Golgi Complex

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Golgi complex:

Golgi complex is also known as Golgi apparatus which is found in most cells. It is another packaging organelle like the endoplasmic reticulum (ER). It was named after Camillo Golgi, an Italian biologist.

General description:

The Golgi complex was first discovered in 1897 by Italian physician Camillo Golgi. The Golgi complex, also termed as Golgi body is a single membrane bound organelle and part of endomembrane system.

Structure of a Typical Animal Cell

Golgi complex characteristics:

  • It consists of five to eight flattened membrane bound sacs called the cisternae.
  •  Each stack if cisternae us termed as Golgi stack. The cisternae in Golgi stack vary in number, shape and organization in different cell types.
  • In unicellular flagellates, however, as many as 60 cisternae many combine to make up the Golgi stack.
  • The number of Golgi complexes in a cell varies according to its function. A mammalian cell typically contains 40-100 stacks.
  • In mammalian cells, multiple Golgi stacks are linked together at their edges.
Golgi apparatus

Golgi complex structure:

  • Each Golgi stack has two distinct faces: a cis face (or the entry face) and a Trans face (or maturing face).
  • Both cis and trans faces are closely associated with special compartments: the cis Golgi network (CGN) and the trans Golgi network (TGN), respectively.
  • TGN was earlier known as GERL (Golgi apparatus- ER-Lysosome).
  • Proteins and lipids enter the cis Golgi network in vesicular tubular clusters arriving from ER and exit from the trans Golgi network. Both networks are thought to be important for protein sorting.
  • Similarly, proteins exiting from the TGN can either move onward and be sorted according to whether they are destined for lysosomes, secretory vesicles or the cell surface, or to be returned to an earlier compartment.
  • The Golgi apparatus is especially prominent in cells that are specialized for secretion, such as the goblet cells of intestinal epithelium, which secrete large amounts of polysaccharide-rich mucus into the gut.
  • In such cells, usually large secretory vesicles are found on the trans side of the Golgi apparatus. Secretory vesicles form from the TGN , and they release their contents to the cell exterior by exocytosis.
  • The membrane proteins and the lipids in these vesicles provide new components for the cell’s plasma membrane, while soluble protein and other compounds inside the vesicles are secreted to the extracellular space.
Functions of Golgi Complex

Functions of Golgi complex:

  • The Golgi complex is often considered the distribution and shipping department for the cell’s chemical products.
  • It modifies proteins and lipids that have been built in the ER and prepares them for export outside of the cell or for transport to other location in the cell.
  • Proteins and lipids from the smooth and rough ER bud off in tiny bubble- like vesicles that move through the cytoplasm until reach the Golgi apparatus.
  • The vesicles fuse with the Golgi membranes and release their internally stored molecules into the organelle.
  • Glycosylation of proteins: N-linked oligosaccharide chains on proteins are altered as the proteins pass through the Golgi cisternae en route from the ER. Further modifications of N-linked oligosaccharide in the Golgi apparatus gives two broad classes of N-liked oligosaccharides, the complex oligosaccharides and the high mannose oligosaccharides. High mannose oligosaccharides have no new sugars added to them in the Golgi body. They contain just two N-acetylglucosamines and many mannose residues. Complex oligosaccharides, by contrast, can contain more than the orginal two N-acetylglucosamines as well as a variable number of galactose and sialic acid residues and, in some cases, fucose.

 Lipid metabolism

Lipid  metabolism:

In addition to its activities in processing and sorting glycoproteins, the Golgi complex functions in lipid metabolism in particular, in the synthesis of glycoproteins and sphingomyelin. Ceramide, which is synthesized in the ER, is converted either to sphingomyelin or to glycolipid in the Golgi complex.
  • The Golgi complex also acts as a major site of carbohydrate metabolism. In plant cells, it acts as the site where complex polysaccharides of the cell wall are synthesized. Complex cell wall polysaccharides such as hemicelluloses and pectins are synthesized in the Golgi apparatus and then transported in vesicles to the cell surface. In animals, most of glycoaminoglycans of the extracellular matrix are synthesized in the Golgi apparatus.
Golgi Functions

Transport of proteins through Cisternae:

  • Several models have been proposed to explain the ordered transport of proteins through the cisternae. Two most accepted models are cisternal maturation model and vesicular transport model.
  • According to cisternal maturation model, stacks of cisternae are dynamic. The cis- most cisterna is assembled by the fusion of transport vesicles from the ER, carrying newly synthesized cargo. Each Golgi cisterna matures as it migrates outward through a stack and at each stage, the Golgi resident proteins that are carried forward in a cisterna are moved backward to an earlier compartment. In this way cisterna progresses into the next one and COPI-coated vesicles function to transport resident Golgi components in the retrograde direction.
  • Vesicular transport model postulates a stable stack of cisternae containing a fixed and ordered array of processing enzymes. The cargo is delivered to each cisterna in turn by vesicles that bud from one cisternae and fuse with the next stack.
Transport of proteins through Cisternae

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