Golgi Apparatus Functions, Definition and Structure

Definition

Golgi apparatus is a cellular organelle in eukaryotic organisms made up of a series of flattened stacked pouches called cisternae. This organelle is also called a Golgi body or Golgi complex. It is an organelle in the cell that is made up of different types of membranes. Some of the membranes are tubules, and some are vesicles. Golgi bodies have a major role in transporting, modifying, and packaging lipids and proteins into vesicles for delivery to specific areas in the cell.

A Golgi apparatus is found in the cytoplasm of the cell next to the endoplasmic reticulum. It is also near the nucleus of the cell. A Golgi body can be called the manufacturing and shipping center of the cell. Many cell types may contain one or many Golgi apparatus. However, plant cells may contain hundreds of Golgi bodies. Golgi complex does not produce proteins but they modify the proteins by the addition of carbohydrates to the proteins, forming glycoproteins. Thus, when the glycoproteins are modified, they are transported by Golgi vesicles which bud off from one end of the Golgi bodies.

The Golgi apparatus is basically the site for the modification, completion, and exportation of secretory proteins and glycoproteins. This organelle is described as having a structure made up of 5-8 flattened disk-shaped cisternae arranged in a stack. Several proteins at some point in their maturation pass through the Golgi complex. Proteins like secretory proteins, glycoproteins, cell membrane proteins, lysosomal proteins, and some glycolipids.

The organelle is involved in the packaging of protein molecules before sending them to their various destination in the cell. Hence, the Golgi complex process and packages proteins and lipids that are synthesized by the cell. Moreso, in plant cells, most of the cell wall material passes through the Golgi apparatus.

The primary importance of the Golgi apparatus in the cell is its ability to deliver vesicles or package various cell macromolecules products to various locations within the cell. Also, this organelle has an important role in tagging vesicles with proteins and sugar molecules. This serves as identifiers for the vesicles so they can be delivered to the appropriate target destination.

In the endoplasmic reticulum, proteins and cellular products are manufactured. There are a number of ribosomes in the rough endoplasmic reticulum that assemble these proteins from instructions contained in messenger RNA (mRNA). These protein products are modified and folded within the rest of the endoplasmic reticulum. However, as these proteins reach the Golgi complex, more modifications are made to them. Then, within vesicles, the products are packages and labeled by other molecules and proteins. The vesicles are then released. Depending on their tags or label, the products are delivered to the target location within the cell.

Where is the Golgi apparatus located?

The Golgi apparatus is located and seen in between the cell membrane and the endoplasmic reticulum. It is near the nucleus and the endoplasmic reticulum. Usually, this organelle seems to be an extension of the endoplasmic reticulum which is slightly smoother and smaller in appearance. This is why the Golgi apparatus can be easily mistaken for the smooth endoplasmic reticulum. Well, even though they look similar, the Golgi apparatus happens to be an independent organelle with different functions.

The Golgi is called a perinuclear body and is near the endoplasmic reticulum. As a result, when proteins come out of the endoplasmic reticulum, they go into the Golgi apparatus for further modification and processing. For instance, carbohydrates are put on some of the proteins, thus forming glycoproteins. These glycoproteins then move out of the Golgi apparatus to the rest of the cell. They do the same inside other vesicles.

The vesicles are made from the Golgi network. Actually, one of the functions of the Golgi apparatus is to generate new vesicles from the existing membrane of the Golgi apparatus. It then put the glycoproteins and other substances that are made in the Golgi network into those vesicles. Then the vesicles filled with the Golgi products move to the rest of the cell. They usually move through the cell to the plasma membrane. The plasma membrane, however, is their final destination.

What does the Golgi apparatus do?

The Golgi apparatus does many tasks in the cell although all tasks are associated with the movement of molecules from the endoplasmic reticulum to their various end destination. Also along the way, it modifies certain products.

Golgi apparatus has multiple sacs that serve as chambers for different chemical reactions. As products move through the Golgi body from the endoplasmic reticulum, they are transferred continuously into new environments. Hence, different reactions take place. In this process, a product can be modified or multiple products can come together to form large macromolecules. So, the many sacs and folds that the Golgi apparatus possess allow many reactions to occur at the same time. Hence, increasing the speed at which an organism can produce products.

