# INTRODUCTION:
-> Golgi apparatus is a vital central organelle in eukaryotic cells .
-> It functions as a cellular "post office" that modifies, sorts and packages proteins and lipids from endoplasmic reticulum to vesicles .
-> The discovery of Golgi apparatus by the Italian cytologist CAMILLO GOLGI in 1898.
-> It is a polar organelle , which means it has directionality.
# HISTORICAL BACKGROUND:
-> Camillo Golgi used staining method for light microscopy specimens pioneered for the study of nerve cells proved to be vital for the discovery of " Internal reticular apparatus" in 1898.
-> Key studies by Nassonov, in which he investigated the localization and movement of secretory granules through the reticular apparatus, conclusively demonstrated that the apparatus was involved in cellular secretion.
-> In 1953, the first electron micrographs of the Golgi apparatus were published by Dalton and Felix.
-> It was first described more than 50 years earlier , has a clear morphological structure, one that included “flattened sacs and groups of vesicles.
# GENERAL STRUCTURE:
-> The Golgi apparatus consists of stacked cisternae composed of flattened membrane discs, usually arranged in stacks of 6-8 cisternae , historically in plants, these were referred to as dictyosomes.
-> The Golgi complex is also polar, with membranes at one end of the stack differing in both composition and thickness from those at the other end.
-> The entry point into the Golgi apparatus , called the Cis face, is closely associated with the endoplasmic reticulum (ER) and acts as the receiving center of the Golgi apparatus.
-> In the middle region of Golgi apparatus is the Medial face located between the cis face and trans face, it is involved mainly in biological modifications.
-> At the opposite end of the Golgi stack is the Trans face, which acts as the shipping center from the Golgi apparatus.
-> A vesicular network is present involved in sorting and secretion known as Trans- Golgi network .
-> Golgi trafficking change according to different cellular cues which leads to the synthesis and secretion of specific Golgi products.
-> Golgi trafficking changes in relation to the functions of other cellular components, environmental signals, cell stage, and overall needs of the cell.
# PLANT VS ANIMAL CELLS:
-> While the basic structure and function is the same between both plant cells and animal cells, there are a few key differences between the two.
* SUBCELLULAR POSITION-
-> Golgi in mammalian cells tends to reside near the nucleus, extending only as far as the perinuclear region in highly secreting cells.
-> Golgi in plant cells however is polydispersed throughout the cytoplasm , with stacks distributed throughout the entire cell.
->While animals typically contain less than 10 Golgi stacks .
-> In plants each individual Golgi stack acts in concert with the other stacks in the cell, forming
a collective network and system of endomembrane trafficking between the Endoplasmic reticulum and the plasma membrane.
* CARGO-
-> The plant cell requires extensive synthesis and secretion of non-cellulosic polysaccharides from the Golgi apparatus in order to form the cell wall, which animal cells do not requires.
-> During cell division , In animal cells, the Golgi apparatus is disassembled into vesicular bodies as membrane trafficking ceases at the onset of division.
-> Conversely in plants cells , Golgi stacks persist throughout the cellular division. The number of Golgi stacks in dividing plant cell can double during metaphase/ anaphase to meet high demand for new polysaccharides at plane of cell division termed as "cell plate" where new cell wall is formed between two daughter cells.
*CYTOSKELETAL ASSOCIATION-
-> Plant Golgi moves along Actin filaments , whereas animal Golgi is more associated with microtubules.
-> Upon disruption of microtubules, animal Golgi loses its organization around the nucleus and disperses throughout the cytoplasm, whereas identical treatment does not affect the organization of plant Golgi.
-> Conversely, actin depolymerization causes extensive disorganisation of plant Golgi with much more subtle effects in animal cells.
# FUNCTIONS:
1) PROTEIN MODIFICATION AND GLYCOLYSATION:-
-> Golgi apparatus secretes polysaccharides, lipids, and proteins and glycosylation and addition of sugar components to proteins and lipids.
- Enhances protein stability .
- Affects protein folding and targeting.
2) VESICULAR TRAFFICKING:-
-> Central hub of intracellular trafficking
-> The secretion process begins when vesicles coated with the specialized coat protein (COPII) and loaded with cargo, namely lipids and proteins, leave the endoplasmic reticulum.
-> These vesicles bud from specific locations of the ER, the ER exit sites, and traffic to the Golgi where they fuse and deliver their cargo. The materials are then shuttled through the Golgi stacks sequentially from cis to trans where they undergo various modifications.
->They consist of three type of vesicles:-
a) Clathrin- coated vesicles:
->A clathrin coat, or cage as it is frequently described, consists of three chains of two different types (heavy/~190 kDa and light/~25 kDa chains) as well as adaptor proteins that mediate the interaction between the clathrin chains and the cargo.
-> Golgi transport via this vesicle type takes place at the trans face of the Golgi and TGN
through a process that includes activation of the clathrin assembly prior to physical deformation of the membrane to form the vesicle.
b) COPII vesicles:
-> COPII coats are composed of four proteins, dimers of Sec23‐24 and Sec13‐31 . They function between the ER and the Golgi by moving material from the ER to the cis face of the Golgi apparatus.
c) COPI vesicles:
-> COPII‐coated vesicles are more associated with the export of secretory material from the
ER to the Golgi, COPI‐coated vesicles are involved with transport of material from Golgi to the ER as well as between Golgi stacks.
-> The COPI coat complex is made up of two subunits that form a coat around budding vesicles by interacting with ARF1Activated ARF1‐GTP is attached to the membrane and recruits the coatomer proteins.
-> Sorting of cargo into these vesicles is thought to be achieved by a family of transmembrane proteins, p24 and p23 . These proteins are implicated in both the formation of the COPI coat and the recruitment of suitable cargo.
Golgi apparatus as a highly dynamic, multifunctional organelle central to secretion, cell wall biosynthesis, protein modification, and intracellular trafficking. While its basic structure is conserved across eukaryotes, plant Golgi apparatus exhibits unique features reflecting plant-specific needs such as cell wall construction and sustained secretion during cell division.
beautifully written
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