What is the Endoplasmic Reticulum?

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Inside all cells, there are number of different organelles, which are specialized for different functions within the cell. Organelles are tiny structures with very specific functions within the confines of a single cell. Some organelles occur in all cells, while others only occur in specific types of cells. The Endoplasmic Reticulum (ER) is an organelle that occurs in all cells. Endoplasmic reticulum (ER) is a network of branching tubules, vesicles (assembly made of different molecules) and flattened sacs that are interconnected. They may serve specialized functions in the cell including protein synthesis, production of steroids, storage and production of glucose etc.

Who introduced the Endoplasmic reticulum?

The membranes of Endoplasmic Reticulum were first seen by Keith R. Porter, a Canadian cell biologist, Albert Claude, a Belgian biologist, and Ernest F. Fullam in 1945.

What is the structure of Endoplasmic reticulum?

The general structure of Endoplasmic Reticulum is an extensive membrane network of sac-like structures held together by the cytoskeleton; the skeleton contained within the ‘cytoplasm’ and is made up of protein. Cytoplasm is the thick liquid residing between the cell membrane holding organelles. The sacs and tubules of Endoplasmic Reticulum are all interconnected by a single continuous membrane so that the organelle has only one large complexly arranged “lumen” (internal space). The lumen of the organelle often takes up more than 10% of the total volume of a You do not have access to view this node. The function of Endoplasmic Reticulum varies greatly depending upon the exact type of ER and the type of cell it resides in.

What are the types of Endoplasmic reticulum?

There are 3 varieties of ER which are as follows.

•  Rough Endoplasmic Reticulum (RER): The surface of RER is studded with protein-manufacturing ribosomes (component of a biological cell which creates protein from amino acids) giving it a “rough” appearance. The ribosomes bound to the RER, at any one time, are not a stable part of this organelle’s structure as ribosomes are constantly being bound and released from the membrane. A ribosome only binds to the ER once it begins to synthesize a protein destined for the secondary pathway, which is a series of steps a cell uses to move proteins out of the cell. The newly synthesized proteins are sequestered in sacs, called cisternae. The system then sends the proteins via small vesicles to the organelle found in most cells (Golgi complex), or in the case of membrane proteins, it inserts them into the membrane. The RER is key in functions such as:
  • Secreted proteins, either secreted constitutively with no tag, or regulated secretion involving basic amino acids in the short peptide chain that directs the transport of a protein (signal peptide)
  • Integral membrane proteins, proteins that are permanently attached to the biological membrane that stay imbedded in the membranes as vesicles exit and bind to new membranes
•  Smooth ER: The SER has functions in several metabolic processes including synthesis of lipids and steroids, metabolism of carbohydrates, regulation of calcium concentration, drug detoxification etc. It is connected to the nuclear membrane, a double membrane forming the surface boundary of an organism’s nucleus. SER is found in a variety of cell types including plants and animals and it serves various functions in each.
• Sarcoplasmic reticulum: The sarcoplasmic reticulum is a special type of smooth ER found in smooth and striated muscle. The only structural difference between this organelle and the SER is the medley of proteins they have, both bound to their membrane and drifting within the boundaries of their lumens. Lumen is the central cavity of a tubular or other hollow structure in an organism.

What are the Various Functions of ER?

•  Transport of proteins: Secretory proteins, mostly glycoproteins, are moved across the ER membrane. Proteins are transported by the endoplasmic reticulum and from there they are carried throughout the You do not have access to view this node.
•  Drug metabolism: The smooth ER is the site at which some drugs are modified by microsomal enzymes.
•  In the generation of glucose: The Smooth ER also contains the enzyme glucose-6-phosphatase which converts glucose-6-phosphate to glucose, a step in gluconeogenesis. Gluconeogenesis is a metabolic pathway that results in the generation of glucose from non-carbohydrate substance. The SER consists of tubules and vesicles that branch forming a network. In some cells there are dilated areas like the sacs of RER. The network of SER allows increased surface area for the action or storage of key enzymes and the products of these enzymes.
•  The fundamental difference between SER and SR in their functions: The SER synthesizes molecules while the SR stores and pumps calcium ions. The SR contains large stores of calcium, which it sets apart and then releases when the muscle cell is stimulated
  
   

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