The Golgi apparatus (also known as the dictyosome or Golgi body) is a cellular organelle that is to be found in many eukaryotic cells. Their main job is as a processing and packaging factory for the important macromolecules vital to cellular functioning like proteins and lipids. The products of the process are often secreted by the cell.
The structure of the Golgi apparatus can be seen as three to eight membranous stacks called cisternae. The stack has five different functional regions and each of these regions uses different enzymes which it uses to modify the contents depending on where the product is going to be sent to. It is thought that the Golgi apparatus have a storage function and are also responsible for constructing complex cell components out of more simple ones.
The key to moving products around in the to different areas, both into and out of the Golgi apparatus is the vesicles. These containers may, for example, transport products from the endoplasmic reticulum into the Golgi apparatus.
Meanwhile other vesicles will be filled at the Golgi apparatus and sent to the cell membrane, for example, where the products may be secreted into the exterior of the cell in a process called exocytosis. Secretory vesicles are used in the nervous system in the release of neurotransmitters with the vesicles moving to the membrane of an appropriately stimulated neuron and releasing their contents into the extracellular space. Vesicles may also be sent to lysosomes for degradation of their contents.
An individual cisterna, one of the half dozen or so that makes up a stack, contain enzymes capable of modifying or helping in some way the proteins that are being transported through the Golgi apparatus. They can add carbohydrates in a process known as glycosylation, for example. Another function is to add phosphates in a process called phosphorylation. They also contain the proteins required for their own maintenance.
The function of the Golgi apparatus can be seen as an important processing and packaging plant, in some ways similar to a Post Office. There are many different macromolecules, particularly proteins but also lipids, that it is required to sort and package and modify and guide in the right direction around the cell or perhaps even out of the cell altogether.
There is still plenty of work to be done in figuring out all of the mysteries of the Golgi apparatus such as the transport mechanism used by proteins as they pass through the Golgi apparatus, for example. But what is clear is that without the Golgi apparatus the cell would quite probably become clogged up with macromolecules, decreasing the efficiency of the cell, degrading its function, and quite probably leading to its death.