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Endoplasmic Reticulum (ER) – Definition, Structure, Functions and Diagram

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Endoplasmic Reticulum (ER) Definition

Endoplasmic Reticulum is an intricate network of tubular membranes found solely in the cytoplasm of eukaryotic cells.

  • Endoplasmic Reticulum (ER) is a membrane-bound network of tubes and sacs present in eukaryotic cells.
  • Two regions comprise the ER: the rough ER and the smooth ER. ER is littered with ribosomes, which are responsible for protein synthesis. Without ribosomes, the smooth ER is involved in lipid synthesis, metabolism, and detoxification.
  • The ER is essential for protein synthesis, folding, and modification. On ribosomes connected to the rough ER, proteins undergo changes including glycosylation and disulfide link formation.
  • The smooth ER participates in numerous metabolic activities, including lipid synthesis and metabolism, calcium storage, and drug and toxin detoxification.
  • The ER is continuous with the nuclear membrane, and together they constitute the endomembrane system, a complex system of membranes.
  • Transport of proteins and lipids from the cytoplasm to other sections of the cell is facilitated by the endoplasmic reticulum (ER). From the ER, vesicles deliver their cargo to their eventual destination.
  • The unfolded protein response (UPR) is a cellular stress response that is initiated when the endoplasmic reticulum (ER) is overrun by unfolded or misfolded proteins.
  • Principal components of cell membranes are phospholipids, which are synthesized by the endoplasmic reticulum (ER).
  • The ER is also involved in carbohydrate metabolism, including glycogen production and modification.
  • The endoplasmic reticulum (ER) is an essential organelle in eukaryotic cells and is involved in a variety of cellular functions, including protein synthesis, lipid metabolism, calcium storage, and detoxification.

Types of Endoplasmic Reticulum (ER)

1. Rough Endoplasmic Reticulum

The Rough Endoplasmic Reticulum (RER) is a section of the Endoplasmic Reticulum (ER) whose surface is covered with ribosomes. The ribosomes are responsible for generating proteins destined for membrane insertion or secretion.

RER is essential for protein synthesis, folding, and modification. As the ribosomes produce polypeptide chains, they are threaded through the membrane of the RER and into its lumen. Within the lumen, freshly produced proteins undergo numerous changes, including folding and carbohydrate chain addition (glycosylation).


Particularly common in cells specialized for protein secretion, such as plasma cells and pancreatic cells, is the RER. These cells have a high need for protein synthesis and require a substantial amount of RER to satisfy their requirements.

The RER is also engaged in the quality control of newly generated proteins, in addition to its role in protein synthesis. A complex system of chaperones and folding enzymes in the ER ensures that proteins are folded and assembled correctly. If a protein fails to fold properly, it may be degraded by ER-associated degradation (ERAD).

In eukaryotic cells, the RER is essential for the synthesis and quality control of proteins. It is able to perform its activities efficiently and effectively due to its intricate structure, which consists of a network of membrane-bound tubes and sacs.


  • The rough endoplasmic reticulum derives its name from its physical appearance.
  • It is a series of interconnected sacs with ribosomes on its outside surface, hence the name.
  • It produces and secretes proteins, hormones, and other chemicals in the liver and glands.
  • Rough ER is abundant in cells where protein synthesis takes place (such as hepatocytes)

2. Smooth Endoplasmic Reticulum

The Smooth Endoplasmic Reticulum (SER) is an area of the Endoplasmic Reticulum (ER) lacking surface ribosomes. The SER is engaged in numerous metabolic activities, including as lipid production, metabolism, and detoxification.

lipid metabolism is among the primary functions of the SER. Important components of cell membranes, such as phospholipids, are synthesized by the SER. The SER is also involved in the metabolism of cholesterol and steroids, among other lipids.

The SER is also engaged in drug and toxin detoxification. Enzymes involved for detoxification are present in the membrane of the SER, and they serve a crucial function in protecting the cell from toxic chemicals.

In addition to its metabolic tasks, the SER is engaged in calcium ion storage and release. When required for cellular functions like as muscle contraction and neurotransmitter release, the SER can rapidly release calcium ions into the cytoplasm from its lumen, where they can be accumulated.

Comparable to the RER, the SER is composed of a network of membrane-bound tubes and sacs. Nonetheless, the absence of ribosomes imparts a tubular look to the SER. The membrane of the SER is made of phospholipids and a variety of proteins, including enzymes and transporters involved in its diverse functions.

Overall, the SER is a vital organelle in eukaryotic cells, participating in a vast array of metabolic and signaling functions. Its intricate structure enables it to perform its functions successfully and efficiently.


  • The endoplasmic reticulum smooth, on the other hand, lacks ribosomes.
  • Endoplasmic reticulum smooth has a tubular shape.
  • It contributes to the formation of phospholipids, the predominant lipids in cell membranes that are crucial to the metabolic process.
  • The smooth ER carries the products of the rough ER to other organelles within the cell, particularly the Golgi apparatus.

