Endoplasmic Reticulum – Structure, Types And Functions

Endoplasmic reticulum is an important cell organelle present in the cytoplasm of eukaryotic cells. The structure of the Endoplasmic reticulum comprises membranous tubules that are interconnected and carry out major cellular functions like protein synthesis, breakdown of carbohydrates, lipid synthesis, storage of calcium, etc. Based on the presence or absence of ribosomes, the Endoplasmic reticulum is divided into RER (Rough Endoplasmic reticulum) and SER (Smooth Endoplasmic Reticulum) respectively.

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.

Defining Endoplasmic Reticulum

Endoplasmic Reticulum is a large organelle that consists of network of membrane and tubular structures whose primary functions include the formation, packaging, and secretion of numerous cellular products.

What is the Endoplasmic Reticulum?

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.

Diagram of Endoplasmic Reticulum

The given diagram depicts both types of Endoplasmic Reticulum- SER and RER as follows:

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 cavity which acts as a passage for secretory products. There are three different kinds of elements present with the Endoplasmic Reticulum:

Cisternae (cisterns)

Some of its keypoints 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

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 snthesis. These help in transporting various cellular materials, including proteins and lipids, to a specific site within the cell.

Tubules

Some of its keypoints 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 two types of Endoplasmic reticulum according to the presence of ribosomes are:

Rough Endoplasmic Reticulum 

The RER is made up of phospholipid bilayers, which is similar to the structure of the plasma membrane. Some of the keypoints 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 number of 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 which 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.

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 signaling, 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.

Difference between SER and RER

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

Features	Rough Endoplasmic Reticulum	Smooth Endoplasmic Reticulum
Appearance	Rough, studded with ribosomes	Smooth, lacks ribosomes
Structure	Consists of flattened sacs called cisternae.	Composed of a network of tubules and vesicles.
Location	Primarily located near the nucleus but can also extend into other parts of the cell.	Found throughout the cell, often adjacent to the nucleus.
Function	Protein synthesis	Lipid synthesis, detoxification, calcium storage
Examples	Plasma cells, pancreatic cells, and cells of the digestive system.	Liver cells, cells in the ovaries and testes.
Other functions	Part of the endomembrane system, involved in membrane synthesis and modification.	May be involved in carbohydrate metabolism.

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 o ribosomes.
3. RER is also involved in the reformation off 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.

FAQs on Endoplasmic Reticulum

1. What is the Endoplasmic Reticulum (ER)?

The endoplasmic reticulum is a membranous organelle found in eukaryotic cells. It consists of an extensive network of membranes and it has two types: Rough endoplasmic reticulum (RER) and Smooth endoplasmic reticulum (SER).

2. What is the difference between RER and SER?

RER has ribosomes on its surface and is primarily involved in protein synthesis and modification, while SER lacks ribosomes and is mainly responsible for lipid synthesis, detoxification, and calcium ion storage.

3. Who first discovered the ER?

The endoplasmic reticulum was discovered by Porter and Thompson in 1945. They found the membranous structures in cell fractions, which later were recognized as part of the endoplasmic reticulum(ER).

4. What is the function of the ER?

The ER is a major site for protein synthesis and transport. It also plays an important role in protein folding, lipid and steroid synthesis, metabolism of carbohydrate and calcium storage.

5. Where is the Endoplasmic Reticulum Located?

The endoplasmic reticulum (ER) is located within the cytoplasm of eukaryotic cells and forms extensive network of membranes that extends throughout the cell. It is found adjacent to the cell nucleus, which is often referred to as the nuclear envelope.