The Golgi apparatus, also often referred to as the Golgi complex, is the organelle found in most eukaryotic cells that is at the center of protein packaging and transport. It is similar to the endoplasmic reticulum in composition and structure – a network of flattened, tubular, lipid membrane sacs called cisternae – except the Golgi apparatus’ cisternae are organized into a stack of multiple cisternae closely packed together. The outermost cisternae are called the cis Golgi network (CGN) and the trans Golgi network (TGN) and the inner cisternae are called the medial compartment of the stack. While many eukaryotic cells typically have between four (4) and eight (8) cisternae in their Golgi apparatus, others have many more. Furthermore, the Golgi complex in some organisms’ cells is actually a combination of several stacks, like the 40 to 100 that are present in mammals (of the animal kingdom). This is one of the reasons that the Golgi apparatus was one of the first organelles to be discovered. Where it is present, it takes up so much space in the cell. Much like the smooth ER, the Golgi complex is very much an accessory organelle – it can be abundant where synthesizing and secretion of substances is essential to the operation of the type of cell and where this is not as much of a need, the cell may be limited to very few stacks or even no Golgi apparatus all together. And the location of the Golgi apparatuses is also very much variable throughout the taxonomy of living organisms.
The Golgi complex is like a transport station of proteins. For all proteins destined for organelles other than the Golgi apparatus itself or for the endoplasmic reticulum, membrane balls containing proteins synthesized in the ER, called vesicles, “bud off” the ER as if they were its daughter particles and then travel to the Golgi complex, like bubble ships, where the vesicles fuse with the Golgi apparatus at the cis Golgi network (CGN); and once the transported protein and the vesicle are part of the Golgi complex, the protein travels through the stack and into the trans Golgi network (TGN), where a new vesicle is formed by budding off of the Golgi complex so that it can carry the protein either to other organelles of the cell or out of the cell all together. If the protein is destined for the Golgi apparatus, it is absorbed and goes no further; and if it is destined for the ER, it is routed there from the CGN, without going through the stack. The Golgi apparatus redirects proteins synthesized in the ER (and other substances) like a post office redirects mail. Each stack of a cell’s Golgi complex have a different package of enzymes that process the proteins that travel through them in a different way, and this processing is how the Golgi apparatus routes proteins, and it is also how the proteins can be modified. While each and every single detail of vesicular transport is disputed by scientists, what’s explained here is a general idea of how it works.
The Golgi apparatus is similar in structure and composition to the endoplasmic reticulum, so much so that membrane balls bud off of the ER containing proteins and then fuse into the Golgi complex, which processes the protein and then redirects the protein in a vesicle that buds off it. This similarity in structure helps Golgi apparatus and the ER work together to transport proteins throughout the cell. The number of stacks of Golgi complex cisternae in the cell vary greatly from one type of cell to another, as do the number of cisternae per stack, as does the location of the Golgi complex itself. And very much like the smooth ER, the Golgi apparatus is very much an accessory organelle.