Today we will understand in detail the Endoplasmic Reticulum structure under biology.
Read Contents This Artical
- Discovery of ER
- Definition of ER
- Occurrence of ER
- Types of ER
- Ultrastructure of ER
- Ultrastructure and chemical composition of ER
- Origin of ER
- Modifications of ER
- Interrelationship between ER and other membranes
- Functions of ER
- Functions of SER and RER
Endoplasmic reticulum is the largest single membrane bound intracellular compartment. It is an extensive network of closed and flattened membrane bound structure. The enclosed compartment is called as ER lumen. ER membranes are physiologically active, interact with the cytoskeleton and contain differentiated domains specialized for distinct functions.
1. Discovery of ER
Discovery Endoplasmic Reticulum was first discovered by Garnier in 1897, and named it as ergastoplasm, but its ultrastructure was first given by Porter, Claude, and Fullam in 1945, and coined the term Endoplasmic Reticulum.
2. Definition of ER
Endoplasmic reticulum is a well-developed electron microscopic network of interconnected cisternae, tubules and vesicles present throughout the cytoplasm, especially in the endoplasm.
3. Occurrence of ER
It is absent in the prokaryotes but present in eukaryotes except germinal epithelium and mature mammalian erythrocytes. Development of ER depends upon the metabolic state and stages of differentiations of the cells e.g. absent in embryonic cells, less developed in spermatocytes, while fully developed in metabolically active cells.
4. Types of ER
ER membranes are differentiated into rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER), depending on whether ribosomes are associated with their cytoplasmic surface. Regions of ER that lack bound ribosomes are called SER. The membranes and luminal space of ER are normally continuous throughout the cell and that RER and SER form an interconnected system. When cells are disrupted by homogenization, The ER break into small vesicles called microsomes (RER- Rough microsomes, SER- Smooth microsomes).
Differences between RER and SER
|BASIS FOR COMPARISON||SMOOTH ENDOPLASMIC RETICULUM||ROUGH ENDOPLASMIC RETICULUM|
|Meaning||Smooth ER appears like containing many circular marks which are the interlocking tubular sheets and they may be varied in look and function as well.||Rough ER looks like the arrangement of the double membranes which are spotted with the ribosomes all over. They appear consisting of the parallel sheets of membrane. Found near Smooth ER is found near the cell membrane.|
|Found near||Smooth ER is found near the cell membrane.||Rough ER is found near the cytoplasm.|
|Originates from||Rough endoplasmic reticulum by giving off the ribosomes.||From nuclear membrane.|
|Ribosomes||They do not have ribosomes.||They have ribosomes.|
|It mainly produces||Lipids and Proteins.||Proteins.|
5. Ultrastructure of ER
Electron microscopic studies showed the presence of three types of elements in ER:
1. Cisternae- These are narrow, two layered and unbranched elements generally present near the nucleus. These lie one upon the other and may be interconnected through helical Teraski ramps (Which give the appearance of parking garage of ER).
These are held by cytoskeletal elements. Ribosomes are attached by a transmembrane glycoprotein called ribophorin. These are abundant in protein forming cells.
Each of cisterna has two surfaces: cytoplasmic or protoplasmic face (is with ribosomes and in direct contact with cytosol) and luminal face (toward the cisternal space and borders the cavities of cisternae).
2. Vesicles- These are oval or spherical elements scattered in the cytoplasm. Size ranges from 25-500 micro-meter in diameter. These are also studded with ribosomes so are also mainly found in protein forming cells.
3. Tubules- These are wider, tubular and branched elements mainly present near the cell membrane. These are without ribosomes so are more in lipid and steroids forming cells and the cells involved in glycogen metabolism.
In liver cells, fine tubules with glycogen granules are called glycosomes.
6. Ultrastructure and chemical composition of ER
Electron microscopic studies revealed that all the components of ER are unit membranes, so are lipoproteinous and trilaminar like the plasma membrane. ER membranes are thicker, with less cholesterol and with the more lipids in comparison to the plasma membrane. Cytochemical studies showed that there are more lipid to protein in SER than RER. In SER, more sphingomyelin and cholesterol while in RER, about 30 types of proteins have been identified. E.g.-
- Synthetases involve in biosynthesis of triglycerides, phospholipids, glycolipids, plasmalogens, fatty acids and steroids.
- NADH-cytochrome C-reductase
- Mg++ activated ATPase
- Nucleotide diphosphatase
- Uridine, guanosine, and ionosine diphosphatase to hydrolyse UDP, GDP and IDP.
7. Origin of ER
Most accepted view regarding the origin of ER is that RER arise as an evaginations of outer nuclear membrane (Palade, 1956) while SER is formed from RER by the loss of ribosomes. This view is supported by following similarities between ER and nuclear membrane:
- Both are similar in chemical composition
- Inter-cisternal space of ER is continuous with perinuclear space.