Structure of Endoplasmic Reticulum (ER)

  • The endoplasmic reticulum membrane is 50 to 60 Ao thick and fluid-mosaic, similar to the unit membrane of the plasma membrane.
  • It has been shown that the membranes of the endoplasmic reticulum contain numerous types of enzymes that are required for various crucial synthetic processes. The essential enzymes are stearases, NADH-cytochrome C reductase, NADH diaphorase, glucose-6-phosphatase, and Mg++ activated ATPase.
  • Endoplasmic reticulum membrane is continuous with the plasma membrane, nuclear membrane, and Golgi apparatus membrane.
  • The cavity of the endoplasmic reticulum is well-developed and functions as a passageway for secretory products.
  • The endoplasmic reticulum (ER) can exist in three distinct configurations: lamellar form or cisternae, vesicular form or vesicles, and tubular form or tubules.
  • The lamellar form of the endoplasmic reticulum (ER) is characterized by a flattened, sheet-like structure formed of membrane-bound sacs called cisternae. This variant of the ER is typically encountered in cells with a high requirement for protein synthesis, including plasma cells and pancreatic cells.
  • The vesicular form of the endoplasmic reticulum contains small, spherical, membrane-bound vesicles that are involved in transport and secretion. These vesicles are capable of detaching from the ER and transporting their payload of proteins and lipids to other areas of the cell.
  • A network of linked, membrane-bound tubes and sacs characterizes the tubular shape of the ER. This variant of the endoplasmic reticulum is engaged in a variety of processes, including lipid metabolism, calcium storage and release, and protein transport and sorting.
  • The three forms of the ER are interconnected and can transition between each other, allowing the cell to carry out a diverse array of functions efficiently and effectively. With its network of membrane-bound tubes, sacs, and vesicles, the endoplasmic reticulum (ER) is able to perform its numerous cellular functions, such as protein synthesis, lipid metabolism, and transport.

Subdomains of the Endoplasmic Reticulum

ER DomainFunctionAssociated Proteins
Rough ERProtein translocation
Protein folding and oligomerization
Carbohydrate addition
ER degradation
Sec61 complex, TRAP, TRAM, BiP PDI, Calnexin, Calreticulin, BiP Oligosaccharide transferase EDEM, Derlin1
Smooth ERDetoxification
Lipid metabolism
Heme metabolism
Calcium release
Cytochrome P450 enzymes
HMG-CoA reductase
Cytochrome b5
IP3 receptors
Nuclear envelopeNuclear pores
Chromatin anchoring
POM121, GP210
Lamin B receptor
ER export sitesExport of proteins and lipids into secretory pathwaySar1p, Sec12p, Sec16p
ER contact zonesTransport of lipidsLTPs

Endoplasmic Reticulum (ER) Diagram

Endoplasmic Reticulum (ER) - Definition, Structure, Functions and Diagram
Endoplasmic Reticulum (ER) Diagram

Endoplasmic Reticulum Shape Generation

  • ER morphology is determined by multiple protein classes.
  • These proteins guarantee that the endoplasmic reticulum (ER) maintains a single polygonal network of tubules and stacked cisternae.
  • Reticulons and DP1/Yop1 proteins are the proteins that mediate ER tubule formation.
  • The hydrophobic segments of these proteins are predicted to form -helical hairpins that span a portion of the lipid bilayer.
  • The highly curved, tubular ER morphology is produced by the insertion of these hydrophobic segments into the cytoplasmic leaflet of the bilayer ER membrane.
  • Reticulons and DP1/YOP1 proteins are involved in the formation of nuclear pores by stabilizing curved membranes.
  • Dynamin-related GTPases of the Atlastin/RHD3/Sey1p family mediate the formation of three-way junctions, which are responsible for the polygonal structure of the tubular ER network.
  • Atlastins contain an N-terminal cytoplasmic GTPase domain, a three-helix bundle, two transmembrane segments that are closely spaced, and a C-terminal amphipathic helix.
  • GTP binding promotes interactions between atlastin oligomers in two adjacent membranes, resulting in the formation of a complex that is tethered.
  • Fusion between ER tubules is dependent on a conformational change in the cytosolic domain, which is triggered by GTP hydrolysis.
  • GDP is released by atlastin following membrane fusion.
  • In two ways, interactions with microtubules can remodel the ER.
  • ER can be pulled along the side of a microtubule by motor proteins.
  • The ER membrane can affix to +TIP attachment complexes that follow the growing microtubule’s terminus.
  • STIM1 is a transmembrane ER protein that binds directly to the +TIP protein EB1 in order to reach the plasma membrane and activate Orai.
  • Actin cytoskeleton interactions can drive ER remodeling.
  • Deficiencies in proteins that regulate the structure of the ER frequently result in disease.
  • In hereditary spastic paraplegias, mutations in atlastin or reticulons cause length-dependent degeneration of the distal portions of the axons of corticospinal upper motor neurons.
  • These mutations contribute to the pathogenesis of amyotrophic lateral sclerosis, which involves both upper and lower motor neuron degeneration.
  • Due to the large size and highly polarized geometry of neurons, shaping and distributing the ER network is particularly crucial.

