Golgi apparatus: what is it and what are its parts and functions?

A summary of the characteristics of this important cell organelle.

Although its name might lead one to think that it is some strange apparatus invented in the land of pizza, the Golgi apparatus is an organelle found in our cells, the Golgi apparatus is an organelle found in our cells that is of key importance to the function of our cells.It is of key importance when it comes to manufacturing substances that will be used by the cell.

Already described by Ramón y Cajal and, later, in more detail by his colleague Golgi, the apparatus bearing the latter's surname acts as a sort of cellular post office: it receives, sorts and sends proteins and other substances. Let's take a closer look.

What is the Golgi apparatus?

The Golgi apparatus is an organelle found in all cells of eukaryotic organisms.and belongs to the cellular endomembrane system. It is formed by about 80 dictiosomes which, in turn, are formed by more or less 50 sacculae or flattened cisternae surrounded by membrane, stacked one on top of the other, in charge of the modification and completion of some proteins.

As a whole, this organelle is in charge of acting as a packing plantIt modifies the vesicles of the endoplasmic reticulum and distributes them to the parts of the cell where they belong. It was first described by the Spanish physician Santiago Ramón y Cajal in 1897 and, later, Camillo Golgi would give detailed descriptions of the apparatus, which would end up being named after him.

Structure

As we have already mentioned, the Golgi apparatus is made up of stacked cisternae, whose number can be variable, being between 4 and 8 and forming the dictyosome. In some cells up to 60 stacked cisternae have been seen.

Between these cisternae there are tube-like connections that allow the passage of substances.. Saccules are flattened and curved, with their convex or external face oriented towards the endoplasmic reticulum. They can be grouped into three regions:

1. cis cis cisterna

The cis cis cisterna of the Golgi apparatus, also called cis-Golgi region, is the closest to the rough endoplasmic reticulum, which receives the transition vesicles that have the proteins that are transformed.

2. Intermediate cisternae

The intermediate cisternae, also called medial region, are the cisternae that are closest to the rough endoplasmic reticulum.are the cisternae found in the intermediate zone of the apparatus, between the cis and trans. This is a transition region, through which the substances that were in the interior of the vesicles will travel.

3. Trans cisterna

This 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 appropriate location.

The Golgi apparatus is constantly having a flow of molecules passing through its interior, from the cis side 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 hormone 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 will go to the outside of the cell by exocytosis, a process which will take place at the plasma membrane.This process will take place at the plasma membrane.

2. Regulated

These are secretory vesicles which, like the constitutive vesicles, are destined to the outside of the cell by exocytosis.These are secreted vesicles which, like the constitutive ones, are destined for the outside of the cell, but not immediately. They will remain stored in the cell waiting for a signal to initiate their exocytosis (regulated secretion).

3. Lysosomal

Their destination is the lysosomes, which are organelles generated in the Golgi apparatus itself. Lysosomes are responsible for breaking down foreign substances from outside the cell (cell digestion). (cell digestion).

Location and size

In animal cells, the Golgi apparatus is an organelle located close to the centrosome, which in turn is usually close to the nucleus. The central position of the Golgi apparatus depends on the organization of the system of microtubules that make up the cytoskeleton, which in animal cells start radially from the centrosome.

In plant cells there is no centrosome. There are numerous, poorly developed structures similar to the Golgi dictyosomesor even individual cisternae scattered throughout the cytoplasm, acting independently.

The size of the dictyosomes and cisternae of this organelle is variable, depending on the cell type in which it is found. In general, their size is between 1 and 3 microns in diameter.

Functions

The Golgi apparatus performs several functions, although research suggests that it may have other functions. The two main ones, as we were already suggesting, have to do with modifying substances and distributing them throughout the organism.

1. Manufacture of substances

The Golgi apparatus is one of the main glycosylation centers of the cell. They are added and modified to carbohydrates, which will become part of glycoproteins, proteoglycans, glycolipids and polysaccharides..

O-bonded oligosaccharides are also added to the hydroxyl groups of amino acids, such as serine, threonine and hydroxylysine, and sulfate groups are added 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 that glycoconjugates are synthesized that will later form part of the cell wall, with the exception of cellulose, which is synthesized in the plasma membrane.

The functions related to carbohydrates are carried out by glycosyltransferases, which are enzymes specialized in the synthesis of glycoconjugates.which are enzymes specialized in adding carbohydrates, and glycosidases, which remove them.

Each cisternae of the Golgi apparatus has a specific role, since they contain special enzymes and carry out the process of substance synthesis as a sequential reaction.

2. Partitioning of substances

In the Golgi apparatus the delivery of molecules from the endoplasmic reticulum, which are already synthesized in the organelle itself, takes place in the Golgi apparatus.o.

Once they have been processed in the Golgi apparatus, the different molecules are selected and packaged in vesicles, which will be directed to different places in the cell.

Recently, it has been discovered that the Golgi apparatus is a calcium storage center, acting as an intracellular signaling platform..

To release the proteins formed in the apparatus, the apparatus produces two types of vesicles.

On the one hand we have the secretory vesicles, which are those that carry proteins to the outside of the cell to be released into the interstitial space or the bloodstream.

On the other hand, we have the storage vesicles or lysosomes, which are vesicles that remain in the cytoplasm until they are exported.

Bibliographic references:

• Farquhar, M. G., Palade, G. E. (1981). The Golgi apparatus (Complex). The journal of cell biology. 91: 77s-173s.
• Glick, B. S., Luni A.. (2011). Models for Golgi traffic: a critical assessment. Cold Spring Harbor perspectives in biology. 3: a005215.
• Vildanova M. S., Wang W, Smirnova E. A. (2015). Specific organization of Golgi apparatus in plant cells. Biochemistry (Moscow). 79: 894-906.
• Witkos T. M., Lowe M. (2017) Recognition and tethering of transport vesicles at the Golgi apparatus. Current opinion in cell biology. 47:16-23.