Golgi Apparatus Definition
The Golgi apparatus is an organelle in eukaryotic organisms that moves molecules from the endoplasmic reticulum to their destination. The organelle also modifies products of the endoplasmic reticulum to their final form. The Golgi apparatus is comprised of a series of flattened sacs that extend from the endoplasmic reticulum.
Golgi Apparatus Overview
The main function of the Golgi apparatus is the ability to deliver vesicles, or packets of various cell products, to different locations throughout the cell. The Golgi also has important functions in tagging vesicles with proteins and sugar molecules, which serve as identifiers for the vesicles so they can be delivered to the proper target. The organelle is also called the Golgi complex or Golgi body.
Typically, proteins and cellular products are manufactured in the endoplasmic reticulum. The rough endoplasmic reticulum has a number of ribosomes, which assemble proteins from instructions contained in messenger RNA. Throughout the rest of the endoplasmic reticulum, these protein products are folded and modified. As they reach the Golgi apparatus, more modifications are made. Finally, the products are packaged within vesicles which are “labeled” by other proteins and molecules. The vesicles are released and based on their tags or labels they are carried to the appropriate location within the cell by the cytoskeleton.
Golgi Apparatus Functions
The Golgi apparatus has many discrete functions. But, all functions are associated with moving molecules from the endoplasmic reticulum to their final destination and modifying certain products along the way. The multiple sacs of the Golgi serve as different chambers for chemical reactions. As the products of the endoplasmic reticulum move through the Golgi apparatus, they are continuously transferred into new environments, and the reactions that can take place are different.
In this way, a product can be given modifications, or multiple products can be combined to form large macromolecules. The many sacs and folds of the Golgi apparatus allow for many reactions to take place at the same time, increasing the speed at which an organism can produce products.
Tagging Cellular Products
Regardless of the product, the vesicles containing the product move from the endoplasmic reticulum and into the cis face of the Golgi apparatus. In layman’s terms, this is the side facing the endoplasmic reticulum. The side furthest from the endoplasmic reticulum is known as the trans face of the Golgi apparatus, and this is where products are headed.
After having any modifications or additions to their structure, the products are packaged in vesicles and tagged with markers that indicate where the vesicle needs to end up. These tags can be molecules, such as phosphate groups, or special proteins on the surface of the vesicle. Once tagged, the vesicle is excreted from the Golgi apparatus, on its way to its final destination.
Finalizing Cellular Products
There are many products that are produced by eukaryotes, from proteins that can carry out chemical reactions to lipid molecules that can build new cell membranes. Some products are meant for the endoplasmic reticulum or the Golgi apparatus itself and travel in the opposite direction of most vesicles. While the endoplasmic reticulum produces most of the products and bases used, it is the Golgi apparatus that is responsible for the final presentation and assembly of products. Often, the environment must be slightly different from that present in the endoplasmic reticulum to obtain certain end products. The many sacs of the Golgi apparatus function to provide many different areas in which reactions can take place in the most favorable of conditions.
In secretory cells, or cells which produce large amounts of a substance that your body needs, the Golgi apparatus will be very large. Consider the cells in your stomach that secrete acid. The acid is produced by reactions in the endoplasmic reticulum and is modified as is goes through the Golgi apparatus. Once to the trans side of the Golgi apparatus, the acid is packaged in a vesicle and sent towards the cell’s surface. As the vesicle joins with the plasma membrane, the acid is released into the stomach, so it can digest your food.
Golgi Apparatus Structure
The image below shows the structure of the Golgi apparatus. The cis face of the organelle is closest to the endoplasmic reticulum. The trans face is the side furthest from the nucleus, which secretes vesicles to various parts of the cell. Further, there are a number of lumens and cisternae through which products flow. These appear as a series of flattened sacs stack on each other, much like the endoplasmic reticulum.
Golgi Apparatus Location
The Golgi apparatus is situated in between the endoplasmic reticulum and the cell membrane. Most often, the Golgi appears to be an extension of the endoplasmic reticulum which is slightly smaller and smoother in appearance. However, the Golgi apparatus can be easily mistaken for smooth endoplasmic reticulum. Although they look similar, the Golgi is an independent organelle which has different functions.
Theory of Golgi Apparatus Function
The most prevalent theory of how the Golgi apparatus forms is the cisternal maturation model. This model suggests that the sacs themselves tend to move from the cis face to the trans face of the Golgi apparatus over time. New sacs are formed closest to the endoplasmic reticulum. These sacs “age” as they move towards the trans face of the Golgi apparatus and their product becomes fully mature.
It may seem like there could never be enough lipids to produce the continual flow of cell membrane needed to continually make transport vesicles between the endoplasmic reticulum and the Golgi apparatus. However, there are constantly segments of cell membrane being produced and recycled by the endoplasmic reticulum, Golgi apparatus, lysosomes, and other organelles in the cell, as well as the outer cell membrane itself. The Golgi apparatus and endoplasmic reticulum work together to produce new cell membrane, as well as recycle the cell membranes of vesicles by merging two membranes when vesicles are absorbed.
The Golgi also creates lysosomes. These sacs contain digestive materials. The sacs are pinched off from the Golgi apparatus, and they are used to process materials which have been phagocytized or to digest organelles which no longer function. The lysosome delivers raw ingredients to the endoplasmic reticulum.
Golgi Apparatus in Plant Cells
While this article primarily discusses the operation of the Golgi apparatus within animal cells, plant cells also have a Golgi apparatus. In fact, plant cells may contain hundreds of these organelles.
Within plant cells, the Golgi apparatus serves the additional function of synthesizing the major polysaccharide molecules which help form the cell wall. To do this, plants often have many more Golgi bodies than an animal cell. Further, plant cells do not contain lysosomes. These digestive organelles are replaced in the plant with the central vacuole, which serves as a large lysosome as well as an organelle to store water. Thus, many vesicles from the Golgi bodies of plants move to the vacuole and fuse their contents with this large organelle.