Endoplasmic Reticulum

Endoplasmic Reticulum
Advance Level Materials

Historical

Garnier, 1900 was first to observe the ergastoplasm in a cell. The term endoplasmic reticulum was coined by Porter, 1948 and later used by Porter and Kallman, 1952. Watson (1955) demonstrated a continuity between outer nuclear membrane and ER.Sjostrand,(1956) used a different type of terminology given hereunder :           

α—Cytomembranes: Used for rough surfaced ER

β—Cytomembranes: Used for infoldings of plasma membrane       ,

γ—Cytomembranes: Used for smooth ER and golgi complex.

These are found to be absent in mature erythrocytes and prokaryotes. Normally it forms 30-60% of membrane system of the cell which increases the internal surface 30-40 times as compared to external surface. Endoplasmic reticulum is quite extensive in metabolically active cells (e.g., cells of pancreas, liver),simple in storage cells (in the form of tubules in adipose cells), reduced in spermatocytes (in the form of a few vesicles), and absent in eggs, mature erythrocytes, embryonic cells, resting cells, prokaryotic cells, etc. 

Ultra Structure

The ER is made-up of three types of structures:

(a)     ER Cisternae: They have a diameter between 400-500 A0. They are flat interconnected sac-like parts of the endoplasmic reticulum. The cisternae are found in bundles where they lie parallel to one another. They occur in the cells actively involved in synthetic activity

(b)     ER Tubules: They have a diameter between 500-1000 A0. They are tube-like extensions which may be connected with cisternae or vesicles to form a reticular system. The tubules can be irregular or regular, branched or un-branched.

(c)     ER Vesicles: They have a diameter between 250-5000 A0. They are oval or rounded sacs. The vesicles appear as small vacuoles. They remain isolated in the cytoplasm. The vesicles are also called microsomes.

All- the three structures are bound by a single unit membrane of 50 A0 thicknesses. Depending upon the presence of ribosomes, the ER has been categories into two types: 

(a)     Rough ER or Granular ER:

         It has ribosomes attached to the C-face (cytoplasmic face) of ER membrane. It has rough membranes because a number of ribosomes occur attached to their outer surfaces. RER is, therefore, also called granular en­doplasmic reticulum. The membrane of the endoplasmic reticulum bears a fine pore in the area of attached ribosome to pass the synthesised polypeptide into the channel of endoplasmic reticulum for transport. RER contains two types of glycoproteins (ribophorin I and ribophorin II) for attachment to ribosomes. On account of the presence of ribosomes, the rough ER is engaged in synthesising proteins and enzymes. It is, therefore, rich in cells which perform active metabolism, and produce proteinaceous substances, e.g., pancreatic acinus cells, plasma cells, fibroblasts, goblet cells. In conjunction with Golgi apparatus, RER helps produce lysosomes. Rough endoplasmic reticulum is mostly made of cisternae. Tubules are very few. 

The RER is key in multiple functions:

*       Lysosomal enzymes with a mannose-6-phosphate marker added in the cis-Golgi network

*       Secreted proteins, either secreted constitutively with no tag, or regulated secretion involving clathrin and paired basic amino acids in the signal peptide.

*       Integral membrane proteins that stay imbedded in the membrane as vesicles exit and bind to new membranes. Rab proteins are key in targeting the membrane, SNAP and SNARE proteins are key in the fusion event.

*       Initial glycosylation as assembly continues. This is either N-linked (O-linking occur in the golgi).

         *       N-linked glycosylation: if the protein is properly folded, glycosyltransferase recognizes the AA sequence NXS or NXT (with the S/T residue phosphorylated) and adds a 14 sugar backbone (2 N-acetylglucosamine, 9 branching mannose, and 3 glucose at the end) to the side chain nitrogen of Asn.

(b)   Smooth ER or Agranular ER: It has no ribosomal association. It has smooth membranes which do not bear ribosomes. It is, therefore, also called agranular endoplasmic reticulum. This type of ER is found in cells engaged in the synthesis and storage of glycogen, fat and sterols (e.g., glycogen storing liver cells, interstitial cells, adrenal cortical cells, adipose cells, muscle cells, retinal cells, etc). It is also commonly found in leucocytes. Smooth endoplasmic reticulum is mostly made of vesicles and tubules. Sphaerosomes are believed to originate from SER.

         Out of total ER contents, about 2/3rd is RER and l/3rd SER. However, the SER and RER are interconvertible.

         The RER is more stable than SER. The RER is basophilic due to the presence of ribosomes. A term microsome was used by Claude, 1943. It probably refers to those fragments of ER which are associated to ribosomes. Thus, they are infact RER fragments. The ER at one end may be connected to plasma membrane and at the other end to the outer unit membrane of nuclear envelope. The ER, thus, compartmentalises the intracellular space. It may pass through the plasmodesmata and connect the adjacent nuclei. Rarely, the ER is present in the form of concentric rings. Such a formation is sometimes described as accessory nucleus.

  • The RER is predominant in cells which are involved in active protein synthesis, e., enzyme secreting cells.
  • The SER is characteristic of cells in which synthesis of non proteinaceous substances, e.g., phospholipids, glycolipids, steroid hormones, etc. takes places like adipose tissue cells, Leydig cells of testes, etc.
  • SER is less stable than RER however, depending upon the metabolic requirement both of them are interconvertible.
  • The granules present on RER are ribosomes as shown in figure above.
  • A modified form of SER is sarcoplasmic reticulum of striated muscles. These are store houses of Ca2+ ions which are necessary for muscle contraction. Similarly, Myeloid body in the pigment cells of frog's retina is possibly modified SER.
  • The space enclosed within above written structures is called Luminal or Cisternal Space.
  • Remember, the enzyme UDPG glycogen transferase is responsible for glycogenesis. This particular enzyme has been found to be bound with glycogen rather than being associated with ER.
  • ER is found to be greatly involved in glycogen and glucose metabolism. The enzyme Glucose-6-phosphatase has be reported in the ER of rat liver cells
  • Microbodies are formed as dialations of ER and frequently show connections with ER cisternae. These are rich in enzyme peroxidase (hence called Peroxisomes) in animals along with catalase and D-amino acid oxidase. However, in plants, these are called as Glyoxysomes the enzymatic content is different here, e., the enzymes of Glyoxylate cycle are present.
  • In testes, ovary and adrenal cortex, SER has a role in the synthesis of steroid hormones
  • ER acts as the member of intracellular transport system for various substances, which may pass across the ER membranes [by diffusion, as well as by active transport. It is suggested that ER membranes has carriers and permeases which are |involved in active transport. 

Chemical Composition

The ER is a lipoproteinaceous structure. The percentage of proteins ranges from 50-70 and those of
lipids from 30-50. Among the lipid contents about 70% are the phospholipids, most of which is, lecithin and cephalin.

(Rest part of study materials after registration).

Details heading of rest part study materials:-

2. Functions
3. Membrane flow
4. Origin
5. Annulate lamella 
6. Supplementary Reading
7. The secretory pathway 
8. Protein sorting by RER
9. Isolation of rough ER
10. The signal sequence of growth hormone
11.Cotranslational targeting of secretory proteins to the ER 
12. Posttranslational translocation of proteins into the ER 
13. Orientations of membrane proteins 
14. Topology of the secretory pathway 
15. Protein folding in the ER 
16. Protein glycosylation in the ER
17. Addition of GPI anchors 
18. Translocation of phospholipids across the ER membrane 
19. Vesicular transport from the ER to the Golgi 
20. Retrieval of resident ER proteins 

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