Endoplasmic reticulum It is a membrane cellular organelle present in all eukaryotic cells. This complex system uses more than half of the membranes in an ordinary animal cell. The membranes continue until they fill the core membrane, forming a continuous element.

This structure is distributed in the cellular polymer in the form of a labyrinth. It is a kind of network of pipes connected by bag-like structures. The biosynthesis of proteins and lipids occurs within the endoplasmic reticulum. Almost all proteins that need to be taken out of the cell first pass through the cell.

The reticular membrane is not solely responsible for separating the interior of this organelle from the cytoplasmic state and mediating the transport of molecules between these cell bodies; It is also involved in the synthesis of lipids, which are part of the plasma membrane of the cell and the membranes of other organelles.

Definition of ER

The endoplasmic reticulum is a continuous membrane system in eukaryotic cells that plays a key role in the biosynthesis, processing, and transport of lipids and Proteins.

The reticulum is divided into even and coarse, depending on the presence or absence of ribosomes in its membranes. The coarse endoplasmic reticulum has ribosomes attached to the membrane (the presence of ribosomes gives it a “coarse” appearance) and the shape of the tubes is slightly straight.

On the other hand, the smooth endoplasmic reticulum is absent from the ribosomes and the shape of the structure is much more irregular. The function of the coarse endoplasmic reticulum is mainly directed to protein processing. In contrast, the flat is responsible for lipid metabolism.

Key points

1. The endoplasmic reticulum (ER) of a cell contains a network of flattened tubules and sacs. The er performs several functions in both plant and animal cells.
2. The endoplasmic reticulum has two main regions: smooth endoplasmic reticulum and rough endoplasmic reticulum. Roughly contains ribosomes attached while smooth does not.
3. Through the bound ribosomes, the rough endoplasmic reticulum synthesizes proteins through the translation process. Rough also manufactures membranes.
4. The smooth endoplasmic reticulum serves as a transition area for the transport of vesicles. It also works in the synthesis of carbohydrates and lipids. Cholesterol and phospholipids are examples.
5. The rougher and smoother elements are typically connected so that the proteins and membranes produced by the rough element can move freely inward to be transported to other parts of the cell.

General Features of the Endoplasmic Reticulum

1. The endoplasmic reticulum is a membrane network present in all eukaryotic cells.
2. It consists of shackles or reservoirs and tubular structures that form a continuity with the core membrane and are distributed throughout the cell.
3. Reticulum lumens are characterized by high concentrations of calcium ions in addition to the oxidizing environment. Both features allow you to perform functions.
4. The endoplasmic reticulum is considered to be the largest organelle in cells.
5. The cell volume of this compartment covers about 10% of the cell interior.

Classification of Endoplasmic Reticulum

Rough endoplasmic reticulum
The rough endoplasmic reticulum exhibits a high density of ribosomes on the surface. It is an area where all processes involved in the synthesis and modification of proteins occur. Its appearance is mainly tubular.

Smooth endoplasmic reticulum
The smooth endoplasmic reticulum has no ribosomes. It is abundant in cell types with active metabolism in lipid synthesis; for example, in testicular and ovarian cells, which are steroid-producing.

Similarly, the smooth endoplasmic reticulum is found in a fairly large proportion of liver cells (hepatocytes). Lipoprotein production occurs in this region.

Compared to the coarse endoplasmic reticulum, its structure is more complex. The abundance of smooth and coarse reticulum depends primarily on the cell type and its function.

Smooth endoplasmic reticulum function
The functions of smooth ER are given below:

1. Helps in the metabolism of various molecules.
2. Helps in lipid synthesis.
3. Allow glycogen synthesis within the cell.
4. Help with steroid synthesis such as cholesterol, progesterone, testosterone, etc.

Rough endoplasmic reticulum function
The functions of rough ER are described below:

1. It provides the site for protein synthesis.
2. It helps in the transport of proteins.

Chemical Composition of ER

Biochemical studies performed on the reticular membrane after isolation by centrifugation have shown that they contain proteins (70%) usually enzymes and lipids (30%). The reticular membrane has a very marked asymmetry since on the cytosolic side we find cytochrome b5, cytochrome P450 reductase, and ATPase Ca ++ dependent. On the light side, we have glucose-6-phosphatase, β glucuronidase, and glycosyl.

