Golgi Apparatus is a membrane-bound cell organelle present in cells of all the eukaryotic organisms. It is also known as Golgi body, Golgi Complex or just Golgi. Due to its role in a cell, the Golgi Apparatus is called a packaging area of a cell because it is responsible for modifying, packaging (into vesicles) and transport of all the secretory proteins to their respective location inside or outside the cell. Golgi Apparatus is a part of the Endomembrane system which includes the Nucleus, Endoplasmic Reticulum, Golgi, Vesicles and Plasma Membrane. This whole endomembrane system work as a team inside a cell and is responsible for secretory protein pathway.
Structure of Golgi Apparatus
- Golgi Apparatus is a membrane-bound cell organelle which is present near the Endoplasmic Reticulum, which is present near the outer membrane of the Nucleus.
- It appears like many flattened pouches present close to one another. This flat-tube/pouch-like structure is called as cisternae.
- The cisternae of the Golgi apparatus is divided into 3 compartments, these are, Cis (or cis face which is present near the endoplasmic reticulum), Medial (centre part of the cisternae) and trans (or trans face, which is present near the plasma membrane).
- Generally, the number of cisternae present in a Golgi Apparatus is only six to eight but in several single-celled organisms, these number can rise up to 60 cisternae per Golgi Apparatus.
- The two parts Cis Golgi network and Trans Golgi network are the outmost pouches of the cis and trans face respectively. Cis Golgi network and trans Golgi network, both perform a different and important function in the Golgi Apparatus.
- The network of Cisternae is stabilized in the cell via cytoplasmic microtubules which are the cytoskeleton of a cell.
- The lumen of the Golgi Apparatus is filled with a matrix which is a fluid protein. The matrix contains many enzymes which play a major role in the modification of the proteins arriving at the Golgi Apparatus.
- Animal cells contain one or few Golgi per cell but plant cells contain hundreds of Golgi Apparatus per cell.
Functions of Golgi Apparatus
Golgi Apparatus is responsible for some of the most important function inside our cell. Without the Golgi Apparatus, the proteins that are formed in the cytoplasm will have no use.
- The proteins, after they are formed, undergoes into a series of modification which is done inside the Endoplasmic Reticulum first and then inside the Golgi Apparatus. This modification works as a tag on the proteins and tells them where to go in the cell. Just like an address on the package makes the package to get successfully delivered, similarly, the modification on the proteins helps them to get delivered to their address inside the cell or outside, depending upon the type.
- The Golgi apparatus also modifies the lipids which are to be inserted into the Plasma Membrane.
- The proteins coming from the Endoplasmic Reticulum is protected inside a vesicle, that vesicle is received by the cis face of the Golgi Apparatus which is near the Endoplasmic Reticulum.
- This vesicle gets fused with the membrane of the Golgi Apparatus and the protein is delivered inside the Golgi. This process of sending of vesicles packed with proteins from Endoplasmic Reticulum to Cis face of the Golgi Apparatus is called as the Anterograde movement. And opposite to this, when the vesicles move form cis face of the Golgi Apparatus to the Endoplasmic Reticulum, it is called a Retrograde movement.
- The proteins inside the matrix of the Golgi Apparatus which are embedded on the wall of the cisternae then modify the proteins and lipids by several steps.
- When the protein or lipid finally reach the Trans end of the Golgi Apparatus, it buds off along the membrane of the trans-Golgi face. The membrane protects the proteins and is called a vesicle.
- The modification on the proteins directs them to go to their particular location. For example, Mannose-6-phosphate is a modification done on their proteins which is destined to go to lysosomes. So any vesicle containing protein tagged with Mannose-6-phosphate inside them will automatically fuse with the membrane of the lysosomes.
- Similarly, the proteins or lipids of the plasma membrane with go and fuse with the plasma membrane and those tagged to go outside of the cell also called as secretory proteins like enzymes, hormones, etc, will be secreted out of the cell.
- Therefore the exocytosis process of the secretory proteins is regulated by the Golgi Apparatus.
- O-linked Glycosylation occurs in Golgi Complex where the synthesis of most of the cell’s saccharides (complex form) are synthesised including GAG’s of proteoglycans.
Models of Golgi Apparatus
Two models of the Golgi Complex were proposed by the scientist.
Cisternae Maturation Model
Here is was said that the cisternae mature gradually from cis to trans, that is, each cisterna matures into next cisternae along with the stack and then finally disperse at the trans Golgi network.
The vesicles may move in the opposite direction. For example, the enzymes that must be moved back to Endoplasmic Reticulum, that is the enzymes resident to either Endoplasmic Reticulum or to Golgi itself must be moved back from the trans face to cis face, called as Retrograde movement.
Vescicle Transport Model
In this model, it is proposed that the cargo is carried in the forward or anterograde direction by vesicle and cisternae remains stable, that is, they are non-dynamic, unlike in the Cisternae Maturation Model.
Here the cisternae remain at the same place in stable compartments and transport of the vesicles take place instead.
In Vesicular Transport, 3 types of vesicles are present.
Types of Vesicles
|COP II Vesicle
|COP I Vesicle
|Clatheirn Coated Vesicle
|These vesicles move cargo from ER to Golgi
|These move cargo from Golgi to ER or from trans Golgi to cis Golgi
|They move cargo from Trans Golgi Network to endosome, lysosome and plasma membrane or outside the cell
|They move in anterograde manner
|They move in retrograde manner
|Also perform endocytosis and move material between endosome and lysosome