Endoplasmic reticulum

ENDOPLASMIC RETICULUM:

                                                 A network of membrane-bound cavities was distributed throughout the cytoplasm and this network created a passage for the transport of substances from one part of the cell to another. This network of membranes is known as "Endoplasmic reticulum".This network is more concentrated in the endoplasm than the ectoplasm, hence the name Endoplasmic reticulum.

It is the transportation system of the cell and functions as a packaging system. The endoplasmic reticulum name was also derived from the fact that in the light microscope, it looks like a "net in the cytoplasm". This membrane is similar to the structure of the plasma membrane as it also has a fluid mosaic structure.

                         The term endoplasmic reticulum was first introduced by "K.R. Porter". According to Porter, the endoplasmic reticulum is a complex finely divided vacuolar system extending from the nucleus throughout the cytoplasm to the margin of the cell. ER acts as a secretory, storage, circulatory, and nervous system for the cell. It also the site of the biogenesis of cellular membranes. This ER varies from cell to cell ie, it is absent from eggs, embryonic cells, RBC and other bacteria. The space inside the ER is called as "lumen" which acts as a passage for the intracellular transport of secretory products.

                                      

   Morphologically, the ER may occur in the following three forms. They are:

1. Lamellar form or cisternae
2. Vesicular form or vesicle
3. Tubular form or tubules

Cisternae:

• The cisternae are long flattened, unbranched sac-like structures having a diameter of approx. 40-50μm, and also arranged in parallel bundles.
• They have ribosomes on their surface and normally found in the secretory cells.
• They play synthetic roles in the cells of pancreas, notochord and brain

Vesicles:

• These vesicles are ovoidal or rounded membrane-bound structures having the diameter of app. 25-500μm and are found at the end of cisternae and tubules.
• They often remain isolated in the cytoplasm and occur in most cells but especially, abundant in the smooth ER.
• Vesicles are surrounded by coating proteins called COPI and COPII. COPI makes the vesicle move towards Golgi apparatus whereas COPII directs vesicles back to the ER.              

                         

Tubules:

• These are smooth-walled and highly walled branched tubular spaces forming the reticular system.
• They have a diameter of 50-100μm.
• They occur in non-secretory cells like striated muscle cells.
• They arise from cisternae and the tubular form of ER is often found to be SER.

                                              The endoplasmic reticulum is classified into two types. They are:

Rough endoplasmic reticulum:

                       In some endoplasmic reticulum, spherical granular structures called ribosomes are attached on the surface. This type of ER is called as "Rough endoplasmic reticulum" or "Granular endoplasmic reticulum". It occurs in most cells which are active in protein synthesis such as liver cells, pancreatic cells, plasma cells, goblet cells etc. The granular type of ER takes basophilic stain due to its RNA contents of ribosomes

                                                     

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Smooth endoplasmic reticulum:

                               This type of ER posses smooth walls because the ribosomes are not attached to its membrane. Hence it is called as "Smooth endoplasmic reticulum" or "Agranular endoplasmic reticulum". It occurs mostly in those cells which are involved in the metabolism of lipids and glycogen. It is found in the adipose cells, interstitial cells, glycogen cells of the liver , conduction fibers of heart, etc. The muscle cells are also rich in smooth ER and known as "Sarcoplasmic reticulum". In the pigmented retinal cells, it exists in the form of tightly packed vesicles and tubes known as the "Myeloid bodies".

                                                                  

Functions:

• The ER provides mechanical support to the cytoplasmic matrix.
• The exchange of molecules by the process of osmosis, diffusion, and active transport occurs through the membrane of endoplasmic reticulum.
• The endoplasmic reticulum contain many enzymes which perform various synthetic and metabolic activities.
• It also take part in lipid metabolism and protein synthesis.
• It also helps in the formation of microbodies and detoxification.
• It helps in the synthesis of cholesterol and steroid hormones.
• It also undergoes Glycosylation and Glycogenolysis.
• It helps in the storage of calcium ions.

                  Courtesy of google images