Golgi apparatus

The Golgi apparatus [ read: Gol - chih ] is one of the organelles of eukaryotic cells and forms a membrane-enclosed reaction chamber within the cell. He is involved in the secretion and other objects of the cell metabolism and was named after the Italian pathologist Camillo Golgi in 1898 in histological researches discovered him in the brain.

Construction

The Golgi apparatus consist of four to six, in some flagellates of up to several hundred, membrane-, mostly flat cavities which are referred to as tanks. Typically each form 3 to 8, often up to 30 of these tanks a stack with 1 micron average diameter, which is referred to as Dictyosom. Depending on the cell type of the Golgi apparatus contains from one to several hundred dictyosomes. The Golgi apparatus is usually located near the nucleus and centrosome, which is ensured by microtubules. In some cells of the Golgi apparatus is not limited to this space, but is distributed throughout the cytoplasm; this is true for most plant cells and some non-plant cells. The relative expansion of the Golgi apparatus in the cell is closely related to its activity and function may vary temporarily within a cell.

At the Golgi apparatus can be observed a clear polarization. The side that faces the endoplasmic reticulum ( ER) and tied-off vesicles receives therefrom, which are occupied by the coat protein COP II is called the cis Golgi network ( CGN ), it is convex. Also vesicles from CGN to the ER can be sent, this the vesicles are with another coat protein (COP I) provided. The side facing the ER from the plasma membrane and rather is referred to as the trans -Golgi network ( TGN), it is concave. This so-called Golgi vesicles are pinched off. In the Golgi network is several smaller cisterns and vesicles, which are connected with each other.

Cisterns between the Golgi networks are called Golgi stacks, each stack containing a specific enzymatic equipment. For the passage of the proteins through the Golgi apparatus, there are two models that are probably both true: For a "walk" the individual cisternae of the cis to the trans side, while the enzymes on opposite vesicular transport for down the following cistern retained be ( model of cistern ripening). Secondly, we observed vesicle through which the proteins to the next tank - are transported ( model of vesicular transport ) - towards the TGN; Thus, the Golgi apparatus is a dynamic system.

If, during the cell to undergo cell division, disintegrates the Golgi apparatus and is divided between the two daughter cells, where it then sits again.

Functions

The functions of the Golgi apparatus are varied and complex, but can be prepared according to present knowledge in three groups:

As described above, receives the Golgi apparatus (mostly from the ER ) vesicles, in which proteins or polypeptides are included; these proteins are further modified here now. Depending on its use, and according to other proteins or protein different sugar moieties ( glycosylation) of different lengths are bound to the protein itself; the structure of the protein is changed. All these modifications take place within the Golgi apparatus, as they would result in the cytoplasm to reactions with other cell organelles and substances, which could mean the immediate death of the cell.

Are the proteins fully modified, they are sorted in the TGN after their destination, pinched off in Golgi vesicles with signaling proteins provided (SNARE proteins) and transported to the place of their destination via cell internal transport mechanisms. Most of the proteins that are modified in the Golgi apparatus, to be transported via exocytosis out of the cell, as can exocytosis the extracellular matrix (ECM ) can be modified, and it is important that all the substances except for the glycosaminoglycan (GAG), hyaluronan (formerly: hyaluronic acid), which forms a significant part of the ECM, are produced in the Golgi apparatus. The modification of the ECM contributes significantly to the intercellular communication and to the stability of the tissues and is therefore one of the most important tasks of the Golgi apparatus. In addition, a cell, for example, repair or enlarge their cell membrane; Simultaneously, the cell is given a possibility to change the structure of the outer membrane, and the metabolism of intercellular communication may be useful.

The Golgi apparatus forms primary lysosomes. Included are lytic enzymes, the activity optimum is at a pH of about 4.5, which means that the inside of the lysosome to be acidified, which is ensured by specifically incorporated into the membrane proton pump. The interior of the lysosome is lined as a protective acid with proteoglycans. Prevent false proteins included the pinching of lysosomes, lysosomal membrane is covered with mannose-6 -phosphate receptors to which the lytic enzymes which have been modified with mannose 6- phosphate binding.

The function of the Golgi apparatus is nearly identical in plant and animal cells, but the most important task of the Golgi apparatus in plants is the production of polysaccharides of the plant cell wall ( pectins and hemicelluloses ). Since these materials have to be produced in very large quantities, thus the enormous quantity of the Golgi apparatus in the plant cell is explained in comparison to the animal cell.