Golgi Apparatus



 GOLGI APPARATUS:

The Golgi apparatus is noticeable with both light and electron microscopes.It is also called Golgi complex.It is a system of membranes like the  ER.Golgi  complex is a complex cytoplasmic structure which is made up of smooth membrane saccules or cisternae, and a network of tubules  with vesicles and vacuoles. It is surrounded by an organelle free  cytoplasm called zone of exclusion or Golgi  ground substance. It was first visualized  by George but named after Camilla  Golgi who recognized  the Golgi complex  in the nerve cells of owl and cat  by means of metallic  impregnation method.

 Discovery : The Golgi apparatus was discovered by an Italian histologist and Nobel Laureate  Camilla Golgi in  1898 in the nerve cells of barn owl and cat.Its electron microscopic structure was described by Dalton and Felix in 1954.


Location 

The Golgi apparatus is present in all eukaryotic cells. It is specially extensive in the secretory cells. It is absent in a few cell types, such as the mammalian red blood corpuscles, sperm cells of bryophytes and pteridophytes, and sieve tubes of plants.It is absent in prokaryotic cells.


      A  cell may have one large Golgi complex or several very small ones. The Golgi complex occupies different positions in different kinds of cells. In secretory and absorption cells, it usually lies between the nucleus and the cell surface where secretions or absorption occurs. In this position, it is said to be  localized.In nerve cells, it surrounds the complexes, called dictyosomes, scattered throughout the cytoplasm. A plant cell generally have 10 to 20 dictyosomes. An animal cell usually contains 3 to 7 dictyosomes. 

    There are many thousands dictyosomes in an insect salivary gland cells, and about 25,000 in the rhizoidal cell of Chara.

    Golgi Ground Substance:The cytoplasm containing the Golgi complex has fewer or no other organelles. It is called Golgi ground substance or zone of exclusion.





STRUCTURE:

Golgi apparatus varies in size and form in different cell types, but usually has similar organization for any one kind of cells.The Golgi apparatus appears as a coarse network under a light microscope. Electron microscope it as a central stack ( pile) of parallel, flattened, inter_ communicating sacs or cisternae and many peripheral tubules and vesicles. 



(i) Cisternae: 

The cisternae vary in number from 3 to 7 in most animal cells and from 10 to 20 in plant cells. They are usually equally spaced in the pile, separated from each other by thin layers of intercisternal cytoplasm. The latter may contain a layer of parallel fibres, called intercisternal elementsthat support the cisternae. The cisternae may be flat, but are often curved. Golgi apparatus has a distinct polarity, the cisternae on its opposite sides differing in thickness and molecular composition. The two poles of a Golgi apparatus are called cis face and trans face,  which act respectively as the receiving and shipping departments. The cis face is usually near the ER. The cisternae are free of ribosomes and have swollen ends. They look like the Smooth endoplasmic reticulum. This suggests that the Golgi apparatus is derived from the smooth endoplasmic reticulum. Secretory materials reach the Golgi apparatus from the SER by way of transport vesicles which bud off from the SER and fuse with the Golgi cisternae on the cis face. From the trans face of the Golgi complex arise secretory vesicles that carry the processed material to their destination.


 (ii) Tubules:

Tubules form a complicated network  towards the periphery and maturing face of the apparatus. They arise due to fenestrations of the cisternae and interconnect the different cisternae. Short, anastormosing tubules arise from the periphery of the cisternae. Some of these enlarge at their ends to from vesicles.

 (iii)Vesicles:

 The vesicles lie near the end and concave surface of the Golgi complex. They are pinched off from the tubules of the cisternae.They are of two types:  smooth or secretory vesicles, which have a smooth surface and contain secretions of the cell; and  coated vesicles, that have rough surface and elaborate membrane proteins. They carry materials to or from the cisternae. 

  All the Golgi elements are filled with a fluid, the Golgi matrix.

Golgian Vacuoles: They are expanded parts of the cisternae  developing  from the concave or maturing  face. Golgian vacuoles contain amorphous or granular substance. Some of  the Golgian vacuoles function as lysosomes. 

Functions:

Golgi apparatus is metabolically very active and many functions have been assigned to it.

(i) Packaging  of Materials: The Golgi complex modifies, sorts and packaged the materials coming from the  ER synthesized in the Golgi itself. Chemical labels are added to send the products to other specific parts of the cell or out of the cell. Molecular identification tags, such as phosphate groups, added to the Golgi products, help in sorting. Packaging involves wrapping the material by a membrane, forming transport vesicles. The transport vesicles budded from Golgi may have external molecules on their membrane which recognize " docking sites" on the surface of surface of specific organelles. The materials meant for export are enclosed in secretory vesicles. The latter release their secretions by exocytosis. The materials so packed include zymogen in pancreatic cells, lactoprotein in mammary gland cells, mucus in goblet cells, pigment granules in pigment cells, collagen in connective tissue cells, hormones in endocrine cells, etc.


 (ii) Synthesis  of Carbohydrates: The Golgi apparatus synthesizes some simple  carbohydrates such as galactose, and certain polysaccharides( hyaluronic acid, cellulose, hemicellulose,pectin compounds) from simple sugars.

 (iii) Formation  of Glycoproteins: The Golgi apparatus links   carbohydrates with  proteins coming from ER to form glycoproteins. This process is called Glycosylation.


