Write a short note on the Endoplasmic reticulum with a neat labeled diagram.

Hint: Endoplasmic Reticulum is a complex network of tubular membranes particularly found in the cytoplasm of the eukaryotic cell. They transpire in two types: a type with a ribosome-studded surface and another with a smooth surface.

Complete answer:
Endoplasmic Reticulum (ER) is a three-dimensional network of intracellular membranes of eukaryotic cells. ER is a large, dynamic structure that obeys various functions in the cell including calcium storage, protein synthesis, and lipid metabolism. The endoplasmic reticulum has two major sections, which are the smooth endoplasmic reticulum and rough endoplasmic reticulum. Rough ER has attached ribosomes while smooth ER does not. The diverse functions of the ER are performed by distinct domains; consisting of tubules, cisternae, and the nuclear envelope.
Tubules: These are the irregular branched tubular structures. They form a network together with other elements. ER tubules are continually fusing and branching resulting in the creation of new three-way junctions.
Cisternae: They are sacs similar to unbranched structures. They are present in stacks or piles parallel with each other. These cisternae in the stack are interrelated with each other. Cisternae include transmembrane Glycoprotein Ribophorin 2, they connect with larger subunits of the ribosome. Cytosolic spaces are located between Cisternae.
Nuclear envelope: They are made up of two lipid bilayers, the inner nuclear membrane, and outer nuclear membrane, and share a common lumen with the peripheral ER. Hundreds of nuclear pores spanning the inner and outer nuclear envelope allow the transport of molecules, including RNAs and proteins. The nuclear envelope is connected to cisternae, which make up part of the peripheral ER.

Additional information:
There are two types of Endoplasmic Reticulum, which are-
Smooth ER (SER) or Smooth Endoplasmic Reticulum: It is basically peripheral in position and connected with the plasma membrane. It plays a role in the production of phospholipids. It transports the products of the rough ER to other cellular organelles, especially the Golgi apparatus.
Rough ER (RER) or Rough Endoplasmic Reticulum: RER is basically internal in position and is connected with the nuclear envelope. It synthesizes and secretes proteins in the liver, hormones, and other substances in the glands. It is prominent in cells where protein synthesis happens (such as hepatocytes).

Note:
The ER was studied with a light microscope by Garnier in 1897. The endoplasmic reticulum performs the functions are-
-They are responsible for the production and secretion of steroid hormones.
-They are also responsible for the synthesis of essential lipids such as phospholipids and cholesterol.
-They are responsible for the metabolism of carbohydrates.
-They release calcium ions, which are necessary for the nervous system and muscular system.