- Both RER and ectokaryotheca of nuclear envelope are studded with ribosomes.
- Fluid present in the ER cisternae and perinuclear space is of similar nature
- Derivation of SER from RER is further proved by the use of radioactive amino acids.
8. Modifications of ER
ER Shows following modifications:
– Sarcoplasmic reticulum
– Myeloid bodies
– Annulated lamellae
1. Sarcoplasmic reticulum- It is a highly modified SER and is found as a network of interconnected and branched tubules running longitudinally along the sarcomere of muscle fibers.
It was first reported by Veratti in 1902. At the level of H-zone, it is called central cisterna and forms a sieve like structure around the myofibrils, while at the level of I-band, there tubule merge with large terminal cisternae.
Functions: It help in distributing energy rich material for muscle contraction, provide channels for conducting the nerve impulses, help in expanding the lactic acid formed during muscle contraction so prevent muscle fatigue, stores calcium ions and pump them into the sarcoplasm to stimulate the muscle contraction.
2. Ergastoplasm- or basoplasm or chromodial substance is an accumulated mass of cis cisternae with ribosomes present in the cytoplasm of some metabolically active cells.
The term ergastoplasm was coined by Garner in 1899. In the cyton of neurons, such bodies are called Nissl’s granules (also called Trigoid granules). It is involved in protein synthesis.
3. Myeloid bodies- These are found in retinal cells. Each myeloid body is a biconvex, about 4-5 micrometer long and is formed of stacks of packed tubules. They are probably related with photoreception.
4. Annulated lamellae- These are found in immature oocytes and spermatocytes and were first reported in the oocytes of sea urchin- Arbacia. Later these were also reported in the amphibians’ oocytes, foetal cells, etc. having high rate of metabolism.
These occurs either in the form of free unstacked vesicles in the cytoplasm or as stacked annulated lamellae near the nucleus.
9. Interrelationship between ER and other membranes
Watson (1955) demonstrated a continuity between the outer nuclear membrane and the ER. ER also shows connections with the plasma membrane and Golgi complex. It is suggested that:
– Ectokaryotheca of nucleus forms vesicle by blebbing.
– These vesicles fuse to form annulated lamellae.
– Annulated lamellae lose their pore complexes, become associated with the ribosomes and form RER cisternae (Wischnitzer, 1974).
– RER produces transition vesicles which fuse to form the plasma-lemma (North Cote, 1971).
– Golgi body also gives rise to secretory granules and primary lysosomes by blebbing. Primary lysosome can also arise directly from ER.
– Plasma-lemma invaginates to form pinocytotic vesicles.
– Pinocytotic vesicles and primary lysosome fuse to form the secondary lysosomes (Novikoff, 1962).
– RER loses the ribosomes to form SER.
10. Functions of ER
Common functions of SER and RER
1. Intracellular transport- ER acts as cell circulatory system and helps in transportation of material inside the cell. Essner and Novikoff in 1962 suggested a directional flow of material as under:
RER → SER → Golgi body → Pr. Lysosome → Exocytosis (out of cell)
2. As cytoskeleton- ER is the major component of cytoplasmic-vacuolar system which act as cytoskeleton and provide mechanical support and a definite shape to the cell.
3. As segregation apparatus- ER act as a segregation apparatus and divides the cytoplasm into two compartments: one lying within ER and other between the ER cisternae.
4. Formation of primary lysosome with hydrolytic enzymes.
5. Synthetic activities- ER has about 30 types of enzymes, mainly synthetases, which are involved in biosynthesis of various biomolecules.
6. Plasmodesmata- ER forms simple or branched tubular extensions, called desmotubules, which pass through the plasmodesmata and connect the ER element of the adjoining plant cells.
7. Storage- ER help in storage of metabolic products such as glycogen.
8. Transportation of genetic informations- ER act as a passage for the transportation of the genetic informations from the nucleus to various cell organelles to control the biosynthesis of proteins, fats and carbohydrates.
9. Formation of cell plate- During cytokinesis in the plant cells, ER provides small sized phragmoplasts which arrange themselves at the equator and later fuse to form the cell plate which later forms the middle lamella.
10. ATP synthesis- ER is the site of ATP synthesis to provide energy for intracellular transportation of materials or RNA metabolism.
11. ER and normal embryonic development- It is evident from the fact that up to notogenesis, embryonic cells are without ER element. But during the formation of fibrous notochord sheaths, there is also formation of large number of ER elements. ER has important role in the differentiation.
12. Photoreception- ER of pigmented epithelial cells of retina act as photoreceptors (also called as myeloid body).
11. Functions of SER and RER
1. Protein synthesis- RER provides two dimensional arrangement of ribosomes and increases the rate approximately 5 to 6 times of protein synthesis.
