Endoplasmic Reticulum

The word endo means "inside", plasmic means "cytoplasm", and reticulum means "network".

Albert Claude and Keith Porter first discovered the endoplasmic reticulum in 1945 by using electron microscopy. They observed the webby and vesicle-like structures in cells and then Porter named them "endoplasmic reticulum".

What is the Endoplasmic Reticulum?

The endoplasmic reticulum is a network of long, compressed sacs, tubules, and vesicles in eukaryotic cells. This network is dispersed throughout the entire cell, and it is assembled by the continuance of the outer membrane of the nucleus. The lumen of the endoplasmic reticulum is the perinuclear space. The membrane of the endoplasmic reticulum is made up of a phospholipid bilayer. The phospholipid bilayer has embedded proteins or channel proteins that facilitate the transport mechanism.

Three Forms of Endoplasmic Retculum

Endoplasmic reticulum exists in three forms as follows:

Cisternal or Laminal Form

The cisternal or laminal form is a combination of long, flattened and unbranched units arranged in stacks having a diameter of up to 40 to 50 micrometers.

Vesicular Form

These are the oval-shaped membrane-bound structures called vesicles. These have a diameter between 25 to 500 micrometers. These originate from rough endoplasmic reticulum having proteins packaged in them.

Tubular Form

The irregular, branched tubes bounded by membrane make the tubular form which is also known as smooth endoplasmic reticulum. Its diameter is 50 to 190 micrometers.

Rough Endoplasmic Reticulum or RER

The rough endoplasmic reticulum is named so because of the presence of ribosomes on its surface. RER gives a rough or granular appearance. The ribosomes are fixed to the surface of the rough endoplasmic reticulum by ribophorin, a type of glycoprotein. The translocon proteins present on the rough endoplasmic surface transport proteins into the lumen of the endoplasmic reticulum. RER are abundantly present in liver and pancreatic cells.

Functions of RER

Protein Synthesis

The ribosomes on the surface of RER make proteins from mRNA. These proteins are then sent to the lumen of RER and get modified and packaged with the help of enzymes and sometime carbohydrate molecules are also attached with these proteins and glycoproteins are formed. These proteins get wrapped into the vesicles and come out of the RER. Here these vesicles are covered with coated proteins which target them to the Golgi bodies with the help of microtubules. The Golgi bodies further modify these vesicles containing proteins and make them able to perform their specific functions.

Smooth Endoplasmic Reticulum or SER

The smooth endoplasmic reticulum lacks ribosomes on its surface, and it gives a glossy appearance, which is why it is named so. It is composed of tubules. SER are involed in lipids metabolim including steroid metabolism and glycogen production and storage. SER are abundantly found in adipose tissues, inerstitial cells, glycogen storing cells of liver, and spermatocytes, etc.

Functions of SER

Due to the absence of ribosomes, the SER has no role in protein synthesis, but it has the most important role in lipid production. It also performs some other functions.

Lipids Production

The acetyl-CoA enters the smooth endoplasmic reticulum, where it is converted into different molecules with the help of different enzymes. For example, many acetyl-CoA molecules combine and make fatty acids that are stored in the cell and can be used for ATP synthesis, can become phospholipids, which become part of the cell membrane, or they me be converted into cholesterol, which is important for maintaining fluidity in the cell membrane and fat emulsification. Cholesterol is also responsible for the formation of sex hormones, for example, it is converted into testosterone in the testes, and estrogen and progesterone in the ovaries. Similarly, cholesterol is converted into cortisol, which is a steroid hormone.

Detoxification

Smooth endoplasmic has a significant role in detoxifying drugs and poisons from the body. They add hydroxyl groups with poisonous and toxic substances and make them water-soluble. These substances are then removed from the body through urine.

Calcium Storage

In muscle cells, there is a huge number of SER also called Sarcoplasmic reticulum, that store calcium in large amounts that is necessary for muscle contraction. In striated muscles, when cells receive a signal from the brain, they release calcium ions from SER which attach to the muscle fibers and stimulate them due to which muscle contracts. Similarly, when the muscles relax, the calcium ions go back to SER.

Overall Functions of ER

RER synthesizes, packs and secretes proteins.
RER pinches off tiny protein-filled vesicles which ultimately fuse to the Golgi body.
SER synthesizes and stores lipids molecules, glycogen, steroids and cholesterol.
ER acts as an intracellular circulatory or transporting system.
ER is a part of the endomembranous system or cytoplasmic vascular system.
ER plays a role in providing mechanical support and maintaining the shape of the cell.
ER forms a nuclear envelope after every nuclear division.

Endoplasmic Reticulum