are important in protein synthesis, even though the membranes are not always necessary for this activity.

The smooth surfaced E.R. is often continuous with the rough surfaced E.R. thereby making the absence or presence of ribosomes the only significant difference between the two. The continuity between smooth and rough E.R. has been repeated by demonstration.

It has been suggested more over that one grows from the other but which from which is uncertain. The ribosomes can readily be dissociated from the endoplasmic reticulum membranes by treatment with deoxycholate.

(ii) Smooth Walled Endoplasmic Reticulum:

The name smooth walled is given to that portion of endoplasmic reticulum that is devoid of ribosomes. Like the rough walled endoplasmic reticulum smooth form shows a characteristic morphology which is tubular rather than cisternae.

This smooth walled endoplasmic reticulum is found in the cells that are active in the synthesis of steroid compounds such as cholesterol, glycerides, and the hormones (testosterone and progesterone). It is studied by Fawcett that are also present in the pigmented epithelial cells of the retina which are involved in the metabolism of vitamin A in the production of visual pigment. Glycogen storing cells of the liver contain the smooth, tubular elements of the endoplasmic reticulum.

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Term Paper # 6. Modification of Endoplasmic Reticulum:

Sarcoplasmic Reticulum:

Sarcoplasmic reticulum, found in the skeletal and cardiac muscles is a highly modified form of smooth ER. It was first reported by Veratti as delicate plexuses in skeletal muscles surrounding the myofibrils. Electron microscopy showed it to be composed of a network of membrane like tubules which run longitudinally in the interfibrillar sarcoplasmic space for the length of each sarcomere.

At the level of H and I bands, these tubules merge with large cisternal structures. At the H band level this cisterna, called the central cisterna, forms a sieve-like structure round the myofibrils. At the level of I band, these tubules merge with the large terminal cisternae, from which transverse tubules extend peripherally to the sarcolemma and are continuous with and are deep invaginations of it.

It is generally believed that sarcoplasmic reticulum plays a role not only in distributing energy-rich material needed for muscular contraction but also in providing the necessary channels for transmitting impulses along the surface and conveying the action potential from the surface to the myofibrils within.

Ergastoplasm:

There are certain regions in the cytoplasm that stain with basic dyes. To these regions various names have been given like chromidial substance, basoplasm, ergastoplasm and so forth. The term ergastoplasm was given by Gamier in 1899 to those cytoplasmic filaments in the cells of exocrine glands which stained readily with basic stains. Weiss (1953) referred to the cisternal elements as ergastoplasmic sacs.

In nerve cells, such areas are called Nissil bodies. Electron microscopic studies reveal it to be an accumulation of ribosomes situated on the parallel lamellae of ER stacks of accumulated freely in the groundplasm. Studies of Caspersson, Bracket and others have demonstrated that the basophilic nature of ergastoplasm is due to the ribonucleic acid. Smooth E.R. areas of cytoplasm are never ergastoplasm.

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Term Paper # 7. Isolation of Endoplasmic Reticulum:

Endoplasmic reticulum can also be isolated mechanically with the help of centrifuge. When tissues or cells are disrupted by homogenisation the E.R. is fragmented into many smaller closed vesicles called microsomes (100 nm diameter), which are relatively easy to purify.

Microsomes derived from rough E.R. are studied with ribosomes and are called rough microsomes. Many vesicles of size similar to that of rough microsomes, but lacking attached ribosomes, are also found or these homogenates. Such smooth microsomes are derived in part from smooth portions of the ER and in part from vesiculated fragments of plasma membrane, Golgi complex and mitochondria (the ratio depending on the tissue).

Thus, while rough microsomes can be equated with rough portions of ER, the origins of smooth microsomes cannot be easily assigned. An outstanding exception is the liver. Because of the exceedingly large quantities of smooth ER in the hepatocyte, most of the smooth microsomes in liver homogenates are derived from smooth ER.

Ribosomes, which contain large amounts of RNA, make rough microsomes more dense than smooth microsomes. As a result, the rough and smooth microsomes can be separated from each other by sedimenting the mixture to equilibrium in sucrost density gradients.

