A cell organelle is the internal component of the cell’s cytoplasm and functions like an organ in a system. Among many cell organelles present in the cell, the endoplasmic reticulum (ER) is the largest membrane-bound organelle of the eukaryotes, but the prokaryotes lack these organelles.
The endoplasmic reticulum is the cell organelle that originates from the cell’s nuclear envelope, synthesizes protein, and transports the proteins for cellular metabolism.
Depending on the cell, the endoplasmic reticulum can occupy 50% of the cell volume. Although the endoplasmic reticulum is present in most eukaryotic cells, some cells like red blood cells and embryonic and egg cells lack it.
ER, is an extensive network of membranes that encloses compartments (called lumen) throughout the cell. ER can be smooth or rough depending on the absence or presence of ribosomes. Smooth ER lacks ribosomes in its membrane, whereas rough ER has ribosomes.
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Discovery of Endoplasmic Reticulum
Emilio Veratti (1902) discovered the sarcoplasmic reticulum in muscle fibers that have similarities with the endoplasmic reticulum of other cells. In comparison, the new organelle was first seen through an electron microscope by Keith Porter after 50 years and then termed an endoplasmic reticulum.
Composition of Endoplasmic Reticulum
The endoplasmic reticulum comprises of continuous membranes from the nuclear envelope, plasma membrane, and Golgi apparatus. The membrane is about 50 Aº thick, thinner than the cell membrane. The membranes have proteins, enzymes, triglycerides, cholesterol, and phospholipids. The main enzymes include stearates, NADH-cytochrome C reductase, glucose-6-phosphatases, glycosyl transferases, NADH-diaphoresis, and Mg++ activated ATPase.
The smaller fragments of ER when the cells homogenize and the cell membrane breaks are the microsomes. The microsomes have lipid content of about 30-50%, of which 70% are phospholipid. About 50-90% of the phospholipid are lecithin and cephalin, i.e., the phospholipid consists of phosphatidylethanolamine, phosphatidylinositol, or phosphatidylglycerol.
The RER (rough endoplasmic reticulum) has ribosome receptor proteins or ribophorins for binding ribosomes. Likewise, cisternae have many permanent proteins like reticuloplasmins.
Structure of Endoplasmic Reticulum
The endoplasmic reticulum’s structural components are divided into cisternae, vesicles, and tubules.
- Cisternae is the lamellar form of ER.
- It is a long flattened unbranched sac-like structure arranged in parallel bundles.
- Its diameter is 40-50 µm.
- The outer surface usually has attached ribosomes, or rough ER (RER) has this type of structure found in secretory cells.
- The vesicles are round or oval-shaped structures with a diameter of 25-250 µm.
- These are membrane-bound and found in abundance in pancreatic cells.
- They are located freely in the cytoplasm or bound to the cisternae and tubules’ end.
- The vesicles are present in most cells and are especially abundant in the smooth ER (SER).
- The tubules are smooth-walled, highly branched tubular structures that form the reticular system in the cisternae and vesicles.
- They have a diameter of 50-190 μm.
- These forms usually occur in non-secretory cells like muscle cells and are often present in SER.
- The function of these part is in membrane movements and fusion between membranes of the cytocavity (cytoplasmic vascular system) network.
Types of ER
ER is divided into two types based on the presence or absence of ribosomes on the outer surfaces. They are; smooth and rough endoplasmic reticulum.
- The walls of smooth ER (SER) are smooth because no ribosomes attach to its membrane.
- The smooth ER or the agranular endoplasmic reticulum and mainly comprises of tubular structures.
- These are present in cells participating in the metabolism of lipids (including steroids) and glycogen like adipose cells, interstitial cells, glycogen storing cells of the liver, spermatocytes, leucocytes, and conduction fiber of the heart.
Rough ER (Ergastoplasm)
- Rough ER (RER) has rough walls due to attached ribosomes to its membranes.
- The membranes consist of ribosome-specific transmembrane glycoproteins celled ribophorins I and II for the attachment of ribosomes involved in polypeptide synthesis.
- These are present abundantly in the cells active in protein syntheses like goblet cells, liver cells, pancreatic cells, and plasma cells.
- These comprise of flattened sacs.
- The RER has basophilic staining properties.
Location of Endoplasmic Reticulum
ER is the extended form of the nuclear envelope and is found throughout the cytoplasm of almost all eukaryotic cells including animal and plant cells. However ER is absent in mature erythrocytes, embryonic cells, and egg cells.
Functions of Endoplasmic Reticulum
The ER acts as the cell’s storage, secretory, circulatory, and nervous system. The RER (rough endoplasmic reticulum) and SER (smooth endoplasmic reticulum) have some common and special functions. The common functions of SER and RER are as follows:
- Mechanical support: The ER divides the fluid content of the cell into different compartments. This property gives the cell mechanical support.
- Transport: ER is the circulatory system of cells. It circulates the proteins, lipids, enzymes, etc., to different parts of the cells.
- Detoxification: The ER of liver cells is involved in detoxifying (removing toxic chemicals like drugs and pollutants). The toxic compounds are converted into non-toxic derivatives and excreted from the cell.
- Glycogenolysis: The conversion of glycogen to glucose occurs in the ER. The membrane of ER also consists of glucose-6-phosphatase, which is the primary enzyme involved in the transformation of glucose to glucose-6-phosphate.
Functions of SER
- SER synthesizes steroid hormones, carbohydrates, lipids (triglycerides and phospholipids), and cholesterol.
- It also synthesizes plasma membranes.
- The SER in male and female reproductive cells produces hormones.
- In the muscle cells, SER assists in the contraction of muscle and stores calcium ions.
Function of RER
- The ribosome of RER is the leading site of protein synthesis because the attached ribosomes are more efficient in protein synthesis than the free ribosomes.
- RER also produces secretory products and buds off vesicles that transport the secretory products.
- The RER of plasma cells (WBC) secretes antibodies.
- The RER present in the pancreatic cells assists in the secretion of insulin.
- The process of glycosylation (linking proteins to sugars to form glycoproteins) takes place in the lumen of RER.
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