| MadSci Network: Cell Biology |
The Golgi Complex (or Golgi Apparatus, or Golgi Body) is a cytoplasmic structure composed of multiple cisternae (pools of solution surounded by membranes) arranged to look like a stack of pancakes hovering over the Endoplasmic Reticulum (ER). The Golgi Complex can be subdivided into three principal parts - cis, medial, and trans - ba sed on proximity to the ER. The cis- and trans- Golgi stacks look discontinuous and irregular as vesicles are constantly joining and leaving, so they are usually referred to as the cis Golgi network (CGN) and trans Golgi netwo rk (TGN), respectively. The medial Golgi can have any number of cisternae, which are referred to collectively as the medial stacks. The function of the Golgi Complex is to modify proteins and then target them to specific sites in the cell.
As proteins are being made through the translation of mRNA on Ribosomes, they are identified by amino acid "tags" which tell the cell where the protein belongs. Many proteins have a signal peptide which tells the cell to insert the protein directl y into the ER during translation. Proteins which enter the ER are modified in several ways, including the addition of large glycosides (branched sugars) to specific amino acid side chains (Asparagines). After processing in the ER is done, the pro teins are shuttled to the CGN via small vesicles.
Once in the CGN, some of the proteins have phosphate molecules added to them, and some proteins are returned to the ER, but most of the proteins await vesicular transport to the medial stacks. As the proteins pass through the medial stacks, the glycoside chains that were added in the ER are further modified in specific ways for specific proteins (this is where the A/B/O blood groups are made), until some proteins (e.g. Antibodies) have very small glycosyl chains and others (e.g. proteoglycans which make up cartillage) have incredibly long glycosyl chains. Then the proteins are transported in vesicles to the TGN. In the TGN, further modification of the glycosides can occur, and once all of the proteins are fully modified, specilized vesicle transporters identify each protein and determine where it belongs. From the TGN (which acts like a traffic cop directing proteins to different locations), proteins are transported in vesicles to the Lysosomes, Secretory Vesicles, and the Plasma Membrane. Almost eve ry protein found floating around or stuck to the membrane outside of a cell got there through the Golgi Complex. Most of the processes of the Golgi Complex have been characterized, but t here are still many things about it that are unknown.
The Golgi Complex is named for Camillo Golgi, who shared the 1906 Nobel Prize in Medicine with Santiago Ramon y Cajal for their combi ned work identifying the Neuron as the cell in the brain responsible for thought. While Golgi was examining neurons stained with silver nitrate (Golgi Stain), he noticed small, dense structures composed of vesicles and fibers (Golgi Bodies), which were l ater named after him.