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

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.

Structure

• 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.

Structure

• 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 Domain	Function	Associated Proteins
Rough ER	Protein translocation Protein folding and oligomerization Carbohydrate addition ER degradation	Sec61 complex, TRAP, TRAM, BiP PDI, Calnexin, Calreticulin, BiP Oligosaccharide transferase EDEM, Derlin1
Smooth ER	Detoxification Lipid metabolism Heme metabolism Calcium release	Cytochrome P450 enzymes HMG-CoA reductase Cytochrome b5 IP3 receptors
Nuclear envelope	Nuclear pores Chromatin anchoring	POM121, GP210 Lamin B receptor
ER export sites	Export of proteins and lipids into secretory pathway	Sar1p, Sec12p, Sec16p
ER contact zones	Transport of lipids	LTPs

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.

FAQ

References

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• Endoplasmic Reticulum. (2017). Cell Biology, 331–350. doi:10.1016/b978-0-323-34126-4.00020-7
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