Endoplasmic Reticulum ¹(ER)

  


 

Endomembrane System:

While  each of the membranous organelles  is distinct  in terms of its structure  and function, many of these are considered  together  as an endomembrane system because their functions are coordinated.The endomembrane system includes endoplasmic reticulum ( ER,  golgi  complex, lysosomes and vacuoles. Since the functions of the mitochondria,  chloroplasts and Peroxisomes are not coordinated with the above components, these are not considered  as part of the endomembrane system. 


Endoplasmic Reticulum¹ (ER)

The endoplasmic reticulum (ER) is noticeable only with an electron microscope. Endoplasmic reticulum is a  3_ dimensional , complicated and interconnected system of membrane _ lined channels running through the cytoplasm and at places it is connected  with plasmalemma as well as nuclear envelope . It is not visible under light microscope but can be observed under electron  microscope. ER divides the  intracellular  space into two distinct  compartments,  i.e., luminal ( inside ER) and extra liminal ( Cytoplasm) compartments.


Normally  it forms 30__ 60% of membrane system of the cell thus increasing  the internal surface 30_ 40 times as compared  to external surface.It 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, erythrocytes,  embryonic cells, resting cells,  prokaryotic cells, etc. In muscle cells, endoplasmic reticulum is called sarcoplasmic reticulum. 







 

Discovery: 

The ER was first noted by Porter, Claude and Fullman in 1945  as a network. It was named RE by Porter in 1953. The endoplasmic reticulum originally seemed to be confined to the endoplasm of the cell, hence its name. It has now been found to extend into the ectoplasm also.

Location.

The ER is present in almost all eukaryotic cells.  A few cells, such as ova, embryonic cells and mature red blood corpuscles, however, lack ER.The Prokaryotic cells also ER. The ER often occupies most of the cytoplasm. 

Amount. 

The ER varies in amount from ce to cell. In spermatocytes, it is represented by a few  vacuole only. In the cells of adipose tissue, it is quite simple, having the form of a few tubules. The cells that are actively synthesizing proteins, such as liver and pancreatic cells and fibroblasts, have abundant ER. The striated muscle fibres have a special type of ER called sacroplasmic reticulum ( SR) 

     Endoplasmic reticulum forms 30 to 60% of the total membranes in a cell. It increases the internal surface 30__ 40 times the outer surface of the cell.

Physical Structure 

Endoplasmic reticulum consists of membrane lined channels or spaces which contain a fluid called endoplasmic matrix .Endoplasmic matrix is quite different  from the   cytoplasmic matrix present outside the reticulum. ER is often an extensive, 3_ dimensional network of intracellular membranes. It is formed of three types of elements: cisterns, tubules and vesicles. 


1. Cisternae¹.

These are flattened, unbranched, sac_ like elements. They lie in stacks ( piles) parallel to one another. The sacs in the stack are interconnected with one another They bear ribosomes on the surface that, therefore, appear rough.The cisternae contain glycoproteins named ribophorin __ I and ribophorin  _ II that bind the ribosomes. There are cytosolic spaces  between the cisternae. 

2. Tubules.

These are irregular branching elements which form a network along with other elements. They are often of ribosomes.
 

3. Vesicles. 

These are oval or rounded, vacuole_ like elements. They often occur isolated in the cytoplasmic matrix.  They are also free of ribosomes. 
 
    All the elements of ER freely communicate with one another, and contain a fluid, called  endoplasmic matrix, in the ER lumen. The endoplasmic matrix is different from the cytoplasmic matrix outside the ER. It is continuous with the  cell membrane, Golgi membranes and the outer membrane of the nuclear envelope.Certain cisternae open out by Pores in the cell membranes. The ER may pass from one cell into another through the Plasmodesmata in the form of  desmotubles. The ER is dynamic as it may fragment and reform its parts, or its parts may fragment and reform its parts, or its parts may fuse. However, the ER has a characteristic form in each type of cells.





Molecular Structure:

The membranes of ER are composed of two layers of phospholipid molecules sandwiched by two layers of proteins molecules like other membranes in the cell. Many kinds of  enzyme proteins are associated with the ER membranes for the various synthetic activities. 