When the separated rough and smooth microsomes of a tissue such as liver are compared with respect to such properties as enzyme activity or poly peptide composition, they are remarkably similar, although not identical. It, therefore, seems that most of the components of the ER membrane can diffuse freely between rough and smooth regions of the E.R. membrane, as would be expected for a fluid, continuous membrane system.

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Term Paper # 8. Enzymes of the Endoplasmic Reticulum Membranes:

The membranes of the endoplasmic reticulum are found to contain many kinds of enzymes which are needed for various important synthetic activities. The most important enzymes are the stearases, NADH- cytochrome C reductase, NADH diaphorase glucose-6-phosphotase-and- Mg⁺⁺ activated ATPase. Certain enzymes of the endoplasmic reticulum such as nucleotide diphosphate are involved in the biosynthesis of phospholipid, ascorbic acid, glucuronide, steroids and bexose metabolism.

The enzymes of the endoplasmic reticulum perform the following important functions:

1. Synthesis of glycerides, e.g., triglycerides, phospholipids, glycolipids.

2. Metabolism of plasmalgens.

3. Synthesis of fatty acids.

4. Biosynthesis of the steroids, e.g., cholesterol biosynthesis, steroid hydrogenation of unsaturated bonds.

5. NADPH₂ + O₂— requiring steroid transformations:

Aromatisatiop and hydroxylation.

6. NADPH, + O₂—requiring steroid transformations:

Aromatic, hydroxylations, side-chain oxidation, deamination, the other oxidation, desulfuration.

7. L-ascorbic acid synthesis.

8. UDP-uronic acid metabolism.

9. UDP-glucose dephosphorylation

10. Aryl-and steroid sulphatase.

Functions of the Smooth ER:

It is a little artificial to try to separate the activities of the rough and smooth ER because of their numerous interconnections. One obvious difference, of course, is participation of the rough ER in protein synthesis by virtue of its associated ribosomes. Smooth ER, on the other hand, is more involved with lipid synthesis.

Cells concerned largely with synthesis of lipids have well developed smooth ER just as cells that synthesize and secrete proteins have well developed rough ER. Thus, cells of the adrenal cortex (which synthesize steroid hormones) have abundant smooth ER.

So do intestinal absorptive cells, for the smooth ER supports triglyceride synthesis from the mixture of fatty acids, monoglycerides, and diglycerides taken up by these cells from the intestine. (Dietary fat is broken down by pancreatic lipase to provide these raw materials.) Fat drops (chylomicrons), comprised almost exclusively of triglyride, can be readily identified within the smooth ER near the intestinal lumen shortly after a meal.

The smooth ER of liver cells has functions that are certainly more prominent there. These fictions include regulation of glycogen breakdown (via glucose 6-phosphatase, an ER- associated enzyme), and oxidation of a variety of naturally occurring foreign substances, the purpose of which is usually to eliminate their biological activity. When applied to drugs, this process is referred to as detoxification.

The reactions that detoxify substances consist of oxidations reductions, hydrolyses, or a covalent linking (conjugaon) to soluble small molecules, particularly conjugation to lucuronic acid, a sugar derivative. These changes either activate the substance or make it more soluble and hence more readily eliminated by the kidneys.

A wide variety of natural and artificial chemicals are affected in this way, including environmental pollutants and many drugs. Birth control pills, for example, were made possible by the discovery of drugs having some of the activities of estrogens and progesterone but which are not rapidly inactivated by the liver as are the natural steroid hormones.

In general, these reactions of the smooth ER serve a valuable protective function and also play a role in the normal handling of fatty acids, bile salts, steroids, and heme, recovered from hemoglobin breakdown. The responsible enzymes are associated with endoplasmic reticulum in several different cell types, and related systems have been identified even in prokaryotes. They are especially prominent, however, in liver.

Most of the bio-transformations in this category are oxidations, primarily because of the multitude of ways in which organic molecules can be oxidised. The first key to understanding the mechanism was a proposal by Howard S. Mason of the University of Oregon Medical School for the existence of “mixed function oxidases” utilising both NADPH and molecular oxygen.

The principal enzyme is cytochrome P450, a heme protein so named because it absorbs light maximally at 450 nm when in the reduced form. This protein is capable of binding substrate and O₂, passing one of the oxygen atoms of O₂ to the substrate in the form of an OH group (an oxidation of the substrate) while the other oxygen atom combines with H⁺ to form water-

Here, S is the substrate and Fe³⁺ the heme-associated with P450. The electrons are delivered to P450 mostly from NADPH by an endoplasmic-reticulum-associated electron transport chain involving flavoproteins and in some cases another cytochrome known as cytochrome b₅.

