Endoplasmic Reticulum

Introduction 

The endoplasmic reticulum refers to tubules and flattened sacs. This network is responsible for a range of functions in animal cells and plant cells. There are two types of endoplasmic reticulum in existence- rough ER and smooth ER. The rough ER is characterised by ribosomes which are attached to the membrane’s on the cytoplasmic side. In contrast, the smooth ER does not have attached ribosomes. To put it simply, you can understand rough ER as a flattened sacs series and the smooth ER as a tubule network. There is space inside the ER that is known as lumen. Let us learn about the structure and types of endoplasmic reticulum in detail. 

Structure of ER

The general structure of the endoplasmic reticulum is known as cisternae, which is like a network of sac-like structures. The holding together of sac-like structures takes place by the cytoskeleton. The enclosing of the cisternal space takes place by phospholipid membrane. This cisternal space is known as lumen.

Lumen is separated from the cytosol while it is continuous with the perinuclear space. The structure of ER facilitates the synthesis and exporting of membrane lipids and proteins. The quantity of rough ER and smooth ER in a cell can undergo slow interchanging from one type to the other. This interchanging depends on the metabolic activities of the cell that change. 

The embedding of new proteins can take place during the interchanging or transforming. This embedding happens in the membrane and brings structural changes. Protein content changes may take place without bringing about noticeable structural changes.

Rough ER

The rough endoplasmic reticulum is an extensive organelle that comprises sacs. These sacs are sealed, flattish, and convoluted. The sacs are continuous with the nuclear membrane. It is known as rough because its outer surface is studded with ribosomes. There is a firm attachment of these ribosomes to the outer cytosolic side of the ER.

In an average liver cell, there is a presence of 13 million ribosomes in the rough ER. Rough ER can be discovered all over the cell. However, their density is higher near the Golgi bodies region and nucleus.

Experts call rough ER ribosome membrane-bound. These ribosomes facilitate the assembling of various proteins in a process known as translation. Production of a high volume of enzymes takes place by the certain pancreas and digestive tract cells. Also, many proteins are produced in the pancreas and the digestive tract cells. 

The polypeptides and amino acids are taken from the cytosol by the rough ER working with membrane-bound ribosomes. The production of proteins takes place for the ER itself, endosomes, lysosomes, plant vacuoles, secretory vesicles, Golgi bodies, and plasma membrane. 

The delivery of some of the proteins takes place in the lumen. Other proteins undergo processing in the ER itself. In the lumen, the addition of sugar groups takes place to some proteins, thereby resulting in the formation of glycoproteins. Furthermore, the addition of some metal groups happens to them. In rough ER, the formation of haemoglobin also occurs.

Smooth ER

Smooth ER, in contrast to rough ER, is more tubular. Formation of the interconnecting network sub-compartment of ER takes place by the smooth endoplasmic reticulum. Its distribution is all over the cytoplasm.

The reason it is known as smooth is that it is not studded with ribosomes. The Smooth Endoplasmic reticulum is almost exclusively for the purpose of lipids manufacturing. In certain cases, smooth ER also regulates lipids metabolism.

In liver cells, smooth ER facilitates the breaking down of glycogen to glucose. The storing of this glycogen takes place as granules on the smooth ER’s external surface. There is also the involvement of Smooth ER in steroid hormones production. This production happens in the endocrine glands and in the adrenal cortex.

The function of Endoplasmic Reticulum

There is a difference between the smooth and rough endoplasmic reticulum function. Let us understand the functions of both types of endoplasmic reticulum below. 

The Smooth Endoplasmic Reticulum function is as follows: 

•     Facilitates the synthesis of lipids and steroids. 
•     Assists in the process of detoxification of drugs. 
•     Serves as a passage for the passing of rough endoplasmic reticulum synthetic products onto the Golgi bodies.
•     It makes possible the contraction of the muscles. This takes place due to the releasing and uptaking of calcium ions. 

The Rough Endoplasmic Reticulum function is as follows: 

• It serves as the site of protein synthesis due to the presence of ribosomes. They exist at places that have an active involvement in protein synthesis. Besides synthesis, their presence is also found in the secretion of protein sites. 
• It is responsible for giving rise to the smooth endoplasmic reticulum. 
• It provides the precursors of the enzymes that play a crucial role in forming Golgi bodies. 

Conclusion

The endoplasmic reticulum is a tubule and flattened sacs network that performs several functions in cells. Rough ER and smooth ER are the two types of the endoplasmic reticulum. The rough ER has ribosomes that are connected to the membrane’s cytoplasmic side. In contrast, the smooth ER lacks connected or attached ribosomes. Cisternae form the general structure of the ER. It is like a network of membranes. There is a difference between the smooth and rough endoplasmic reticulum function.