Endoplasmic Reticulum Types:

The endoplasmic reticulum is of two types: smooth or  agranular and rough or granular. The two may occur in the same or different kinds of cells. When present in the same cell, the two are continuous. 
 

   (i) Smooth ER( SER)or Agranular Endoplasmic Reticulum ( AER): 

It consists mainly of tubules and vesicles. It is free of ribosomes. It usually lies near the cell membrane. It is well developed in the muscle cells, adipose tissue cells, adipose tissue cells, interstitial cells, glycogen _ storing liver cells, adrenal cortex cells, spermatocytes, leucocytes, and the cells that synthesize and secrete steroids.

  It  has smooth membranes  which do not bear ribosomes. It is found in the 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, muscular cells, retinal cells, etc.and also found in leucocytes. It is mostly made of vesicles  and tubules.Sphaerosomes are believed  to originate from SER.

1.L. cisterna = box or chest.

 (ii) Rough ER( RER) or Granular Endoplasmic Reticulum ( GER): 

It mainly consists of cisternae. It has ribosomes attached on its cytoplasmic surface.This makes the surface look rough or granular. The RER often occurs deep in the cytoplasm. It is particularly highly developed in the cells that synthesize and secrete proteins. These include liver cells, pancreatic cells, goblet cells,  plasma cells¹, fibroblasts, etc. The proteins in the RER developed fine Pores to let the polypeptide chains formed on ribosomes to enter its lumen.The ER lumen serves as a compartment to contain substances which must be kept separate from the cytosol. In the ER lumen, the enzymes modify the  proteins. 


  It has rough membranes  a number of ribosomes  are attached  to their outer surfaces. The membrane  of the endoplasmic reticulum  bears a fine pore in the area where ribosomes  is attached  to pass the synthesised  polypeptide into the channel of endoplasmic reticulum for transport.  RER lies in the region of the  Cytoplasm that appears  to be pedophilic and is  called ergastoplasm. This basophilic staining  property  of RER is due to its RNA contents of ribosomes.  RER contains  two types of glycoproteins viz,  ribosomes  I  and  ribosomes  II  for attachment  to ribosomes.Due to the presence of ribosomes , the rough ER is engaged  in synthesising proteins  and enzymes.  It is, thus, rich in cells performing active metabolism, and producing proteinaceous substances  e.g., pancreatic acinus cells, plasma cells, fibroblasts, goblet cells.Rough endoplasmic reticulum is mostly  made of cisternae with very few tubules. Transitional endoplasmic reticulum is RER without  ribosomes .Depending on the  metabolic requirements of the cells, the RER and SER are interconvertible.







      Origin .

The ER appears to arise from the outer membrane of the nuclear envelope by outfolding, or from the plasma membrane by unfolding, or from the preexisting ER. The smooth ER seems to arise from the rough ER by detachment of ribosomes. 

Note1.Plasma   cells   produces  antibodies  

Functions:

Endoplasmic reticulum serves many functions. These may be listed in three groups.

(a) Common Functions of Smooth and Rough ER. These are as under_ 

(i) Transport  of Materials: The ER facilities transport of materials from one part of the cell to another, thus forming the cell's circulatory system. 

(ii) Formation of  Destmotubles: Tubular ER extension, celled desmotubles,  extend through the Plasmodesmata to make ER  continuous in the two adjacent plant cells.



 (iii) Support:The ER acts as an intracellular supporting framework, the cytoskeletonthat also maintains the form of the cell.

 (iv) Localization of Organelles:It keeps the cell organelles properly stationed and distributed in rotation to one another.


(v) Surface for synthesisThe ER offers extensive surface for synthesis of a variety of materials. 

 (vi) Location  of Enzymes: ER membranes contain a variety of enzymes ( ATPase, dehydrogenase, phosphatases, etc.) to catalysis synthetic activities. 

(vii) Storage of Materials: The ER provides space for temporary storage of synthetic products such as glycogen.


 (viii) Exchange of Materials: The ER helps in the exchange of materials between the cytoplasm and the nucleus. 

