Book : Cell Biology
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Posted by: CHELSEA
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Writer CHELSEA

Sarcoplasmic reticulum

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The sarcoplasmic reticulum (SR), from the Greek σάρξ sarx ("flesh"), is smooth ER found in myocytes. The only structural difference between this organelle and the smooth endoplasmic reticulum is the medley of proteins they have, both bound to their membranes and drifting within the confines of their lumens. This fundamental difference is indicative of their functions: The endoplasmic reticulum synthesizes molecules, while the sarcoplasmic reticulum stores calcium ions and pumps them out into the sarcoplasm when the muscle fiber is stimulated. After their release from the sarcoplasmic reticulum, calcium ions interact with contractile proteins that utilize ATP to shorten the muscle fiber. The sarcoplasmic reticulum plays a major role in excitation-contraction coupling.

Function of Endoplasmic Reticulum:

  • The endoplasmic reticulum (ER) is an organelle found in the cells of eukaryotic organisms. It is an interconnected network of flattened sacs or tubes encased in membranes. These membranes are continuous, joining with the outer membrane of the nuclear membrane. ER occurs in almost every type of eukaryotic cell except red blood cells and sperm cells. 
  • Endoplasmic reticulum has two types, rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). Rough ER is studded with ribosomes, the site of protein synthesis. This type of ER is especially prominent in certain kinds of cells like hepatocytes where active protein synthesis occurs. Smooth ER doesn't have ribosomes and is very important to the process of metabolism. 
  • Endoplasmic reticulum has a number of jobs within the cell. This includes the folding and transport of various proteins, specifically carrying them to the Golgi apparatus. Someother proteins, mostly the glycoproteins, move across the ER's membrane. 
  • The ER is also responsible for marking these proteins that it transports with a signal sequence. Other proteins are headed outside the ER, so they are packed into transport vesicles and moved out of the cell via the cytoskeleton. 
  • Basically, the ER is the transportation system of the eukaryotic cell, and its proteins are contained within it until they are needed to move.



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