(iv) Formation of Lipoproteins Lipids and  proteins coming from the ER are completed into lipoproteins in the Golgi apparatus.The process is termed liposylation.

 (v) Addition  to Cell Membrane: The Golgi apparatus adds material to  Plasma membrane As the secretory vesicles, discharge their contents by exocytosis, their membranes are incorporated into the cell membrane. This enlarges the cell membrane. Since endocytosis removes a part cell membrane, the latter's enlargement by exocytosis is only temporary. 

 (vi) Membrane Transformation: The Golgi apparatus changes the membranes from ER type to one with the characteristics of  plasma membrane as they through it.

(vii) Formation of Cell WallIn plant cells, the Golgi complex
 ( a) synthesizes pectin and some other carbohydrates necessary for the formation of  cell wall,  and 
( b) produces some secretions, such as mutilate, gums, etc.

  (viii) Formation  of Lysosomes: The Golgi complex gives rise to primary lysosomes by budding from the trans face of cisternae. 

  (ix) Acrosome Formation: The Golgi complex gives rise to the acrosome in an animal sperm.

 (x) Storage of Secretions: The Golgi complex is also thought to store cell secretions, such as proteins and lipids. 
 
(xi) Formation of  Yolk (Vitellogensis):   The Golgi complex produces yolk in the eggs. 

(xii) Formation  of Nematocysts and Trichocysts: The Golgi apparatus forms nematocysts in Hydra and trichocysts in Paramecium. 

 (xiii) Lipid Transport : Fatty acids and glycerol absorbed from the intestine are resynthesised to lipids in the Smooth ER,  coated in protein and then transported through the Golgi complex to the plasma membrane to leave the cell, mainly to enter the lymphatic system.

 (xiv) Absorption: The Golgi apparatus absorbs  materials from the environment. For example, cells of intestinal lining use Golgi apparatus to absorb lipids from the intestinal lumen.

(xv) Location of Enzymes: A variety of enzymes are  localized in the Golgi complex to help in the cell's biochemical reactions. 

(xvi) Formation of Plasma Membrane:Membranes of the vesicles coming from the Golgi complex link up at the site of cytokines cytokinesis to form new plasma membrane for the daughter cells.

Molecules to be processed in the Golgi complex move from one cisternae to the next in sequence, carried by transport vesicles. The latter arises from the peripheral expanded parts of Golgi cisternae.In each cisterna, the molecules are modified  by enzymes and then enclosed in fresh transport vesicle, which fuses with the next cisterna having enzymes for the  next biochemical step.The last cisterna completes modification, sorts and packages the molecules into final  transport vesicles. 


Chemical Composition: 

The membranes of the Golgi apparatus resemble the cell membrane in molecular structure. They consist of a phospholipid bilayer sandwiched by two protein monolayers. A variety of enzymes are associated with the Golgi membranes. 


Origin: 

The Golgi apparatus originates  from the RER that has lost its ribosomes. From this RER, arise transport vesicles that contain Golgi membrane and fuse with the saccule on the cis face of Golgi apparatus. This is why this face is also called the forming face.


Functions:

1): Glandular cells depends upon Golgi complex for concentrating and pouring  their secretion to the outside through secretion vesicles which contain zymogen granules. They are sent out of the cells through exocytosis or reverse pinocytosis. 

2): Golgi complex helps in membrane transformation and membrane recycling. It converts one type of membrane ( e.g., that of ER) into other types ( e.g., selectively permeable plasma membrane, differentiated membrane of lysosomes).

3): Proteins synthesised by the rough endoplasmic reticulum reach the cisternae of the Golgi  apparatus where they combine with carbohydrates to form  glycoproteins. 

4): Special simple carbohydrates e.g.,Sialic acid and galactose  are synthesised inside the Golgi Complex. 

5): Most of the complex carbohydrates , other than glycogen and starch, are synthesised  inside the Golgi complex, e.g., pectic compounds, mucopolysaacharides, hyaluronic acid, chondroitin sulphate, hemicellulose, cellulose, etc.


6): Golgi apparatus  mediates the production of hormones  by endocrine  glands.

7): Golgi acids and glycerol absorbed by intestinal  epithelium are transferred as fat to lateral through Golgi complex. 

8):  Fatty acids and glycerol absorbed  by intestinal  epithelium are transferred as fat to lacteal through  Golgi complex. 


9): Acrosome an important  constituent of the tip of animal sperms which helps in digesting  away the covering sheath of the egg or ovum during fertilization is synthesised by Golgi complex with the help of its vesicles. 

10): Golgi apparatus functions as the center around which yolk is deposited in animal oocytes. The process is called vitellogenesis.

11): Golgi apparatus  helps in the formation  of root hair from their mother cells.


12): Some of the vesicles  or vacuoles of the Golgi  apparatus store digestive enzymes obtained through endoplasmic reticulum in the inactive state thus functioning  as primary lysosomes. 

13): Hypnotoxin of nematoblasts is formed by Golgi  apparatus. 

14): Membranes of the vesicles produced by Golgi  apparatus join in the region of cytokinesis to produce new plasmalemma.


15): Pectin compounds of middle lamella and various polysaccharides of the cell wall secreted by  Golgi complex are brought  to the area of new wall synthesis by secretion vesicles. 

16): In chick embryo, the retinal pigment is synthesised by Golgi complex. 



































































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