Cytochrome P450 and other elements of the microsomal electron transport chain seem to be membrane-bound proteins present mostly on the cytoplasmic surface of the smooth endoplasmic reticulum. Their concentration, however, varies with need.

Phenobarbital can cause the amount of smooth ER to more than double, but the amount of P450 per cell may at the same time increase fivefold. Although there are apparently several versions of P450 with somewhat different specificities, there is broad overlap so that induction with one substrate increases the ability of the ER to detoxify other substrates handled by the same system even though the other substrates may not themselves be effective inducers of P450.

Thus, for example, treatment of a human with more than on ER- metabolised drug at a time often requires careful adjustment of dosage.

Functions of the Rough ER:

It has long been assumed that proteins destined for secretion from the cell are synthesized on ER-bound ribosomes while cytoplasmic proteins are translated for the most part on free ribosomes. This separation is not hard and fast, for even cells that secrete little or no protein typically have significant amounts of rough ER. However, when radioactive amino acids are injected into a cell that makes secreted proteins, the radioactivity very quickly becomes recoverable in the rough microsomal fraction, thus supporting out generalisation.

What is more, further investigation has revealed that this rough ER associated protein is immune to proteases. Not only does translation occur on membrane-bound ribosomes, but the secretory protein, instead of passing into the cytoplasm, appears to pass instead into the cisterna of the rough ER and hence into microsomes where it is protected from experimentally added proteases.

From the rough ER, secreted proteins pass sequentially to smooth surfaced ER, Golgi apparatus, secretion vesicles, and finally to the exterior of the cell.

The questions to be answered here are:

(1) How does mRNA of the secretory proteins bind specifically to membrane-bound ribosomes, and

(2) How does the protein get from the ribosome to the interior of the R?

The answers to those critical questions appear to be at and, thanks in large part to pioneering experiments performed by Gunter Blobel at Rockefeller University and David Sabatini at New York University School of Medicine.

Synthesis of Secretory Proteins:

The obvious first question is whether there is anything different about membrane-bound ribosomes. If they are the same as free ribosomes, then the specific association between mRNA and rough ER must be due to a feature peculiar to the mRNA or to the protein made from it. In fact, that is where we must look, because it appears that ribosomes are selected non-specifically for membrane association.

Electron microscopy reveals that membrane-bound ribosomes are attached by their large, 60S subunit, with the small or 40S subunit sitting on top like a snowman. There is no considerable evidence for the existence of specific binding site within the membrane capable of attaching only to the 60S subunit.

Evidence for specific ribosomal binding sites was obtained by experiments in which rough microsomes were stripped of their ribosomes and then exposed to free ribosomes obtained either in this way or from the cytoplasmic pool.

The result was reattachment of ribosomes to roughly the original concentration. Equivalent treatment of smooth microsomes results in much lower rates of ribosomal attachment, though the affinity of the attached ribosomes is comparable, leading one to conclude that the same type of binding site is involved but present in much lower concentration.

Control experiments with plasma membranes and Golgi membranes showed weak interaction with ribosomes and low capacity for attachment. Still further evidence for specific binding sites was obtained in experiments where the stripped microsomes were exposed to very mild proteolytic activity, destroying their capacity to accept ribosomes by damaging the ribosomal binding site.

The presumed ribosomal binding sites (or receptors) appear as membrane proteins on freeze-fracture, extending well into and possibly through the lipid bilayer. These proteins, like most other membrane proteins, enjoy considerable freedom to diffuse laterally, so that “patching” and even a sort of “capping” of bound ribosomes can occur in vitro at temperatures above the phase transition of the membrane.

This mobility of course, would facilitate formation of the polysome and probably translation, which requires that the mRNA and ribosome move with respect to each other. Absolute freedom of movement is apparently not allowed in vivo, however, otherwise the distinction between rough and smooth ER would disappear.

