Endoplasmic Reticulum

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The Endoplasmic Reticulum (ER) is an organelle found in all Eukaryotic cells.  It is made up of a network of interconnected tubules and cisternae held together by the cytoskeleton and is roughly 100nm in depth. The organelle has many distinct layers which fold and are shaped to create a maze-like structure. The ER and nuclear membrane form and enclosed internal space called the ER lumen.  The ER is involved in many general functions in the cell.  Its main function is the synthesis of both lipids and proteins and serves as site of production for most of the cells organelles, including itself [1]; Specialized Regions of ER serves as a store of Ca2+ that is used in cell signalling and muscle contraction [2].

Distinct regions of the ER become highly specialized to meet the needs of the functional demands of the cell.  There are three types are Endoplasmic Reticulum, Smooth Endoplasmic Reticulum (SER), Rough Endoplasmic Reticulum (RER), and Sarcoplasmic Reticulum.  RER is coated in ribosomes giving it its rough appearance and is involved in protein synthesis. Regions of ER that lack ribosomes are called SER, however most ER is usually partly smooth and partly rough. The SER is where the vesicles carrying newly synthesied proteins bud off  as well as various other metabolic processes like drug detoixification, carbohydrates and steroid metabolism and Ca2+ regulation[3]. Sarcoplasmic Reticulum is modified smooth ER specialised for the pumping and storing of Ca2+ storage [4] [5] [6].

The endoplasmic reticulum (ER) is a feature of all eukaryotic cells, thats main functions are its role in lipid and protein biosynthesis and as a store for calcium within the cell. It is a network of internal membranes called cisternae which are continuous with the outer membrane [7]. The ER can be rough (rough endoplasmic reticulum) in which ribosomes coat the surface of the ER [8] ,which synthesize proteins in the ER. In mammalian cells, synthesis of proteins is co-translational, which means proteins are captured from the cytosol by the ER and imported into it before they are fully synthesized [9]. Cells that produce a lot of specific proteins to be secreted have an abundance of rough ER [10]. Modifications of proteins to be secreted such as glycosylation or formation of disulphide bonds take place in the lumen of the ER [11]. The ER can also be smooth (smooth endoplasmic reticulum) which lack a coating of ribosomes. Transport vesicles bud off from the smooth ER, which carry the newly synthesized proteins and lipids and transport their contents to the Golgi apparatus [12]. Here they are modified further, before being transported to their final destination [13]. As the ER is also a store of calcium, the ER contains a calcium pump which transports calcium ions from the cytosol into the ER lumen [14]. Cells which require rapid responses to extracellular signals, such as muscle cells, require transport of calcium ions into the cytosol from the ER (which is the uptaken back into the ER.) This is why muscle cells have many modified smooth ER, named the sarcoplasmic reticulum, so that calcium is available to trigger myofibril contraction during muscle contraction [15].

References

  1. E Bittar, N Bittar, (1995) p190-211
  2. Alberts B, (2002) p723
  3. Alberts B, (2002)
  4. Alberts B, Johnson A, Lewis J, Raff M, Roberts K and Walter P. (2002) Molecular Biology of the Cell, Fifth Edition, New York, Garland Science.
  5. Lodish H, (2003) Molecular Cell Biology, 5th Edition. W. H. Freeman
  6. Bittar E, Bittar N (1995). Cellular Organelles, Elsevier Science, P190-211
  7. Alberts,Johnson,Lewis,Raff,Roberts,Walter(2008)Molecular Biology of the Cell,5th Edition,New York:Garland Science p723
  8. Alberts,Johnson,Lewis,Raff,Roberts,Walter(2008)Molecular Biology of the Cell,5th Edition,New York:Garland Science p724
  9. Alberts,Johnson,Lewis,Raff,Roberts,Walter(2008)Molecular Biology of the Cell,5th Edition,New York:Garland Science p724
  10. Lodish,Berk,Kaiser,Krieger,Scott,Bretscher,Ploegh,Matsudaira(2008) Molecular Cell Biology,6th Edition,New York:W.H.Freeman and Company p376
  11. Lodish,Berk,Kaiser,Krieger,Scott,Bretscher,Ploegh,Matsudaira(2008) Molecular Cell Biology,6th Edition,New York:W.H.Freeman and Company p376
  12. Alberts,Johnson,Lewis,Raff,Roberts,Walter(2008)Molecular Biology of the Cell,5th Edition,New York:Garland Science p725
  13. Lodish,Berk,Kaiser,Krieger,Scott,Bretscher,Ploegh,Matsudaira(2008) Molecular Cell Biology,6th Edition,New York:W.H.Freeman and Company p15
  14. Alberts,Johnson,Lewis,Raff,Roberts,Walter(2008)Molecular Biology of the Cell,5th Edition,New York:Garland Science p725
  15. Alberts,Johnson,Lewis,Raff,Roberts,Walter(2008)Molecular Biology of the Cell,5th Edition,New York:Garland Science p726




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