ENDOPLASMIC RETICULUM | MANISH MEVADA

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CELL : STRUCTURE AND FUNCTIONS

ENDOPLASMIC RETICULUM

ENDOMEMBRANE SYSTEM 

• It is a grouping of some membrane organelles which function in close coordination with one another, viz, endoplasmic reticulum, Golgi complex, lysosomes and vacuoles. 
• Functions of other organelles are not coordinated. 
• They are not part of endomembrane system, e.g. , plastids, mitochondria, peroxisomes, glyoxisomes, etc. 

Endoplasmic Reticulum ( ER ) 

• It was discovered independently by Porter (1945) and Thompson (1945). The name was given by Porter in 1953. 
• Endoplasmic reticulum is a 3 - dimensional, complicated and inter connected system of membrane - lined channels that run through the cytoplasm. 
• At places, it is connected with plasmalemma as well as nuclear envelope.
• Plasmodesmata contain it in the form of desmotubules.
• It is not visible under light microscope but can be observed under electron microscope.
• The endoplasmic reticulum divides the intracellular space into two compartments luminal (inside the endoplasmic reticulum) and extra-luminal (rest of the cytoplasm).  
• The extent of endoplasmic reticulum varies from cell to cell.  
• Normally it forms 30-60% of the membrane system of the cell which increases the internal surface 30-40 times as compared to the external surface.  
• Endoplasmic reticulum is quite extensive in metabolically active cells (eg, 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.  

Types

• Depending upon the nature of its membranes, endoplasmic reticulum is of two main types smooth and rough.  
• The two types of ER may be continuous with one another, plasma membrane and nuclear envelope.  
• Endoplasmic reticulum may develop from pre-existing E.R. plasmalemma or nuclear envelope. 

Smooth Endoplasmic Reticulum (SER).  

• 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 comsmonly found in leucocytes.  
• Smooth endoplasmic reticulum is mostly made of vesicles and tubules.  Sphaerosomes are believed to originate from SER.  

Rough Endoplasmic Reticulum (RER).  

• 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 synthesized polypeptide into the channel of endo plasmic 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 synthesizing proteins and enzyme It is, rich in cells which are actively engaged in protein synthesis and secretory acitivity, e.g.  , pancreatic acinus cells, plasma cells, fibroblasts, goblet cells.  
• In conjunction with Golgi apparatus, RER helps to produce lysosomes.  
• RER is mostly made of cisternae.  
• Tubules are very few.

Structure 

• Endoplasmic reticulum consists of membrane lined channels or spaces. The channels or spaces contain a fluid called endoplasmic matrix, which is quite different from cytoplasmic matrix present outside the reticulum. 
• The membranes of endoplasmic reticulum are 50-60 Å thick. 
• Endoplasmic reticulum can exist in three forms ( Fig . 8.31 ) — cisternae , vesicles and tubules . 

Cisternae 

• They are flat interconnected sac - like parts of the endoplasmic reticulum which are 40–50 nm in diameter
• The cisternae are found in bundles where they lie parallel to one another. 
• They occur in the cells actively involved in synthetic activity 

Vesicles 

• They are oval or rounded sacs of 25–500 nm in diameter.
• The vesicles appear as small vacuoles.
• They remain isolated in the cytoplasm.
• The vesicles are also called microsomes .

Tubules 

• 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 unbranched with a diameter of 50-100 nm .

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Manish Mevada

M.Sc, M.Phil, B.Ed

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