Endoplasmic
Reticulum
The endoplasmic reticulum (ER), is a cell organelle present in all
eukaryotic cells. It is made up of numerous tubes and sac-like structures
present in the cytoplasm of the cell and is related to various vital processes
which we will understand further. The endoplasmic reticulum is involved in a
variety of functions, including protein synthesis, lipid metabolism, and
calcium storage, which shows the complex functioning of ER and its importance
in living eukaryotic cells.
The ER can be divided into two types based on its appearance: -
smooth endoplasmic reticulum (SER) and rough endoplasmic reticulum (RER), each
performing a specific function.
The ribosome present on the RER gives it a rough appearance, which
plays a major role in protein synthesis.
The SER does not contain ribosomes on its surface hence the smooth
appearance. It plays a crucial role in the process of detoxification, calcium
homeostasis, and lipid metabolism, beneficial for various cellular activities.
Structure
of Endoplasmic Reticulum
The endoplasmic reticulum (ER), is a dynamic cell organelle
composed of a complex network of membranous structures. It is composed of two
main parts: the rough ER (having ribosome on it) and the smooth ER. The tubules
are interconnected to each other forming a complex structure in the cytoplasm
of the cell, enabling the ER to carry out its functions.
RER: Rough Endoplasmic Reticulum
The RER's surface (cytoplasmic side) is covered with ribosomes,
giving it the characteristic look of a beaded structure. These ribosomes, which
are frequently referred to as protein synthesis factories of cells, are in
charge of the translation of messenger RNA (mRNA) into polypeptide chains
(protein) in order to carry out genetic transcription. The RER differs from the
smooth endoplasmic reticulum in appearance due to the presence of ribosomes,
which give it a bumpy or "rough" look.
The RER is primarily involved in the protein synthesis process.
These developing polypeptide chains are threaded through the ER membrane and
into the lumen of the membrane as ribosomes create proteins. Proteins go
through a number of intricate activities within the ER lumen that are crucial
for their correct folding, quality assurance, and modification. Chaperons are
involved in transforming the primary structure protein into its folded
structure ensuring the protein gets its native conformation.
SER: Smooth Endoplasmic Reticulum
SER as the name suggests is smooth in appearance as it lacks
ribosomes on its surface. This SER is involved in a wide range of various
cellular processes, including detoxification and lipid metabolism. It takes
part in the manufacturing of steroids like hormones and cholesterol as well as
lipids like phospholipids, which are essential parts of cellular membranes. The
SER also participates in the metabolic process of lipids, such as the
beta-oxidation of fatty acids, which breaks down fatty acids into Acetyl CoA.
Another crucial function of the smooth endoplasmic reticulum is
detoxification. It contains various enzymes that digest and metabolize various
toxic and foreign substances, which ultimately makes it more water soluble and
easily removed from the body.
Dynamism in connectivity
The ER's structure is a dynamic, interconnected system as opposed
to a static entity. A continuous network of ER tubules, sacs, and vesicles is
present throughout the cytoplasm. Several ER functions, such as the movement of
chemicals inside the cell, depend on this interconnection. Through the vesicles
that branch out from the ER, proteins produced in the RER can be transferred to
other regions of the cell or even beyond the cell.
Functions
of the Endoplasmic Reticulum
The ER is involved in various cellular processes, including
protein folding, lipid metabolism, and calcium storage. These processes
collectively help in maintaining the cell's functioning and survival.
1. Protein Synthesis and Folding
The rough endoplasmic reticulum (RER), which has ribosomes on its
surface, is known for its distinctive studded appearance. These ribosomes are
involved in protein synthesis, translating the messenger RNA (mRNA)--based
genetic information into polypeptide chain sequences that ultimately result in
protein formation. These polypeptide chains are threaded into the ER lumen as
they leave the ribosomes. These developing polypeptides are helped by chaperone
proteins within the ER to fold correctly and adapt their native shape for best
functioning.
2. Post-translational changes
Proteins can experience post-translational changes, such as
glycosylation when they fold within the ER. In this procedure, certain protein
amino acid residues are modified by the insertion of carbohydrate chains.
Protein trafficking, stability, and identification by other cellular components
all depend on glycosylation. The function and diversity of proteins in numerous
cellular processes are influenced by the ER's role in glycosylation.
3. Lipid Metabolism
The most important function of smooth endoplasmic reticulum (SER)
is lipid metabolism. It is involved in the production of lipids, such as
phospholipids, which are a major component of cell membranes, and steroids.
Additionally, the SER participates in lipid metabolism procedures such as fatty
acid production and destruction (beta-oxidation). The preservation of membrane
integrity, energy storage, and the production of bioactive chemicals all depend
on these lipid-related processes.
4. Calcium Storage and Signalling
Compared to the cytoplasm, the lumen of the ER acts as a calcium
ion store, retaining higher calcium concentrations. For cellular signalling,
this calcium storage is essential. The stored calcium ion is released in the
cytoplasm when targeted with the necessary signal molecules. Calcium ions act
as secondary messengers in various cellular processes such as muscle
contraction, cell signalling and secretion etc.
5. Detoxification and Drug Metabolism
The SER plays a crucial role in cellular health because of its
role in detoxification. It contains enzymes that digest
various medications, poisons, and other foreign substances,
making them more water soluble and easily excretable. The liver cells, where
the SER plays a large role in the metabolism of many substances that enter the
body, are where this process is most noticeable.
6. Membrane Biogenesis and
Intracellular Transport
The ER is an essential component of cell membrane biogenesis. As
ER is involved in lipid and protein synthesis which are essential components
cell membranes (phospholipids and channel proteins). The ER also serves as a
hub for intracellular transportation. Proteins and lipids are transported by
vesicles that branch out from the ER to various locations within the cell or to
the cell surface for secretion.
7. Unfolded Protein Response (UPR)
ER stress is a state in which protein folding within the ER is disturbed.
Unfolded protein response (UPR), a cellular defense mechanism aimed at
reestablishing ER homeostasis, is triggered by ER stress in cells. A number of
signalling pathways are involved in the UPR, which boosts protein folding,
lowers protein synthesis, and starts the breakdown of misfolded proteins.
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