Golgi apparatus

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Diagram of the endomembrane system in a typical eukaryote cell

Micrograph of Golgi apparatus, visible as a stack of semicircular black rings near the bottom. Numerous circular vesicles can be seen in proximity to the organelle

Diagram of secretory process from endoplasmic reticuli (orange) to Golgi apparatus (pink). Please click for full labels.

The Golgi apparatus (also called the Golgi body, Golgi complex, or dictyosome) is an organelle found in typical eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi and was named after him. The primary function of the Golgi apparatus is to process and package macromolecules synthesised by the cell, primarily proteins and lipids. The Golgi apparatus forms a part of the endomembrane system present in eukaryotic cells.

[edit] Structure

The Golgi apparatus is composed of a stack of flattened, membrane-bound sacs known as cisternae. Between five and eight are usually present, however as many as sixty have been observed^[1]. Surrounding the main cisternae are a number of spherical vesicles which have budded off from the cisternae.

The cisternae stack has three functional regions: the cis face, medial region, and trans face. Vesicles from the endoplasmic reticulum fuse with the facing cis face and subsequently progress through the stack to the trans face, where they are packaged and sent to the required destination. Each region contains different enzymes which selectively modify the contents depending on where they are destined to reside^[2]

[edit] Function

Cells synthesise a large number of different macromolecules required for life. The Golgi apparatus is integral in modifying, sorting, and packaging these substances for cell secretion (exocytosis) or for use within the cell. It primarily modifies proteins delivered from the rough endoplasmic reticulum, but is also involved in the transport of lipids around the cell, and the creation of lysosome. In this respect it can be thought of as similar to a post office; it packages and labels "items" and then sends them to different parts of the cell.

Enzymes within the cisternae are able to modify substances by the addition of carbohydrates (glycosylation) and phosphate (phosphorylation) to them. Proteins are also labelled with a signal sequence of molecules which determine their final destination. For example, the Golgi apparatus adds a mannose-6-phosphate label to proteins destined for lysosomes.

Vesicles which leave the rough endoplasmic reticulum are transported to the cis face of the Golgi apparatus, where they fuse with the Golgi membrane and empty their contents into the lumen. Once inside they are modified, sorted, and shipped towards their final destination. As such, the Golgi apparatus tends to be more prominent and numerous in cells synthesising and secreting many substances: plasma B cells, the antibody-secreting cells of the immune system, have prominent Golgi complexes.

Once the proteins reach the trans face, the trans-Golgi network (TGN), they are placed into coated transport vesicles and bud off to reach their final destinations. The form of the vesicle is determined by the type of protein and the label it acquired.

[edit] Transport mechanism

The transport mechanism itself is not yet clear; a number of hypotheses currently exist:

• Cisternal progression: the Golgi apparatus itself moves, building new cisternae at the cis face and destroying them at the trans face.
• Static compartments: small vesicles transport the proteins from one cisterna to the next, while the cisternae remain unchanged.

It has also been 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 mechanism.

[edit] References