The Golgi apparatus, named after its discoverer Camillo Golgi (1843-1926), is the central delivery system of all but the simplest eukaryotic cell. Its main function is to modify proteins synthesized by Ribosomes.
Most of the transport vesicles that leave the endoplasmic reticulum (ER), specificly rough ER, are transported to the Golgi apparatus, where they are modified, sorted and shipped towards their final destination. Golgi Apparatus is present in all cells but tends to be more prominent where their is a lot of substances, such as enzymes, being sectreted.
The structure and internal function of the Golgi apparatus is quite complex and subject of scientific dispute. The Golgi apparatus consists, like the ER, of membranous structures. It is made up of a stack of flatterned cisternae and similar vesicles. Its cis face is the side facing the ER, while the trans face is directed towards the plasma membrane (Fig. 1). Cis and trans face have a different membranous composition.
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 internal space of the Golgi apparatus). The proteins are then transported through the Golgi apparatus 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 budding (small vesicles transport the proteins from one cisterna to the next, while the cisternae remain unchanged). Once the proteins reach the trans face, they are embedded into transport vesicles and brought to their final destinations.
An example is the formation of glycoproteins (used in cell membranes). Vesicles from the ER have proteins. In the Golgi Apparatus carbohydrates are attached to them creating glycoproteins. After they have been secreted in to the cell the vesicles fuse to the cell membrane and release their contents.
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) Expelled proteins. (15) Cytoplasm. (16) Extracellular space.