Golgi Apparatus: What It Is And What Are Its Parts And Functions
Although its name may lead one to think that it is some strange device invented in the country of pizza, The Golgi apparatus is an organelle found in our cells and which has a key importance when it comes to manufacturing substances that will be used by the cell.
Already described by Ramón y Cajal and, later, more detailed by his colleague Golgi, the apparatus that bears the latter’s surname acts as a kind of cellular post office: it receives, classifies and sends proteins and other substances. Let’s look at it further.
The Golgi apparatus is an organelle found in all cells of eukaryotic organisms, and belongs to the cellular endomembrane system. It is made up of about 80 dictiosomes which, in turn, are made up of more or less 50 flattened saccules or cisterns surrounded by membrane, stacked on top of each other, responsible for the modification and completion of some proteins.
As a whole, This organelle is responsible for acting as a packaging plant, modifying the vesicles of the endoplasmic reticulum, in addition to distributing them to the parts of the cell where they affect. It was first described by the Spanish doctor Santiago Ramón y Cajal in 1897 and, later, Camillo Golgi would give detailed descriptions of the device, which would end up being named after him.
Structure
As we were already commenting, the Golgi apparatus is made up of stacked cisternae, the number of which can be variable, being between 4 and 8 and forming the dictyosome. In some cells up to 60 stacked cisterns have been seen.
Between these cisterns there are tube-shaped connections that allow the passage of substances The saccules are flattened and curved, with their convex or external face facing the endoplasmic reticulum. They can be grouped into three regions:
1. Cis tank
The cis cisterna of the Golgi apparatus, also called the cis-Golgi region, is the closest to the rough endoplasmic reticulum, which receives the transition vesicles that contain the proteins that are transformed.
2. Intermediate tanks
The intermediate cisterns, also called the medial region, are the cisternae that are found in the intermediate zone of the apparatus, between the cis and the trans. This is a transition region, through which the substances that were inside the vesicles will travel.
3. Trans cistern
It is the region closest to the plasma membrane and, in fact, its chemical composition is very similar. It is linked to the smooth endoplasmic reticulum.
Vesicles from the endoplasmic reticulum fuse with the cis-Golgi, passing through all the dictyosomes until they reach the trans-Golgi. Here they are packaged and sent to the corresponding place.
The Golgi apparatus is constantly having a flow of molecules that pass through its interior, from the cis to the trans side, passing through the intermediate cisternae
This makes it an organelle in constant renewal, which affects its size and organization. It should be noted that in those cells that have a secretory function, such as hormonal glands, the organelle will be highly developed.
Types of vesicles
Vesicles that transport substances through the Golgi apparatus can be of three types:
1. Constitutive
They contain proteins that end up outside the cell through exocytosis a process which will occur in the plasma membrane.
They are secretory vesicles that, like the constitutive ones, are destined for the outside of the cell, but not immediately. They will remain stored in the cell waiting for some signal to initiate their exocytosis (regulated secretion).
3. Lysosomal
Its destination is lysosomes, which are organelles generated in the Golgi apparatus itself. Lysosomes are responsible for breaking down foreign substances from outside the cell (cellular digestion).
Location and size
In animal cells, the Golgi apparatus is an organelle that is located near the centrosome, which in turn is usually near the nucleus. The central position of the Golgi apparatus depends on the organization of the microtubule system that make up the cytoskeleton, which in animal cells start radially from the centrosome.
In plant cells there is no centrosome. There are numerous structures similar to Golgi dictiosomes that are poorly developed or even individual cisternae dispersed throughout the cytoplasm, acting independently.
The size of the dictiosomes and the cisternae of this organelle is variable, depending on the type of cell in which it is found. Generally, their size is between 1 and 3 microns in diameter.
Features
The Golgi apparatus performs several functions, although research suggests that it may perform others. The two main ones, as we were already suggesting, have to do with modifying substances and distributing them throughout the body.
1. Manufacturing of substances
The Golgi apparatus is one of the main glycosylation centers of the cell. They are added and modified to carbohydrates, which will be part of glycoproteins, proteoglycans, glycolipids and polysaccharides
O-linked oligosaccharides are also added to the hydroxyl groups of amino acids, such as serine, threonine, and hydroxylysine, and sulfate groups to glycosaminoglycans.
Other processes that also take place in this organelle are: palmitoylation, methylation and phosphorylation, to name a few.
They are fundamental organelles for plants, since it is here where the glycoconjugates are synthesized that, later, will form part of the cell wall, with the exception of cellulose, synthesized in the plasma membrane.
Carbohydrate-related functions are carried out by glycosyltransferases which are enzymes specialized in adding carbohydrates, and glycosidases, which eliminate them.
Each tank of the Golgi apparatus has a specific role, since they contain special enzymes and carry out the process of synthesis of substances as a sequential reaction.
2. Distribution of substances
In the Golgi apparatus the distribution of molecules from the endoplasmic reticulum occurs, already synthesized in the organelle itself either.
Once they have been processed in the Golgi apparatus, the different molecules are selected and packaged into vesicles, which will go to different places in the cell.
It has recently been discovered that the Golgi apparatus is a calcium storage center, acting as an intracellular signaling platform
To release the proteins that are formed in the apparatus, it produces two types of vesicles.
On the one hand we have secret vesicles, which are those that carry proteins to the outside of the cell so that they are released into the interstitial space or the bloodstream.
On the other hand, we have storage vesicles or lysosomes, which are vesicles that remain in the cytoplasm until they are exported.