Functions of Endoplasmic Reticulum (ER)

As noted previously, the endoplasmic reticulum is divided into two categories, and both types serve distinct functions:

Smooth Endoplasmic Reticulum Function

  • Smooth ER is responsible for the synthesis of phospholipids and cholesterol, among other important lipids.
  • Also responsible for the synthesis and secretion of steroid hormones is Smooth ER.
  • It is also responsible for glucose metabolism.
  • The store and release of calcium ions by the ER. They are extremely crucial for the neurological and musculoskeletal systems.

Rough Endoplasmic Reticulum Function

  • The bulk of rough ER functions are related to protein synthesis.
  • Moreover, the rough endoplasmic reticulum is essential for protein folding.
  • Additionally ensures quality control (regarding correct protein folding).
  • Protein sorting is the second most critical activity after protein synthesis and folding.


What is endoplasmic reticulum?

In eukaryotic cells, the endoplasmic reticulum (ER) is a membrane-bound organelle. It is a network of flattened sacs, tubules, and cisternae that stretch throughout the cytoplasm and are attached to the nuclear envelope. Rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER) are the two forms of ER (SER).
Ribosomes cover the surface of RER, giving it a rough appearance. It is indispensable for protein synthesis and processing. Before being delivered to their final destination, newly produced proteins undergo folding, modification, and quality control in the RER lumen.
SER is devoid of ribosomes and has a more uniform look. It participates in a number of metabolic activities, including lipid synthesis, detoxification, and calcium ion storage. It also regulates intracellular calcium levels and is involved in the generation of steroid hormones.
Endoplasmic reticulum is a highly dynamic organelle that is necessary for numerous cellular functions, such as protein synthesis, lipid metabolism, and intracellular signaling.

List the types of endoplasmic reticulum.

There are two types of endoplasmic reticulum:
1. Rough endoplasmic reticulum (RER): It is studded with ribosomes on its surface, giving it a rough appearance. RER plays a crucial role in protein synthesis and processing. Newly synthesized proteins enter the RER lumen, where they undergo folding, modification, and quality control before being transported to their final destination.
2. Smooth endoplasmic reticulum (SER): It lacks ribosomes and has a smoother appearance. SER is involved in various metabolic processes, such as lipid synthesis, detoxification, and calcium ion storage. It also plays a role in the regulation of intracellular calcium levels and the production of steroid hormones.

List the functions of the endoplasmic reticulum.

In eukaryotic cells, the endoplasmic reticulum (ER) is a membrane-bound organelle. It serves multiple crucial tasks within the cell, including:
Protein synthesis: Attached to the membrane of the ER are ribosomes that contribute to the production of proteins. These proteins are then transferred to other cellular compartments or secreted outside the cell.
Lipid synthesis: The ER is also involved in the synthesis of phospholipids and steroids, among other lipids. These lipids have key functions in cell membranes, energy storage, and signaling.
Detoxification: The ER plays an important function in the detoxification of drugs and other foreign substances in the body. In a process known as biotransformation, enzymes in the ER convert these compounds so that they can be removed from the body.
Calcium storage: The endoplasmic reticulum (ER) is also involved in the storage and release of calcium ions, which are essential for numerous cellular functions, including muscle contraction and signal transduction.
Cell signaling: The endoplasmic reticulum (ER) is engaged in multiple signaling pathways, such as the unfolded protein response (UPR) and the ER stress response, which help the cell adapt to stress and maintain homeostasis.
Overall, the ER is critical to the correct functioning of the cell, as it is involved in numerous cellular processes that are essential to its survival.

What is the Endoplasmic Reticulum (ER)?

The Endoplasmic Reticulum (ER) is a cellular organelle that plays a crucial role in the synthesis, modification, and transport of proteins and lipids.

What are the two types of ER?

There are two types of ER: Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER). The RER is covered in ribosomes, while the SER lacks ribosomes.

What is the function of RER?

The RER is responsible for the synthesis and modification of proteins that are destined for secretion or for use in the cell membrane.

What is the function of SER?

The SER is involved in lipid metabolism, detoxification of drugs and toxins, and the storage and release of calcium ions.

How does the ER contribute to protein synthesis?

The ER provides the platform for the synthesis of proteins by ribosomes, which are located on the surface of the RER. Newly synthesized proteins are then transported to the Golgi apparatus for further processing and distribution.

How does the ER play a role in lipid synthesis?

The SER is responsible for the synthesis of lipids, including cholesterol and phospholipids, which are essential components of cell membranes.

How does the ER participate in detoxification?

The SER is involved in the detoxification of drugs and toxins by modifying them to make them more soluble and easier to excrete from the body.

What is the role of ER in calcium storage and release?

The ER plays a critical role in the storage and release of calcium ions, which are important for cell signaling and muscle contraction.

How is ER structure maintained?

The unique morphology of the ER is maintained by a variety of proteins, including reticulons, DP1/Yop1, and atlastins, which ensure that the ER remains a single polygonal network of tubules and stacked cisternae.

What happens when there are defects in ER structure or function?

Defects in ER structure or function can lead to a variety of diseases, including neurodegenerative disorders and metabolic disorders, highlighting the crucial role of the ER in cellular function and health.


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