Golgi Apparatus

 Golgi apparatus

Golgi apparatus or Golgi body is a membrane-bound pouch-like structure present in the eukaryotic cells. It is the center of the secretory pathway in the cytoplasm which is responsible for the transport, modification, and packaging of proteins and lipids into vesicles. It ranges from one to several within the cell.

Golgi Apparatus Structure

Golgi apparatus has three different components; cisternae, vesicles, and tubules. It has two ends i.e. the cis face that is facig endoplasmic reticulum. It has a convex shape and acts as the receiving side of the Golgi apparatus and the trans face that is facing the cell memmbrane. It has a concave shape and acts as the shipping side of the Golgi apparatus.

Cisternae

Cisternae are several stacks of parallel, flattened sacs ranging from 4 to 8 in number. Usually, these cisternae are equally spaced in start separated from each other by thin layer of inter-cisternal cytoplasm. their lumen is not attached to each other.  In plant cells, many cisterni are termed as Dictyosomes. 

Vesicles

Vesicles are small bulges coming out of the Golgi apparatus that transport proteins and lipids to different locations within the cell. The vesicles that import into the Golgi apparatus are called primary vesicles, while those that exit the Golgi apparatus are known as secondary vesicles. The vesicles may be smooth vesicles and coated vesicles.

Tubules

These are the small round structures found on the periphery of cisternae. Some of these tubules magnify to form vesicles.

Matrix

The fluid-filled center of the Golgi apparatus is called a matrix, where all protein modifications, such as sorting, chemical tagging (like phosphorylation), and carbohydrate modifications, are carried out.

Golgi  Apparatus Functions

Secretion

There are many different functions of the Golgi apparatus but secretion is one of the primary roles played by the Golgi apparatus. There are multiple ways of secretion such as constitutive secretion where the Golgi apparatus does not require any signal from outside and regulated secretion where a particular signaling molecule binds to the surface receptor of the cell as a result of which cell signaling process takes place inside the cell that tells the Golgi apparatus to pack certain proteins into vesicles are fused to the cell membrane for secretion.

Synthesis

The Golgi apparatus is an important site for carbohydrate synthesis. This includes the synthesis of glycosaminoglycans (GAGs), long unbranched polysaccharides. Golgi apparatus then attaches these polysaccharides to the proteins synthesized by the endoplasmic reticulum to make proteoglycans. The enzymes of the Golgi apparatus polymerize the GAGs by a xylose link into the protein molecules.

Sulphation

Sulphation is a kind of modification that the Golgi apparatus does. Golgi involves the sulphation of certain molecules passing through its lumen via a sulfotransferase enzyme that gain their sulfur from the PAPS enzyme. The sulphation process is important for proteoglycans' signaling abilities and gives them a negative charge.

Apoptosis

The Golgi apparatus has a potential role in apoptosis due to the presence of several members of BCL2 in it. It can trigger stress signaling pathways if the stress becomes irreparable due to which cell death occurs. The Golgi apparatus also has a role in lysosomes synthesis which are responsible for apoptosis.

The Golgi apparatus can also prevent the cells from apoptosis by regulating stress, maintaining calcium levels in the cells and caspase inhibition. A newly characterized protein termed as Golgi anti-apoptotic protein (GAAP) exclusively residing in the Golgi apparatus protects the cells from apoptosis.

Protein glycosylation 

One of the particular functions of the Golgi apparatus is protein processing which involves the modifications and synthesis of carbohydrate portions of glycoproteins. One of the important aspects of this processing is the modification of N-linked oligosaccharides.

This processing is carried out by four steps as follows:

• Deletion of 3 additional mannose residues
• Sequential addition of N-acetylglucosamine
• Deletion of 2 more mannoses
• Insertion of fucose and 2 more N-acetylglucosamine

Vesicular Transport

Vesicles leaving the rough endoplasmic reticulum are sent to the cis face of the Golgi apparatus where the inner molecules of vesicles are inserted into the lumen of the Golgi apparatus. Molecules in the lumen are modified and sorted to transfer them to their target locations. These molecules get placed on either of the three vesicles such as exocytic vesicles, secretory vesicles and lysosomal vesicles. These vesicles then bud from the trans Golgi network to deliver their contents to the appropriate cellular locations.

Cell -Specific Functions

Some cell-specific functions fo Golgi apparatus are the following:

• Cell wall and cell plate development in plant tissues.
• Acrosomal development in sperm cells.
• Zymogen secretion in the exocrine cells of pancreas.
• Secretion and transformation of lipids in the liver cells.