Structure of Endoplasmic Reticulum

The physical architecture of the endoplasmic reticulum is a continuous membrane system consisting of interconnected bags and tubules. These membranes extend to the core, forming a single lumen.

The ER is formed by a set of vesicles and tubules organized in a network. The membranes of the RE form a continuous sheet delimiting a space internal: the light of the RE or tank.

The lattice is built with multiple domains. Distribution is associated with other organelles, various proteins, and cytoskeleton components. These interactions are dynamic.

Structurally, the endoplasmic reticulum consists of a nuclear power plant and a peripheral endoplasmic reticulum composed of tubules and sacs. Each structure is associated with a specific function.

The nucleus, like all biological membranes, consists of a lipid layer. The interior space delimited by this insider is shared with the peripheral network.

Sacks and tubule
The sacs that make up the endoplasmic reticulum are flat and are usually stacked. They contain curved areas at the edges of the films. The tubular network is not a static entity; can grow and rebuild.

The sack and tubule system is present in all eukaryotic cells. However, it varies in shape and structure depending on the cell type.

The reticulum of cells with important functions in protein synthesis consists mainly of verses, whereas the cells most involved in lipid synthesis and calcium signaling consist of a larger number of tubules.

Examples of cells with a large number of sacs are pancreatic and B cell-secreting cells, and muscle cells and liver cells also have a network of visible tubules.

Role of Endoplasmic Reticulum / Functions of Endoplasmic Reticulum

The endoplasmic reticulum is involved in several processes that involve protein synthesis, smuggling, and folding, and modifications such as disulfide crosslinking, glycosylation, and the addition of glycolipids. In addition, it is involved in the biosynthesis of membrane lipids.

Recent studies have linked the reticulum to cellular stress responses and may even induce apoptosis processes, although the mechanisms have not been fully elucidated. All of these processes are described in detail below:

Protein Shop
The endoplasmic reticulum is closely related to the protein trade; especially proteins that need to be sent outside, the Golgi apparatus, lysosomes, the plasma membrane, and logically those that belong to the same endoplasmic reticulum.

Protein secretion

1. The endoplasmic reticulum is the behavior of cells involved in the synthesis of proteins that must be performed from the cell.
2. This function was clarified by a group of researchers in the 60s who studied pancreatic cells, which are responsible for secreting digestive enzymes.
3. This group, led by George Palade, managed to search for proteins using radioactive amino acids.
4. In this way, it was possible to monitor and locate proteins using a technique called autoradiography.
5. Radiolabeled proteins can be traced to the endoplasmic reticulum.
6. This result indicates that the reticulum is involved in the synthesis of proteins whose ultimate target is secretion.
7. The proteins are then transferred to a Golgi apparatus where they are “packaged” into vesicles whose contents are secreted.

Fusion

1. The secretion process occurs because the membrane of the vesicles can fuse into the plasma membrane of the cell (both are lipid-type). In this way, the contents can be released outside the cell.
2. That is, secreted proteins (and also proteins directed to lysosomes and the plasma membrane) must follow a specific pathway involving a coarse endoplasmic reticulum, a Golgi apparatus, secreting vesicles, and finally the extracellular space.

Membrane proteins

1. Proteins intended for incorporation into a biomembrane (plasma membrane, Golgi apparatus membrane, lysosome, or reticulum) are first inserted into the reticulum membrane and are not immediately released to light. They must follow the same pathway for secreted proteins.
2. These proteins may be located within the membranes in the hydrophobic sector. Several 20 to 25 hydronic amino acids in this region can interact with the carbon chains of phospholipids. However, how these proteins are added is variable.
3. Many proteins cross the membrane only once, while others do so repeatedly. Likewise, in some cases, it may be a carboxyl or amino terminus.
4. All protein domains that refer to reticulum light are found outside the cell at its final location.