It was suggested that ER membrane are involved not only in the synthesis of proteins for intracellular use (e.g. hemoglobin) but also play important role in the synthesis of proteins of extracellular use (e.g. tryptocollagen, antibodies, and plasma proteins).
RER is also involved in post translational modification of the proteins. These modifications are followings: ]
– N-linked glycosylation of proteins- N-linked glycosylation is the attachment of a sugar molecule to a nitrogen atom in an amino acid residue in a protein. In RER, this process involves the addition of a large performed oligosaccharide precursor to a protein.
The precursor oligosaccharide is linked by a pyro-phosphoryl residue to dolichol (a long chain poly-isoprenoid lipid), firmly embedded in RER membrane and act as a carrier for the oligosaccharide.
The oligosaccharide is transferred from the dolichol carrier to an asparagine residue on a polypeptide catalyzed by oligosaccharyl transferase.
– Protein Folding- Protein folding is done by the chaperons which prevents misfolding of a newly synthesized polypeptide chain. An important chaperon of Hsp70 family is located in the lumen of ER is BiP. BiP binds newly synthesized proteins as they are translocated into the ER and maintain them in unfolded state.
Two other chaperons in ER apart from BiP, calnexin and calreticulin, prevents premature and incorrect folding of proteins.
– Formation of disulfide bond- Disulfide bond do not form in cytosol due to reducing environment, which maintains cysteine residues in their reduced form. Disulfide bond formation in ER lumen is facilitated by the enzyme Protein-Disulfide-Isomerase (PDI). PDI also catalyzes rearrangement of disulfide bond.
2. Formation of nuclear envelope- Porter and Machado in 1960, reported that nuclear membranes break into a number of fragments which merge with ER elements during later stages of prophase mitosis and mitosis and meiosis. Nuclear membrane is reformed from the cisternae of ER during telophase of cell division.
3. Formation of SER from RER by the loss of ribosomes.
4. Formation of transition vesicle- RER forms the transition or transport vesicles which shuttle the proteins to the cisternae of Golgi apparatus for their condensation into secondary vesicles. These transition vesicles are surrounded by coating proteins.
1. Synthesis of membrane lipids- The SER produces most of membrane lipids, including both phospholipids and cholesterol.
Phospholipids are amphipathic molecules and their synthesis from fatty acyl-CoAs and glycerol 3-phosphate take place at the cytosolic leaflet of ER membrane and catalyzed by membrane associated enzymes. Fatty acids are firstly converted into CoA esters.
Fatty acid from fatty acyl-CoA are esterified to the phosphorylated glycerol backbone, forming phosphatidic acid, whose two long hydrocarbon chain anchor the molecule to the membrane.
2. Synthesis of steroid hormones- Amount of SER has been found to be well developed in those cells which are involved in biosynthesis of steroid hormones e.g.
corticoids in adrenal cortex, testosterone in the interstitial cells of testes of opossum and estrogen in the follicular cells of the mature ovarian follicle.
Large quantities of enzymes like esterases, cholesterol synthetase, adenosine triphosphatase etc. have been associated with SER tubules of these cells.
Role of SER in cholesterol synthesis is evident from the use of labelled acetate and association of intermediate compounds like squalene and lanosterol with SER in liver cells of rat.
3. Detoxification of xenobiotic compounds- SERs in the hepatic cells are the major site for removal of xenobiotics (synthetic compounds not found in nature). Cytochrome P-450 present in the SER catalyzes this process.
Cytochrome P-450 are monooxygenases and constitute a superfamily of Heme-containing enzymes that occur in nearly all living organisms. Monooxygenase catalyzes the incorporation of a single atom of molecular oxygen into a substrate with the reduction of the other atom to water.
Claude demonstrated that there is considerable hypertrophy of SER when drugs like Phenobarbitol, steroid hormones, carcinogens like 3-methylcholantrene and 3, 4-benzpyrene are administered.
4. SER and glycogenolysis- Porter reported that glycogen granules are concentrated more in those areas of liver cells which have well developed SER. Luck in 1961 reported that enzyme Uridine diphosphatase glucose
(UDPG)- glycogen transferase was associated with the glycogen granules and not with the tubules of SER. He suggested that SER is associated with glycogenolysis. It was confirmed by Rosen in 1964, who reported that depletion of glycogen granules in the liver cells after the birth probably due to enzyme glucose-6-phophatase.
5. Nerve impulse conduction- SER in striated muscle fibers, called sarcoplasmic reticulum, transmits the nerve impulse from the sarcolemma throughout the length of muscle fibers.
6. SER is involved in synthesis of Ascorbic acid.
7. Fat oxidation- SER membranes have enzymes to regulate the initial reactions in the oxidation of fats.