It is probable, based on other experiments, that the association between mRNA and ribosomes-i.e., polysome formation -begins while ribosomes are free. They later attach to the ER, depending on availability of ribosome binding sites. This association, according to some data, is fostered by binding of the mRNA to membrane; other data point to association between the new polypeptide chain and membrane. Both, factors may be important.

Work from Sabatini’s laboratory demonstrated that the 3′ end of mRNA from membrane-bound polysomes can remain attached to the membrane even after the ribosomes have been dissociated and stripped away. Other experiments suggest that certain mRNA can associate with microsome formed from stripped rough ER even in the absence of ribosomes.

The 3′ end of mRNA is of course, the end that gets translated first. It is also the end carrying, in eukaryotic cells, a 150-200 nucleotide segment of poly A. However, the poly A is found on mRNA of both types, free as well as membrane bound. Hence, it is probable that mRNA ending for secreted proteins contains near this segment a sequence that binds specifically to ER-or, more likely, to protein receptors within the ER.

Another line of investigation, carried out in large part by Blobel and his colleagues, points to an affinity between membrane and an N-terminal segment of those polypeptide chains stained for secretion. Translation of the immunoglobulin chain, for instance, produces first a 20- amino- acid segment that is later cleaved from the chain. This segment is largely hydrophobic in character, giving it an affinity for membrane.

Comparable segments, have en identified at the N-terminus of other incomplete secretory proteins. Removal of these segments, at least in the case of the immunoglobulin light chain, is catalysed by enzymes of the rough ER and takes place even before completion of the polypeptide chain.

These two proposals for directing secretory protein mRNA to rough ER-namely, attachment of the mRNA itself, and affinity between the nascent (incomplete) polypeptide and membrane—are not mutually exclusive. Both may function to help ensure translation of these mRNAs specifically at the rough ER.

Isolation of Secretory Proteins by the ER:

The next problem is how the polypeptide chain that is destined for export gets into the cisternae of the ER. It does so very quickly, as demonstrated by Sabatini and Blobel in 1970, when they showed that mild proteolysis of rough microsomes did not destroy the nascent polypeptide chains but divided them into two parts. The incomplete C- terminal segment was protected by the ribosome on which it was being synthesized. The N-terminal segment was protected by the membrane of the ER. Hence, passage into the ER cisterna takes place during translation, leaving only a small segment exposed to the cytoplasm at any one time.

How the polypeptide chain gets through the lipid bilayer is not so clear, but it is quite reasonable to propose that the membrane protein serving as a ribosomal receptor also has a channel through its core that opens into the cisterna of the ER.

The channel would have to be no larger than some of the water and cation pores associated with certain proteins of the plasma membrane, for there is free rotation about most of the non-peptide bonds in the peptide chain. Hence the chain has great flexibility, permitting the amino acids to snake their way single file through the proposed pore.

Folding into secondary and tertiary structures also begins while the polypeptide is still in the process of synthesis. Formation of such structures would prevent return of the peptide through the narrow orifice from hence it came, thus trapping it in the cisternae of the rough ER.

Glycosylation of Proteins:

Nearly all proteins destined for secretion are glycoproteins; notable exception is albumin, actually the most common protein in serum. Glycosylation—i.e., the addition of carbohydrate—begins even before the polypeptide chain is complete. The sugar molecules appear to be added one at a time, transferred usually from the nucleotide UDP, which serves as carrier.

Glycosylation is ordinarily still in progress when e polypeptide chain is completed and released into the cisterna of the rough ER. However, if the glycoprotein is to maintain a terminal galactose, fucose, or sialic acid, those sugars be added in the Golgi apparatus where the appropriate sugar transferases are localised.

The stepwise addition of sugars to proteins from an UDPgar intermediate is well documented. However, in recent years it has become clear that animal cells like cells of bacteria A plants, have a second and quite different way of glycosyling proteins.

In this second pathway, the sugar chains are assembled in a stepwise manner from nucleotide- sugars, but the assembly is not on the protein itself. Rather, it is on a lipid, either retinol (vitamin A) or on one of a family of lipids called dolichols. The latter system has been particularly well studied.

In the lipid-linked assembly of certain oligosaccharides, transfer is initially to dolichol phosphate, creating a high energy pyrophosphate (— P—O—P—) linkage. The first sugar in the chain is typically N-acetylglu- cosamine (GlcNAc).

