There are two kinds of cells structures: prokaryotic and eukaryotic. The latter one makes up the units of life, be it animals or plants, or fungi or protists. Basically, a eukaryotic cell consists of a well-formed nucleus and membrane-bound organelles, unlike the prokaryotes, which are devoid of both.
About 50% of the total surface of the eukaryotic cell is constituted by the cell organelles known as endoplasmic reticulum (ER). They are the production sites for lipids and essential proteins, which are required for the biochemical processes of plants and animals. However, this organelle is constituted of three layers: rough endoplasmic reticula (synthesizes proteins), smooth endoplasmic reticulum (synthesizes and metabolizes lipids and steroids, and regulates calcium concentration), and sarcoplasmic reticulum (regulates calcium levels).
ER is an intense network of interconnected tubules, vesicles, and fluid-filled sacs surrounding the nuclear membrane, which spreads all across the cell cytoplasm. If you were looking for a rough endoplasmic reticulum analogy, then you can very well relate it to a highway with intermittent stations, where the proteins are being synthesized and transported to the cell membrane.
Rough Endoplasmic Reticulum (RER) Facts
Rough ER is made up of convoluted and sealed sacs, which are studded with membrane-bound ribosomes on the outer surface. For instance, in an average liver cell, there are about 13 million ribosomes present on the outer layer of its structure. In a cell, RER is spread throughout, but is found densely near the nucleus and the Golgi apparatus (an important cell organelle, also known as Golgi complex). The branched tubules of this organelle expand when the cell is active in protein synthesis.
In other times, the reticulum branches out, and the cisternae (flattened disk-like structure of Golgi apparatus) dilate to form fluid-filled sacs in the cell. The entire structure has an interior space, which is also known as endoplasmic reticulum cisternal space, or lumen, which comprises 10% of the cell volume. Both RER and the Golgi complex work in sync, where the functions of the latter are synthesizing and transporting proteins.
Ribosomes studded on the outer surface of the RER assemble the proteins, and this process is termed as translation. Precisely, the RER segregates the polypeptides and amino acids from cytosol (the intracellular fluid), and assembles the proteins. It is here that the proteins are synthesized for other cell organelles like plasma membrane, Golgi complex, secretory vesicles, plant vacuoles, lysosomes, endosomes, etc. Sometimes, inside the lumen of ER, these proteins attach themselves with certain sugar groups to form glycoproteins, as per the cell requirements. Another interesting fact of RER is that hemoglobin is produced out of the four polypeptide chains present in this cell organelle.
Some of the major functions of RER are listed below:
- Assembling proteins from cytosol.
- Folding the proteins into three dimensional shapes (occurs in lumen with the help of ribosomes)
- Post synthesis, this cell organelle checks the quality of the proteins by rejecting the ones with incorrect folding structure
- Transfers the synthesized proteins to Golgi apparatus for further processing, from where the proteins are distributed to requisite locations in the cell.
Severe health concerns like emphysema and cystic fibrosis can occur, if during synthesis process, the proteins are not folded correctly, and also aren't quality checked due to some reasons. Therefore, rough ER plays a stellar role in numerous biochemical processes involving the cell.