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Golgi apparatus


In cell biology, the Golgi apparatus, Golgi body, Golgi complex, or dictyosome is an organelle found in most eukaryotic cells, including those of plants and animals (but not most fungi). The name comes from Italian anatomist Camillo Golgi, who identified it in 1898. Its primary function is to process proteins targeted to the plasma membrane, lysosomes or endosomes and those that will be secreted from the cell, and sort them within vesicles. Thus, it functions as a central delivery system for the cell.

Most of the transport vesicles that leave the endoplasmic reticulum (ER), specifically rough ER, are transported to the Golgi apparatus, where they are modified, sorted and shipped towards their final destination. The Golgi apparatus is present in most eukaryotic cells, but tends to be more prominent where there are a lot of substances, such as enzymes, being secreted.

Structure

Figure 1: Image of nucleus, endoplasmic reticulum and Golgi apparatus: (1) Nucleus, (2) Nuclear pore, (3) Rough endoplasmic reticulum (RER), (4) Smooth endoplasmic reticulum (SER), (5) Ribosome on the rough ER, (6) Proteins that are transported, (7) Transport vesicle, (8) Golgi apparatus, (9) Cis face of the Golgi apparatus, (10) Trans face of the Golgi apparatus, (11) Cisternae of the Golgi apparatus, (12) Secretory vesicle, (13) Plasma membrane, (14) Exocytosis, (15) Cytoplasm, (16) Extracellular space.

The structure and internal function of the Golgi apparatus is quite complex and is the subject of scientific dispute. The Golgi apparatus consists, like the ER, of membranous structures. It is made up of a stack of flattened cisternae and similar vesicles. The cis face is the side facing the ER, the medial region is in the middle while the trans face is directed towards the plasma membrane (Fig. 1). The cis and trans faces have different membranous compositions.

Function

The transport vesicles from the ER fuse with the cis face of the Golgi apparatus (to the cisternae) and empty their protein content into the Golgi lumen. The proteins are then transported through the medial region towards the trans face and are modified on their way.

The transport mechanism itself is not yet clear; it could happen by cisternae progression (the movement of the apparatus itself, building new cisternae at the cis face and destroying them at the trans face) or by vesicular transport (small vesicles transport the proteins from one cisterna to the next, while the cisternae remain unchanged). Lately, it is also proposed that the cisternae are interconnected and the transport of cargo molecules within the Golgi is due to diffusion, while the localisation of Golgi resident proteins is achieved by an unknown mechansim.

Once the proteins reach the trans face, they are embedded into coated transport vesicles and brought to their final destinations. An example is the modification of glycoproteins (used in cell membranes). Vesicles from the ER contain simplified glycosylated proteins. In the Golgi Apparatus, carbohydrates are attached and removed from these glycoproteins, creating a diversity of carbohydrate structures on the proteins. After they have been secreted in to the cell the vesicles fuse to the cell membrane and release their contents.

As well as protein modification, Golgi apparatus is involved in the transport of lipids around the cell as well creating lysosomes -- organelles involved in digestion.


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