Thus we have (where P is phosphate):

UDP—GIcNAc + dol—P → dol—P—P—GIcNAc + UMP

Thereafter the chain is lengthened one sugar at a time. When the chain is nearly complete, it is transferred intact to the protein, using cleavage of the pyrophosphate to drive the reaction.

Why should two so very different mechanisms for protein glycosylation exist? All the evidence is not in but it appears probable that the lipid-linked system functions in the glycosylation of membrane proteins. The assembly of carbohydrate chains onto dolichol seems to take place, at least in part, on the cytoplasmic side of the membrane, rather than in the cisternae of the ER.

Although membrane proteins seem to be synthesized largely on membrane-bound ribosomes (one season for the presence of rough ER in non-secretory cells), one can imagine that their hydrophobic nature causes strive to become incorporated into the membranes of the endoplasmic reticulum rather than confined to the cisternae within.

Transfer from dolichol might then take place at the surface of the ER, which would also explain how membrane proteins come to be glycosylated in such an asymmetric way—i.e., on only one side. One of the functions of carbohydrate on secretory proteins, then, might be to provide a hydrophilic coating to help keep the proteins in the cisternae of the ER.

In any case, membrane proteins, like those destined for export, pass from the rough ER to smooth surfaced ER adjacent to the Golgi apparatus and then ordinarily to the Golgi apparatus itself where terminal sugars may be added in a stepwise fashion.

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Term Paper # 9. Other Functions of Endoplasmic Reticulum:

Many functional interpretations of endoplasmic reticulum based on the polymorphic aspects of its components in a variety of cells and its different stages of activity. More reliable interpretations are based on the isolation studies just positioned.

The following functions are based on the same known facts together with hypothesis:

a. Mechanical Support:

ER contributes to the mechanical support of the cytoplasm by dividing the fluid contents of the cell into compartments; this makes possible the existence of ionic gradients and electrical potentials along ER membranes. This concept has been specially applied to sarcoplasmic reticulum.

b. Exchange of Ions and any Other Fluid:

The membranes of the endoplasmic reticulum may regulate the exchange between the inner compartment or cavity and the cytoplasmic matrix. The following statistic given an impressive idea of the surface area available for exchange: 1 gm of liver contains about 8 to 12 square meter of endoplasmic reticulum. After isolation, microsomes expand or shrink according to the osmotic pressure of the fluid. Diffusion and active transports may take place across the membrane of the endoplasmic reticulum.

c. Intracellular Circulation:

The endoplasmic reticulum may act as a kind of circulatory system for intracellular circulation of various substances, membrane flow may be an important mechanism for carrying )articles, molecules and ions into and out of the cells by way & vascular system. The “pinocytosis”, a “cellular drinking” also takes place by endoplasmic reticulum.

By this mechanism, particles attached to the surface of the cell or suspended in the fluid medium can be incorporated into the cytoplasm. The similar mechanism but working in a reverse direction can affect the transport of a particle from the interior of the cytoplasm to the outer medium.

The continuities observed in some cases between the endoplasmic reticulum and the nuclear envelope suggests that the membrane flow may also be active at this point. This flow would provide one of the several mechanisms for export of RNA and nucleoproteins from the nucleus to the cytoplasm.

d. Protein Synthesis:

Proteins may be synthesized to be utilised within the cell or these may have to be exported outside the cell to the site of their utility. It is the latter kind of proteins in whose synthesis; endoplasmic reticulum plays an important role.

For instance, rough endoplasmic reticulum, which has attached ribosomes carries synthesis of secretory proteins on these ribosomes and export them. Synthesis of tropocollagen, serum proteins and secretory granules are some examples of secretory proteins.

