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. Possible modifications include glycosylation or phosphorylation. The proteins are also labelled with a sequence of molecules according to their final destination.

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 mechanism.

Once the proteins reach the trans face, they are embedded into coated transport vesicles and brought to their final destinations. The form of the vesicle is determined by the type of protein and the label it acquired.

An example of the Golgi complex’s functioning 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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