Endoplasmic Reticulum

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• The endoplasmic reticulum (ER) is an irregular network of branching and fusing membranous tubules, around 40 to 70 nm in diameter, and many flattened sacs called cisternae (s., cisterna). 
• The nature of the ER varies with the functional and physiological status of the cell.    
• In cells synthesizing a great deal of protein to be secreted, a large part of the ER is studded on its outer surface with ribosomes and is called rough endoplasmic reticulum . 
• Other cells, such as those producing large quantities of lipids, have ER that lacks ribosomes. This is smooth endoplasmic reticu­ lum (SER).
•  The endoplasmic reticulum has many important func­ tions.
•  Not only does it transport proteins, lipids, and other materials through the cell, it is also involved in the synthesis of many of the materials it transports.
•  Lipids and proteins are synthesized by ER-associated enzymes and ribosomes.
•  Poly­ peptide chains synthesized on RER-bound ribosomes may be inserted either into the ER membrane or into its lumen for transport elsewhere. 
• The ER is also a major site of cell mem­ brane synthesis.