The Golgi apparatus is a tiny, cellular organelle with a big job. Usually located near the nucleus, it is responsible for modifying, packaging, and distributing proteins and other macromolecules. In animal cells, there is usually only one Golgi apparatus, but plant cells often have more.
The Golgi apparatus is made up of a series of pancake-shaped, connected, membrane-bound chambers called cisternae. These cisternae are layered one on top of the other in stacks, which can be seen using ordinary microscopic techniques. There are usually four or five of these cisternae in a Golgi body, but in cells that perform major secretion functions, there can be as many as sixty. The side of the Golgi apparatus nearest the rough endoplasmic reticulum (ER) is called the "cis" face. The side opposite the rough ER is called the "trans" face.
Proteins are assembled by the ribosomes of the rough ER, and then packaged into little parts of the ER membrane. These membrane packages pinch off, creating a small spherical pouch called a vesicle. The vesicles then make their way through the cytosol (the fluid inside the cell), to the nearby Golgi apparatus.
Proteins enter the Golgi body through the cis face when their vesicle fuses with the Golgi membrane and opens up, emptying its contents into the cisternae. From there, they begin to make their way through each section of the Golgi apparatus until they reach the trans face. Along the way, they are processed based on their ultimate function and destination.
Some proteins will get a molecule called a polysaccharide attached to them in a process called "glycosylation." Polysaccharides are chains of sugar molecules, such as galactose or mannose. These polysaccharides act as either a functioning part of the molecule, or as a marker to help direct the protein to its ultimate destination. Once a protein has a polysaccharide attached to it, it is called a glycoprotein. Other proteins will have a phosphate molecule attached in a process called "phosphorylation." Still other proteins will have some small pieces removed to make a functioning protein.
Once the protein or other molecules reach the trans pole of the Golgi body, they are sorted and segregated by type, and by where they are going to be distributed. Some of the molecules will be immediately packaged for secretion out of the cell. These proteins get enclosed by a part of the trans membrane forming a new vesicle, which then buds off and begins to migrate toward the cell membrane. Components of the cell membrane recognize the vesicle and bind it. The vesicle then fuses with the cell membrane and opens up to the outer side of the cell, releasing its contents in a process called "exocytosis."
Other molecules will be packaged in vesicles and stored in the cell until needed. Molecules such as hormones and neurotransmitters are commonly processed this way. This is called "regulated secretion," because they are not released until a signal is received that they are needed.
Although this discussion has focused on proteins, the Golgi apparatus also processes other types of molecules such as lipids and carbohydrates. In plants, the Golgi apparatus produces the sugars that make up the cell walls.
There is still much that is not known about the Golgi apparatus. It is the subject of a great deal of current research efforts. One question researchers are trying to answer is just how the molecules being processed by the Golgi apparatus move through the cisternae. Two theories are currently being studied. The first theory, called the "Cisternal Maturation Model," says that the cisternae actually move, with a new cis face being formed by vesicles from the rough ER, and the trans face being destroyed as it releases its processed molecules. The second theory is called the "Vesicular Transport Model." This theory states that the Golgi apparatus is a solid, connected body with small vesicles, called shuttle vesicles, inside it that carry the molecules to be processed from the cis side, through the various sections, to the trans side.
With a better understanding of the way the Golgi apparatus works, in the future, medical science might be able to treat the various diseases caused by protein defects.