 (ix) Muscle ContractionSarcoplasmic reticulum helps in muscle contraction by regulating Ca++ ions concentration in the sarcoplasm.

(X) Endoplasmic reticulum  provides a large surface for physiological activities. 

(xi) It functions as cytoskeleton by providing mechanical support to cytoplasmic  matrix.

(xii) It keeps the various  organelles  in their position. 

(xiii) It  acts as a mean of quick intracellular  transport.

(Xiv) As sarcoplasmic reticulum ( S.R.): It conducts impulses from the surface to the deeper parts while in other cells, endoplasmic reticulum  conducts information from cell exterior to inside and from one part of the cell to another,  e.g., cytoplasm  to nucleus and vice versa. ( According  to Power _ Sarcoplasmic reticulum is SER).

( xv) At the time of differentiation of nuclei, after telophase, it provides membranes to nuclear  envelope. 

(xvi) It provides precursors  of different secretory substances to  Golgi  apparatus. 

( xvii) ER gives membranes  to  the Golgi apparatus   for the envelope formation  of vesicles and lysosomes. 


(xviii) It takes part in detoxification  of toxic chemicals  and helps in complexing of proteins  and lipids to form lipoproteins. 

(xix) ER gives rise to vacuoles. 

(xx) The membranes of endoplasmic reticulum  contain a number of enzymes e.g., ATP_ ase, reductases, dehydrogenases, phosphatases for various  metabolic activities  and cytochromes that take part in electron  transport. 

(xxi) Microbodies are also thought to arise from ER.

(b) Functions of Rough ER. These are as under___

(i) Surface for RibosomesThe RER Provides space and ribophorins for the attachment of ribosomes to itself.

 (ii) Surface for Protein Synthesis: The RER offers extensive surface on which protein synthesis can be conveniently carried on by ribosomes. 

 (iii) PackagingThe proteins formed on ribosomes pass into the ER lumen where they are modified.  For example, they may be phosphorylated or converted into glycoproteins. Then the modified proteins move on into the transitional area, where the ER buds off membranous sacs, the  transport vesicles, carrying the proteins to the  Golgi apparatus. Here, they are further processed and packaged into secretory vesicles for export by exocytosis at the plasma membrane. 

 (iv) Smooth ER Formation: The RER gives rise to the smooth ER by loss of ribosomes. 

 (v) Formation of GlycoproteinsLinking of sugars to proteins to form glycoproteins starts in the RER and is completed in Golgi  complex.

 (vi) Synthesis of Enzymes  Precursors: The RER produces enzyme precursors for the formation of lysosomes by Golgi Complex. 

(vii) Membrane Formation: Enzymes of ER use  substrates from cytosol to form new phospholipids. The latter may be inserted into the ER membrane for its growth. Parts of ER can pinch off as vesicles which may fuse with the membranes of other organelles, such as Golgi apparatus. 



(c) Functions of Smooth ER: These are given below__ 


 (i) Surface for Synthesis: The SER provides surface for the synthesis of lipids, including phospholipids,cholesterol, steroid hormones ( sex hormones, adrenal hormones), acerbic acid and visual pigments.

 (ii) Glycogen Metabolism:The SER carrier enzyme bodies, the glycosomes,  for glycogen metabolism ( glycogenesis and glycogenolysis) in the liver cells.

  (iii) Detoxification:The SER brings about detoxification in the liver, i.e., converts harmful materials ( drugs and poisons) into harmless ones for excretion by the cell. Detoxification usually involves adding hydroxyl groups to drugs, making them more soluble and easier to flush from the body. Barbiturates are disposed of by SER in this manner. 

 ((vi) Formation of OrganellesThe SER produces   Golgi apparatus, lysosomes and  vacuoles. 

 (v) Calcium Storage: SER in striated and cardiac  muscle cells surrounds the contractile elements and stores calcium. It releases calcium ions to bring about contraction, and then it withdraws them to bring about relaxation. In other cells, most of the calcium is stored in the mitochondria.  



(vi) Transport Route:  Proteins shift from RER through SER to Golgi complex for further processing. 




































































Stomach               Pancreas

RNA                    Nucleic Acids




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