Endoplasmic Reticulum

Endoplasmic Reticulum is a large organelle that consists of a network of membranes and tubular structures whose primary functions include the formation, packaging, and secretion of numerous cellular products. Based on the presence or absence of ribosomes, the Endoplasmic Reticulum is divided into RER (Rough Endoplasmic Reticulum) and SER (Smooth Endoplasmic Reticulum), respectively.

Characteristics of Endoplasmic Reticulum

Characteristics of the endoplasmic reticulum are given below:

• The Rough Endoplasmic Reticulum is primarily concerned with protein synthesis. On the other hand, the Smooth Endoplasmic Reticulum does not have ribosomes and thus, is responsible for lipid synthesis.
• If there is any disruption in the functioning of the ER, it may lead to diseases like Parkinson's and Cystic Fibrosis.
• The endoplasmic reticulum, one of the numerous cell organelles found in the cell, is structured in a network made up of tubular membranes and compressed sacs found within the cytoplasm.
• It is absent in prokaryotes but is present in all eukaryotes except mammalian Red Blood Cells and germinal cells.
• The endoplasmic reticulum is quite large in size and extends across the cell, and is present above the nucleus.

Structure of Endoplasmic Reticulum

The endoplasmic reticulum membrane is around 50 to 60 A° thick and has a fluid-mosaic structure, similar to the plasma membrane. It is continuous with the plasma membrane, nuclear membrane, and Golgi apparatus membranes. The endoplasmic reticulum contains a cavity that acts as a passage for secretory products. There are three different kinds of elements present in the Endoplasmic Reticulum:

Cisternae (cisterns)

Some of its key points are:

• These two-layered, unbranched, thin components are found close to the nucleus. These are joined and stacked one atop the other.
• They act as the main site for protein synthesis, folding, and modification in the rough endoplasmic reticulum (RER).
• Their size is around 40-50 µm in diameter.

Vesicles

Some of its key points are:

• Vesicles are small, spherical sacs that branch off from the ER and carry chemicals to other organelles or to the cell's surface.
• These are round and oval and are usually related to protein synthesis.
• These help in transporting various cellular materials, including proteins and lipids, to a specific site within the cell.

Tubules

Some of its key points are:

• Tubules are long, cylindrical structures that make up part of the ER network and provide flexibility to the ER structure
• These tubules are involved in the extension and connection of the ER throughout the cell and facilitate the movement of materials within the organelle.

Types of Endoplasmic Reticulum 

There are two types of Endoplasmic Reticulum, according to the presence of ribosomes:

1. Rough Endoplasmic Reticulum 

The RER is made up of phospholipid bilayers, which are similar to the structure of the plasma membrane. Some of the key points are given below:

1. RER have ribosomes attached to its cytoplasmic surface. These ribosomes are involved in protein synthesis.
2. The membranes of the RER have several flattened sacs called cisternae, which are interconnected and allow the exchange of material and information.
3. Inside the cisternae is a central space or lumen that serves as a site where protein synthesis, folding, and modification occur.
4. After synthesis and modification, proteins are packaged into transport vesicles. These vesicles bud off from the RER and carry the proteins to their respective destinations within or outside the cell.

2. Smooth Endoplasmic Reticulum

The SER shares a similar membranous structure with the RER, consisting of a network of tubules and cisternae. However, it lacks ribosomes on its surface, giving it a smooth appearance.

1. A major function of the SER is lipid synthesis and produces phospholipids and steroids, which are essential components of cell membranes.
2. The SER contains enzymes involved in detoxifying drugs and toxins.
3. The SER is responsible for storing and releasing calcium ions (Ca²⁺). These ions play pivotal roles in various cellular processes, including muscle contraction, cell signalling, and enzyme activation.
4. The SER is involved in various metabolic processes and can serve as a storage site for certain molecules, such as glucose.

SER vs RER

The difference between Rough Endoplasmic Reticulum and Smooth Endoplasmic Reticulum is highlighted below:

Functions of Endoplasmic Reticulum

The endoplasmic reticulum contributes significantly to the proper functioning of eukaryotic cells. The functions of different types of Endoplasmic Reticulum are as follows:

Rough Endoplasmic Reticulum

The functions of the Rough Endoplasmic Reticulum are discussed below:

1. It helps increase the rate of protein synthesis by synthesising proteins.
2. SER are formed from RER by the loss of ribosomes.
3. RER is also involved in the reformation of the nuclear membrane during telophase.
4. Rough endoplasmic reticulum is involved in the formation of vesicles, which help in carrying chemicals from the ER to the Golgi Apparatus.

Smooth Endoplasmic Reticulum

Below are the functions of the Smooth Endoplasmic Reticulum:

1. The major role of SER is related to lipid synthesis.
2. It is involved in the synthesis of sex hormones like testosterone and estrogens.
3. The process of Glycogenolysis is also aided by the Smooth Endoplasmic Reticulum.
4. Sarcoplasmic reticulum, which is a special type of SER, is found in muscle cells and helps in contraction.
5. It also helps in detoxification of the body by eliminating drugs like phenobarbitol and carcinogens.

Related Articles:

• Cell organelles
• Ribosomes
• RNA Translation
• Endomembrane System