Golgi Apparatus Structure and Function

The Golgi apparatus is a cell organelle of most eukaryotic cells, although not all of them. Eukaryotic cells are those which have a true nucleus and are fund in all plants, animals and fungi. While both plant and animal cells have a Golgi apparatus, they are more properly formed in animal cells with larger cisternae. Plant cells also have Golgi apparatus, but they tend to be greater in number and have smaller cisternae. Yeast cells vary in Golgi structure.

Located in the cell's cytoplasm, the Golgi apparatus is a set of structures arranged near the plasmatic membrane, the endoplasmic reticulum and the nucleus. It is distinguished by having a moon shape made up of several sacs, the number of which varies depending on the type and origin of the cell.

The Golgi apparatus is derived from the endomembrane system. This system helps compartmentalize cells into different functional organelles. It has a very compact structure which is seen in stacked folds which allow for greater efficiency. We can better understand this structure by looking at the Golgi's component parts in the sections below.

For more background information on how different cells function, take a look at our related guide to the definition and function of cell organelles.

We have already mentioned that Golgi apparatus have cisternae, but we need to explain what these and other parts of the Golgi apparatus are, as well as their functions. For this reason, we need to look at the different parts of the Golgi apparatus:

• Cisternae: they are groups of flattened membranous sacs which are important for the packaging and modification of proteins. They usually group from 4 to 8 vesicles or sacs. Each one is independent of each other, unconnected and bound by a membrane. A cell can have several of these stacked sacs, but it depends on the type and state of the cell. The cisternae are located on fibrous proteins. This collection of proteins is known as the Golgi matrix.
  
  
• Vesicles: vesicles transit within the different cisternae. Those that go in are called incoming vesicles, and those that are sent out are called outgoing vesicles. They are made from the membrane of the Golgi apparatus.
  
  
• Lumen: the region found at the center of the cisterns.
  
  
• Cis-face and trans-face: each lumen has two faces. The cis-face of the Golgi lumen is the side that receives vesicles and the trans-face is the side that sends them out. In the middle of both poles are the intermediate cisternae. Given the name of these faces, the movement of the vesicles has a direction from cis to trans. The cis-face is usually closer to the endoplasmic reticulum. Each face has a different polarity, thickness, and molecular composition.

In some ways, we can think of the Golgi apparatus as a kind of cellular distribution center. This is because it is responsible for packaging, labeling, storing and shipping the products that come from the endoplasmic reticulum, another organelle found inside the cell. Said products that it processes can be:

• Proteins
• Lipids

They are usually modified by eliminating sugar monomers, as well as substituting or modifying portions of these organic compounds. They are also done with the purpose that they can be used within cellular activities.

For example, carbohydrates can be added to proteins so they become glycoproteins. Glycolipids, proteoglycans and polysaccharides can also be formed. This process is called protein glycosylation and it has the function of taking them to other cellular structures or to allow them to be secreted. This secretion is very important in the glandular system which is partly responsible for the regulation of various organisms.

Packaging function

The process that occurs within the Golgi apparatus is structured the following order:

1. First it receives vesicles with macromolecules inside which come from the endoplasmic reticulum.
   
   
2. These vesicles attach to cisternae on the cis-face of the Golgi apparatus with each cisterna completing different stages of processing and packaging.
   
   
3. They then mature and are transported to the trans-face. When they reach this area they leave the Golgi apparatus and transport the products to other areas within the cell. In most cases, the plasma membrane is the final destination of the already processed vesicles.

There is also a movement towards the opposite direction in which the molecules are recycled to be reinserted towards the cis-face. The process is not static as during maturation the cisternae have been found to move.

Labeling function

In order to know where the molecules have to be sent, if they have already been processed or if they should be stored, there is a labeling system that works with molecular identification tags. This is where phosphate groups are added. These serve as shipping labels to orient the Golgi apparatus. After this labelling, the products are stored and sent to another organelle.

Function of manufacture and secretion

Another important function is the manufacture of macromolecules such as polysaccharides that are excreted to complete important functions. One type of polysaccharide that is created by the dictyosome is fruit pectin, which is widely used in various industries as a thickener. This organelle also assists in the formation of primary lysosomes, which are the organelles that complete cell digestion.

Having a secretory function, cells specialized for secretion have a large number of Golgi apparatus. It is found in all eukaryotic cells of the human body except certain red blood cells. This is because red blood cells in mammals lose organelles including nuclei and Golgi once they mature.

To learn more about the different types of cells of living organisms with our guide to the difference between eukaryotic and prokaryotic cells.

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