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Endoplasmic Reticulum – Structure, Types and Functions

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Endoplasmic reticulum (ER) is a membranous organelle present in the cells of eukaryotic organisms, including animals, plants, and fungi. Endoplasmic Reticulum is a complex and interconnected network of tubules and vesicles that plays a critical role in various cellular processes, including protein synthesis, lipid metabolism, calcium storage, and detoxification. ER is composed of two distinct regions: the rough endoplasmic reticulum (RER), which is studded with ribosomes on its surface, and the smooth endoplasmic reticulum (SER), which lacks ribosomes. Together, these two regions of the ER work in coordination to carry out a wide range of essential cellular functions, making the ER a vital organelle for the overall functioning and survival of eukaryotic cells.

What is the Endoplasmic Reticulum?

The Endoplasmic Reticulum Structure is a network of membranes that are distributed throughout the cell and connect to the nucleus. The endoplasmic reticulum is the continuous folds starting from the outer nuclear membrane. It is of two types Smooth and Rough endoplasmic reticulum. Prokaryotes and red blood cells, for example, lack any kind of ER. Cells that synthesize and release a lot of proteins will need a lot of ER.

The endoplasmic reticulum gets its name from the fact that it looks like a “net in the cytoplasm” under a light microscope. Some ER membranes remain connected to the plasma membrane and the nuclear envelope. ER can be either rough or smooth. Rough ER has attached ribosomes on its outer surface, whereas smooth ER does not have attached ribosomes.

The endoplasmic reticulum serves as the cell’s secretory, storage, circulatory, and nervous systems. It is also the site of cellular membrane biogenesis. Endoplasmic reticulum membranes have been found to contain a wide range of enzymes required for a variety of important synthetic activities.

Diagram of Endoplasmic Reticulum

Endoplasmic Reticulum


Structure of Endoplasmic Reticulum

  • The endoplasmic reticulum membrane is 50 to 60 A° thick and fluid-mosaic, similar to the plasma membrane unit membrane.
  • The endoplasmic reticulum membrane is continuous with the plasma membrane, nuclear membrane, and Golgi apparatus membranes.
  • Stearases, NADH-cytochrome C reductase, NADH diaphorase, glucose-6-phosphatase, and Mg2+ activated ATPase are the most important enzymes.
  • The endoplasmic reticulum cavity is well-developed and serves as a passage for secretory products.
  • The endoplasmic reticulum can take three different forms:
    • Vesicle
    • Cisternae
    • Tubules


  • The vesicles are oval membrane-bound vacuolar structures with 25 to 500 nm diameters.
  • They are found in most cells but are especially abundant in the SER, where they frequently remain isolated in the cytoplasm.

Cisternae (cisterns)

  • RER is typically found as cisternae in cells with synthetic functions, such as those in the pancreas, notochord, and brain.
  • They continue to be arranged in parallel bundles or stakes.
  • The cisternae are unbranched, long, flattened tubules with a diameter of 40 to 50 nm.


  • Tubular ER is frequently found in SER and is dynamic in nature, i.e., it is associated with membrane movements, fission, and fusion between cytocavity network membranes.
  • They typically range in size from 50 to 190 nm and can be found in almost all cells.
  • Tubules are branched structures that, along with cisternae and vesicles, comprise the reticular system.

Types of Endoplasmic Reticulum 

There are two types of Endoplasmic reticulum present based on the presence of ribosomes:

Rough Endoplasmic Reticulum 

  • Rough ER (RER) is also included in the ribosomes section and plays an important role in protein synthesis and packaging.
  • The protein synthesis process begins when mRNA travels from the nucleus to a ribosome on the RER’s surface.
  • Ribosomes bind to the ER membrane, making it “rough.” The RER is also connected to the nuclear envelope that surrounds the nucleus.
  • Because of the direct relationship between the perinuclear space and the ER lumen, molecules can pass through both membranes.
  • As the ribosome constructs the amino acid chain, it is forced into the RER’s cisternal space. When protein production is finished and the proteins have been collected, the RER pinches off a vesicle.
  • Some proteins are used within the cell, while others are released into the intercellular space.
  • The vesicle, which is a small membrane bubble, can travel into the cell membrane or the Golgi apparatus.

Endoplasmic Reticulum Smooth

  • Because the ribosomes are not attached to the membranes of this type of endoplasmic reticulum, its walls are smooth.
  • The smooth endoplasmic reticulum is found primarily in cells involved in the metabolism of lipids (including steroids) and glycogen.
  • Adipose cells, interstitial cells, glycogen-storing cells of the liver, heart conduction fibers, spermatocytes, and leucocytes are examples.

Functions of Endoplasmic Reticulum

  • It is primarily in charge of transporting proteins and other carbohydrates to other organs, such as lysosomes, the Golgi apparatus, the plasma membrane, and so on.
  • They also provide increased surface area for cellular reactions.
  • They play an important role in the formation of the skeletal structure.
  • Proteins, lipids, glycogen, and other steroid hormones such as cholesterol, progesterone, and testosterone require them for synthesis.
  • They aid in the formation of nuclear membranes during cell division.
  • The ER performs a variety of functions, including material transport between different parts of the cell.
  • It eliminates waste and cleanses the cells. They also degrade worn-out cell components by digesting foreign or undesirable materials andonar
  • The membrane-bound sacs contain digestive enzymes that break down foreign material that enters the cell.

FAQs on Endoplasmic Reticulum

Q1: What are the functions of the endoplasmic reticulum? 


  • It is in charge of carbohydrates metabolism.
  • It is also in charge of producing essential lipids like phospholipids and cholesterol.
  • The ER secretes calcium ions, which are required by the nervous and muscular systems.

Q2: What is the endoplasmic reticulum? 


The Endoplasmic Reticulum is a complex network of tubular membranes found only in the cytoplasm of eukaryotic cells. The endoplasmic reticulum is a reticulum or network of tiny tubular structures scattered throughout the cytoplasm.

Q3: Mention the functions of the rough endoplasmic reticulum. 


  • The majority of rough ER functions are related to protein synthesis.
  • Protein sorting is the third most important function after protein synthesis and protein folding.
  • The rough endoplasmic reticulum is also important in protein folding. 

Q4: Mention the functions of the smooth endoplasmic reticulum. 


  • The smooth ER is in charge of the synthesis of essential lipids like phospholipids and cholesterol.
  • It is also in charge of carbohydrate metabolism. These are essential for the nervous and muscular systems.
  • Smooth ER is also in charge of steroid hormone production and secretion.

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Last Updated : 14 Apr, 2023
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