The protein molecules synthesized on attached ribosomes are discharged and penetrate into the cavity of ER, where they are stored or exported outside. During the transport of these products, three types of membranes ER-Golgi membrane plasma membrane should interact and remain interconnected or disconnected due to fusion and fission respectively.

e. Synthesis of Lipid:

The cells in which active lipid metabolism takes place are found to contain large amount of the smooth type of endoplasmic reticulum. According to some workers as Christensen and Claude the smooth type of endoplasmic reticulum is related with the synthesis and metabolism of the lipids.

f. Synthesis of Glycogen:

The smooth endoplasmic reticulum of the glycogen storing cells of the liver and the cells of certain plants is found to be associated with the synthesis, storage and metabolism of the glycogen. But, Porter and Peter have suggested that smooth type of endoplasmic reticulum is related to glycogenolysis (digestion of glycogen) and not to glycogenesis (synthesis of the glycogen).

g. Detoxification:

Smooth ER is also involved in the detoxification of many endogenous and exogenous compounds. Prolonged administration of certain drugs (phenobarbitol) results in the increased sensitivity of enzymes related to detoxification, as well as other enzymes, and a considerable hypertrophy of the SER. This is also applicable to administered steroid hormones.

h. Synthesis of Cholesterol and Steroid Hormones:

Cholesterol is an important precursor of steroid hormones. The major site of cholesterol synthesis is the ER. In liver cells he SER is believed to be concerned with both the synthesis and storage of cholesterol.

In the testis, ovary and the adrenal cortex the SER has a role in the synthesis of steroid hormones. The enzymes catalysing biosynthesis of androgens have been located in the SER. There is a strong correlation between the amounts of SER in cells and the capacity to synthesize steroid hormones.

i. Amphibian Development:

There are evidences to suggest that the ER contributes in several ways to the development of the amphibian embryo.

j. Cell Differentiation:

Some specific instances of development have been studied in details which more or less confirm the contention that the ER is important in the process of cell differentiation. Not only this much, ER also plays role in co-ordinating the differentiation.

k. Formation of Microbodies:

Closely related with the ER are microbodies, which are small granular bodies filled with an electron dense substance and limited by a single membrane. Microbodies are formed as dilations of the ER and frequently show connections with the ER cisternae. They are rich in the enzymes peroxidase (and are hence, also called peroxisomes), catalase, and D-amino acid oxidase. In plant cells the enzymatic content is different and the bodies are called glyoxy-somes because they include enzymes of the glyoxylate cycle.

l. Enzyme Activities and Cellular Metabolism:

Numerous enzymes mainly those involved in the metabolism of steroids (cholesterol and glycerides), phospholipids and hormones (testosterone and progesterone) are associated with the membranes of smooth endoplasmic reticulum. These membranes provide an increased inner surface for various metabolic reactions and they themselves take an active part in them by means of attached enzymes. This facilitates free union of enzymes with their substrates.

Role of Endoplasmic Reticulum in Intra-Cellular Impulse Conduction:

The existence of endoplasmic reticulum separating the cytoplasm into two compartments makes possible the existence of ionic gradients and electrical potentials across these intracellular membranes.

The idea has been applied to the sarcoplasmic reticulum, a specialised form of smooth surfaced endoplasmic reticulum found in striated muscles which is now being considered as intracellular conducting system. On the basis of some evidences, it has been postulated that the sarcoplasmic reticulum transmits impulses from the surface membrane into deep regions of the muscle fibres.

Formation of Plasmodermata:

Electron microscopic studies suggest that the endoplasmic reticulum in plants plays a special role in the interconnection of cells through the cytoplasmic strands called plasmodermata.

Role of Endoplasmic Reticulum during Cell Division:

During cell division, some of the elements of reticulum contribute in the formation of the new nuclear membrane after karyogamy. The nuclear membrane breaks up into fragment; in the early part of the division which finally disintegrate into small vesicles.

These vesicles move towards the pole of the spindle as the metaphase starts, where they are indistinguishable from the elements of ER. From the polar ends of the cell, elements of ER as well as the fragmented vesicles migrate into the regions around the chromosomes, which are grouping at the poles. Most of these elements of ER join or fuse around each group of daughter chromosomes to form a new nuclear envelop.

i. Transportation of Message from Genetic Material:

ER provides passage for the genetic material to pass from the nucleus to the various organelles in the cytoplasm, thereby controlling the synthesis of proteins, fats and carbohydrates.

ii. ATP Synthesis:

ER membranes are the sites of ATP synthesis in the cell. The ATP is used as a source of energy for all the intracellular metabolism and transport of materials.

iii. Formation of Cell Organelle:

Most of cell organelles like Golgi complex, mitochondria, lysomes, nuclear membrane and cell plate etc. are usually developed from endoplasmic reticulum.