# INTRODUCTION:
-> The 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 the endoplasmic reticulum into vesicles.
-> The discovery of the Golgi apparatus was made by the Italian cytologist CAMILLO GOLGI in 1898.
-> It is a polar organelle, which means it has directionality.
# HISTORICAL BACKGROUND:
-> Camillo Golgi used a staining method for light microscopy specimens, pioneered for the study of nerve cells, which proved to be vital for the discovery of the " Internal reticular apparatus" in 1898.
-> Key studies by Nassonov, in which he investigated the localisation 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 and 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 centre of the Golgi apparatus.
-> In the middle region of the 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 centre from the Golgi apparatus.
-> A vesicular network is present, involved in sorting and secretion, known as the trans-Golgi network.
-> Golgi trafficking changes 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 are the same between 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 require.
-> 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 plant cells, Golgi stacks persist throughout the cellular division. The number of Golgi stacks in dividing plant cells can double during metaphase/ anaphase to meet high demand for new polysaccharides at the plane of cell division termed as "cell plate", where a 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 organisation around the nucleus and disperses throughout the cytoplasm, whereas identical treatment does not affect the organisation of plant Golgi.
-> Conversely, actin depolymerisation causes extensive disorganisation of plant Golgi with much more subtle effects in animal cells.
# FUNCTIONS:
1) PROTEIN MODIFICATION AND GLYCOSYLATION:-
-> Golgi apparatus secretes polysaccharides, lipids, and proteins and performs glycosylation and the 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 specialised 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 types 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 the 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 ARF1. Activated 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 is a highly dynamic, multifunctional organelle central to secretion, cell wall biosynthesis, protein modification, and intracellular trafficking. While its basic structure is conserved across eukaryotes, the 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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