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Golgi Apparatus
The Golgi apparatus is an important cell
organelle present in eukaryotes. It is also known as the Golgi body or
Golgi complex. This complex membrane structure, (which was discovered by
Camillo Golgi, an Italian scientist and physician) plays a key role in the
selection, processing, and packaging of molecules going to various cellular
locations.
The Golgi apparatus, which is found in the
cytoplasm of most eukaryotic cells, is distinguished by its characteristic
structure made up of flattened cisternae that are joined to form a complex and
dynamic system. In terms of functionality, the secretory and endocytic pathways
both heavily rely on the Golgi apparatus. It helps in the modification of
protein structure with the help of glycosylation, phosphorylation, and
proteolytic cleavage, before transporting it to the cells where it is needed.
The production of vesicles that carry
chemicals to various compartments inside the cell or for secretion outside the
cell requires the Golgi apparatus as well. These vesicles branch out from the
Golgi stacks, each of which has a specific functional role in the distribution
and processing of chemicals. It is also involved in the synthesis of lipids and
various complex carbohydrates, which are essential components of cell membranes
as well as other cellular structures.
The Golgi apparatus is essential for sorting
molecules that need to undergo lysosomal breakdown along with the involvement
in post-translational modification protein and molecular trafficking.
Cellular trash and undesired materials are
packaged into lysosomes, which contain enzymes that can break them down into
recyclable components, as part of this complex sorting process.
Numerous proteins and enzymes found in the
membranes of the Golgi apparatus control its complex operations. These proteins
help in the movement of molecules, the holding up of the Golgi structure, and
various other enzymatic processes necessary for post-translational
modifications.
Structure of Golgi apparatus
It has a complex and distinctive structure.
The Golgi apparatus, which consists of a collection of flattened, membrane sacs
called cisternae, has a dynamic architecture that supports its variety of
tasks.
Stacks of cisternae that are divided into
different compartments according to their functions and physical traits make up
the Golgi apparatus in most cases.
The cis-Golgi network (CGN), cis-cisternae, medial-Golgi cisternae, trans-Golgi network (TGN), and
trans-cisternae are some of these compartments. The CGN serves as a
place where newly synthesised proteins from the ER are received because it is
located closest to the endoplasmic reticulum (ER). The TGN, which is situated
at the other end of the stack, is in charge of classifying and guiding
molecules to different cellular locations.
The proteins that mediate the cisternae's
structural integrity and functional activities are incorporated in the lipid
bilayers that make up the cisternae themselves. The adjacent cisternae are
connected to each other with the help of a tubular structure which helps in the
continuity of the structure and allows the exchange of materials. The diverse
processing and sorting functions that take place within each compartment are
reflected in the different architecture and composition of the cisternae in the
Golgi apparatus.
The Golgi apparatus' capacity to change and
process molecules as they move through its cisternae is one of its
distinguishing characteristics. A variety of enzymes enmeshed in the Golgi
membranes carry out these changes. Glycosylation, the process in which
carbohydrates are added to proteins and lipids to produce glycoproteins and
glycolipids, is one common alteration. As molecules pass through the many
cisternae, each of which is furnished with a unique set of enzymes responsible
for a different change, this process takes place successively.
Another crucial component of the Golgi
apparatus' structure and operation is vesicular transport. Small transport
vesicles sprout from the borders of the numerous cisternae as molecules are
processed there, conveying certain payload molecules to where they are needed.
These vesicles are protein-coated structures that act as cargo and transport
substances to their destination place. Once generated, these vesicles transport
chemicals to and from the plasma membrane, lysosomes, and endosomes as well as
the Golgi stack.
The Golgi apparatus is not a static
organelle, even though it typically retains its stack-like form. Cisternae are
constantly growing, maturing, and dispersing, making them extremely active. The
constant mobility of molecules and vesicles inside the Golgi stack is
indicative of this dynamic character. Additionally, during some cellular
processes, such as mitosis, the Golgi apparatus can split into smaller
vesicular structures before reforming once cell division is finished.
Functions of Golgi apparatus
In eukaryotic cells, the Golgi apparatus is
involved in a variety of tasks that are essential for the normal functioning of
the cell.
Its primary function is modifying and
distributing the molecules, which is vital for cellular functioning. The
various functions of the Golgi apparatus are:-
1)Protein
Processing and Modification
The Golgi apparatus plays a crucial role in
the post-translational modification of proteins. The endoplasmic reticulum
(ER) which is in continuity with this Golgi structure releases newly
synthesized proteins into the Golgi apparatus, where they undergo a number of
modifications. Such as phosphorylation (addition of phosphate groups to
particular amino acids), and glycosylation (addition of carbohydrates to
proteins to create glycoproteins).
Only properly folded proteins are processed
and transported further thanks to the assistance of the Golgi apparatus, which
also promotes protein folding and quality control.
2)Lipid metabolism
and processing
Along with proteins, the Golgi
apparatus also contributes to the metabolism and processing of lipids. It is
involved in the process of lipid biosynthesis, such as glycolipids and
sphingolipids, which are the major components of the cell membrane. Before
being incorporated into cellular membranes or transferred to other organelles,
these lipids are first created and changed in the Golgi cisternae.
3)Sorting and
trafficking of molecules
The Golgi apparatus is essential for
organising molecules and directing them to the right locations within the cell.
Cargo molecules are brought in from the ER, and they are sorted according to
particular signals or tags. Cargo molecules are bundled into vesicles for
transportation to various locations, and the trans-Golgi network (TGN)
functions as a central sorting station. These vesicles can transport molecules
to their destination place which can be endosomes, lysosomes, outside the cell,
cell membrane or even back to the ER. If the cargo substance is needed to be
degraded it is sent to lysosomes for degradation, or to the plasma membrane for
secretion.
4)Lysosome
Formation
Lysosomes are organelles that contain enzymes
that can degrade cellular debris and large molecules. Lysosome formation occurs
in the Golgi apparatus. Hydrolytic enzymes are packaged into vesicles called
lysosomes by the Golgi apparatus. These enzymes are first created in the ER and
then changed there before being transported to the lysosomes. Lysosomes are
essential for cellular recycling as they contain hydrolytic enzymes which can,
digest the damaged organelles and pathogens.
5)Exocytosis and
Secretion
The secretory pathway, which involves
the export of molecules from the cell, is at the heart of the Golgi apparatus.
Before being put into secretory vesicles, proteins that are created in the ER
are further processed and matured in the Golgi apparatus. Following their
fusion with the plasma membrane, these vesicles discharge their contents
outside of the cell. these vesicles can contain enzymes, hormones, antibodies
or various other chemical substances necessary for intercellular communication
and other cellular functions.
6)Cell Wall
Formation in Plant Cells
The Golgi apparatus is essential for the
development of cell walls in plant cells. It creates and changes elements that
make up the cell wall matrix, including cellulose, pectin, and other
carbohydrates. After being delivered to the cell surface, these elements are
subsequently incorporated into the developing cell wall, giving the plant cell
structural support and defence.