Folding and protein processing

1. Protein molecules have the three-dimensional conformation necessary to perform all of their functions.
2. DNA (deoxyribonucleic acid) transfers transcriptional information to an RNA molecule (ribonucleic acid). 
3. Next, RNA is translocated to proteins through a translation process. The peptides are transferred to the retina while the translation process is in progress.
4. These amino acid chains are arranged in a three-dimensional manner within the reticulum of proteins called chaperones: the Hsp70 family protein (heat shock proteins or heat shock proteins); number 70 refers to its atomic mass, 70 KDa), called BiP.
5. The BiP protein can bind to the polypeptide chain and mediate its folding. Likewise, it is involved in the assembly of the various subunits that make up the quaternary structure of proteins.
6. Proteins that are not properly folded are retained by the reticulum and adhere to BiP or degrade.
7. When a cell is exposed to stress conditions, the network responds to it and as a result, proper folding of the proteins does not occur. The cell can turn to other systems and produce proteins that maintain reticulum homeostasis.

Formation of disulfide bridges

1. The disulfide bridge is a covalent bond between sulfhydryl groups that are part of the structure of the amino acid cysteine.
2. This interaction is crucial for the function of certain proteins; It also determines the structure of the proteins that express them.
3. These links cannot be formed in other cell bodies (e.g., cytosol) because it does not have an oxidizing environment that favors the same formation.
4. The formation (and breakdown) of these bonds involve the enzyme: protein disulfide isomerase.

Glycosylation

1. In the reticulum, the glycosylation process occurs at specific asparagine residues. Like protein folding, glycosylation occurs while the translation process is in progress.
2. Oligosaccharide units consist of fourteen sugar residues. They are transferred to an asparagine by an enzyme called oligosaccharyltransferase, which is located in the membrane.
3. Although the protein is in the reticulum, three glucose and one mannose residue are removed. These proteins are introduced into a Golgi apparatus for further processing.
4. On the other hand, certain proteins are not anchored to the plasma membrane by some hydrophobic peptides.
5. In contrast, they are attached to certain glycolipids that act as an anchoring system and are called glycosylphosphatidylinositol (abbreviated GPI).
6. This system is assembled in the reticulum membrane and involves the binding of GPI to the terminal carbon of the protein.

Lipid synthesis

1. The endoplasmic reticulum plays a key role in lipid biosynthesis; especially the smooth endoplasmic reticulum.
2. Lipids are an essential part of the plasma membranes of cells.
3. Lipids are highly hydrophobic molecules, so they cannot be synthesized in aqueous media. Therefore, its synthesis takes place in conjunction with existing membranes.
4. The transport of these lipids occurs in vesicles or transport proteins.
5. Membranes of eukaryotic cells consist of three types of lipids: phospholipids, glycolipids, and cholesterol.
6. Phospholipids are glycerol derivatives and are major components. These are synthesized in the region of the reticulum membrane that points to the cytosolic face. Different enzymes are involved in the process.
7. The membrane grows due to the integration of new lipids. Due to the presence of the enzyme Flipase, growth can occur on both sides of the membrane. This enzyme is responsible for transferring lipids from one side of the bilayer to the other.
8. Methods for the synthesis of cholesterol and ceramides also occur in the reticulum. The latter goes to the Golgi apparatus to produce glycolipids or sphingomyelin.

Calcium Storage

1. The calcium molecule is involved as a signaling agent for various processes, either by fusion or by combining proteins with other proteins or nucleic acids.
2. There are calcium concentrations inside the endoplasmic reticulum of 100 to 800 μM.
3. Calcium channels and receptors that release calcium are found in the reticulum.
4. Calcium release occurs when phospholipase C is stimulated by activating G protein-coupled receptors (GPCRs).
5. In addition, the elimination of phosphatidylinositol 4,5-bisphosphate occurs in diacylglycerol and inositol triphosphate; the latter is responsible for the release of calcium.
6. Muscle cells have an endoplasmic reticulum that specializes in the sequencing of calcium ions called the sarcoplasmic reticulum.
7. It is involved in the processes of muscle contraction and relaxation.

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