Endolassmic Reticulum

The endoplasmic reticulum (ER) is a large membrane-bound compartment spread throughout the cytoplasm of eukaryotic cells. It is divided into three categories  that include the nuclear envelope (NE), peripheral ER cisternae, and an interconnected tubular network. The nucleus is surrounded by a system of two concentric membranes, called the inner and outer nuclear membranes. The outer nuclear membrane is continuous with the endoplasmic reticulum, so the space between the inner and outer nuclear membranes is directly connected with the lumen of the endoplasmic reticulum . In addition, the outer nuclear membrane is functionally similar to the membranes of the endoplasmic reticulum.

There are two regions of the ER that differ in both structure and function. One region is smooth and other is  rough endoplasmic reticulum.  Rough ER because it has ribosomes attached to the cytoplasmic side of the membrane. The rough ER is a series of flattened sacs (cisternae). The space inside of the ER is called the lumen.

Typically, the smooth ER is a tubule membrane vesicles (network) and  network increases the surface area for the storage of key enzymes and the products of these enzymes ,this tubular membrane vesicles varies cell to cell  .

Smooth ER is actively engaged in steroid synthesis from cholesterol ,Lipid biosynthesis, pigment production ,carbohydrate metabolism, detoxification of metabolic wastes.

Carbohydrate metabolism : In the liver, Smooth ER facilitates the conversion of stored glycogen to glucose. The enzyme glucose-6-phosphatase present in Smooth ER takes parts in gluconeogenesis by converting glucose-6-phosphate to glucose.

Lipid Biosynthesis: Steroid hormones are produced in Smooth ER present in the adrenal and endocrine glands. Smooth ER also produces cholesterol and membrane phospholipids, which are used for membrane formation.
One of the most important functions of Smooth ER is to detoxify the body from metabolic wastes and drugs. Drug detoxification, as well as detoxification of wastes generated in metabolic processes takes place in Smooth ER present in the liver cells. It contains enzymes that catalyze a number of reactions that can make lipid-soluble drugs and metabolic wastes water-soluble, so that they can be easily expelled from the body.
The ER is also involved in cell signaling by releasing stored calcium ions (Ca ² ) into the cytosol.The Ca ² ions act as "second messengers" that turn on several cellular systems.

In muscles the smooth ER assists in the contraction of muscle cells, and in brain cells it synthesizes male and female hormones in cells.

Rough Endoplasmic Reticulum: The rough ER have ribosomes attached throughout the surface. These are present in cells, which are active in protein synthesis. The rough endoplasmic reticulum manufactures membranes and secretory proteins. In certain leukocytes (white blood cells), the rough ER produces antibodies. In pancreatic cells, the rough ER produces insulin. When cells are disrupted, the ER fragments into small vesicles called microsomes. The microsomes derived from the rough ER (rough microsomes) are lined with ribosomes on their outer surface.

By the transcription process mRNA form from DNA, mRNA translate polypeptide chain in ribosomes.The cytoplasmic surface of the rER contains a protein called the SRP receptor that binds to the SRP, anchoring the ribosome with its growing peptide to the rER membrane. As translation proceeds, the peptide is fed into the lumen of the rER where the signal sequence is cut off and additional changes called post-translational modifications.

Newly synthesized polypeptides in the membrane and lumen of the ER undergo five principal modifications before they reach their final destinations:

1.Formation of disulfide bonds

2.Proper folding

3.Addition and processing of carbohydrates

4.Specific proteolytic cleavages

5.Assembly into multimeric proteins

Endoplasmic reticulum of plant cell possesses some additional functions, which is not found in animal cells. The additional function involves cell to cell communication between specialized cells and also it serves as a storage site for proteins.