As the endoplasmic reticulum makes most of the products and bases used, the function of the Golgi apparatus is to take charge of the final presentation and assembly of the products. To obtain certain end products, the environment must be slightly different from the surroundings in the endoplasmic reticulum. However, the many sacs of the Golgi body function as they provide several different areas for reactions to take place in the most favorable conditions.

Furthermore, when secretory proteins move through the Golgi, a lot of chemical modifications take place. Most importantly is the modification of carbohydrate groups. Several secretory proteins are glycosylated in the endoplasmic reticulum. Specific enzymes in the Golgi apparatus modify the oligosaccharide chains of the glycoproteins. This is done by extracting certain mannose residues and adding other sugars like sialic acid and galactose. These enzymes are collectively called glycosidases and glycosyltransferases.

Moreso, some of these secretory proteins will not be able to be transported if their carbohydrate groups are modified incorrectly or not allowed to form. However, the carbohydrate groups in some cases are needed for the stability or activity of the protein. Plus, they are also necessary for targeting the molecule for a specific destination.

Also, there are proteases within the Golgi body or secretory vesicles. These proteases split several secretory proteins at specific amino acid positions. This usually results in the activation of the secretory protein. For instance, By removing a series of amino acids, inactive proinsulin is converted to active insulin.

In the structure of the Golgi apparatus, the part of the organelle that is nearest to the endoplasmic reticulum is the cis face. Then the trans face is the side of the organelle that is furthest from the nucleus. Trans face, however, secretes vesicles to various parts of the cell. Moreso, there are several lumens and cisternae through which products flow. Hence, appearing as a series of flattened sacs stack on each other, looking like the endoplasmic reticulum.

Who discovered Golgi apparatus?

An Italian cytologist named Camillo Golgi observed and discovered the Golgi apparatus in 1897. In this cytologist’s early studies of the nervous tissue, he had established a staining technique. He called this technique reazione nera which means black reaction. Today, this technique is known as the Golgi stain. Nervous tissue is fixed with potassium dichromate in this staining technique. Then suffused with silver nitrate.

Camillo Golgi identified an internal reticular apparatus as he examined the neurons that he stained using his black reaction. This structure, however, became known as the Golgi apparatus. Though as at that time, some scientists questioned whether the structure was real. They attributed the structure to be free-floating particles of Golgi’s metal stain. However, in the 1950s, when the use of electron microscope came into vogue, the existence of the Golgi apparatus in the cell was confirmed.

Golgi apparatus (Golgi bodies) Diagram

Golgi apparatus diagram

Golgi Apparatus structure

The Golgi apparatus structure is polarized. It consists of a cis face and a trans face. The cis face or forming face is near the transitional region of the rough endoplasmic reticulum. Whereas, the trans face also called maturing face is near the cell membrane. These cis and trans faces are distinct biochemically and the enzymatic content of each segment is marked differently. However, the membranes of the cis face are thinner generally than the others.

When the Golgi apparatus is viewed under the electron microscope, it appears to be made up of stacks of flattened structures. These flattened structures contain many vesicles which contain secretory granules. Morphologically, the Golgi apparatus in plants cell is very similar to the one in animal cells, although it is very pleomorphic. For instance, in some cell types, the Golgi complex appears compact and limited. Whereas, in other cell types it is spread out and reticular (net-like). However, it is typical for the Golgi apparatus to appear like a complex array of interconnecting vesicles, tubules, and cisternae.

  • Vesicles

Golgi vesicles have a major function of sorting many of the proteins and membrane constituents of the cell. It sorts and directs them to their appropriate destinations. Hence, these vesicles are usually referred to as the traffic police of the cell. In order to carry out this task, the vesicles contain different sets of enzymes in different types of vesicles. These enzymes react with and modify secretory proteins that pass through the Golgi lumen (or membrane proteins) and glycoproteins. However, there are three types of vesicles in the Golgi apparatus. They are:

  • Transitional vesicle
  • Secretory vesicles
  • Clathrin-coated vesicles
  1. Transitional vesicles: These vesicles are small membrane limited. They are said to form blebs from the transitional endoplasmic reticulum. As they migrate and converge to the cis face of the Golgi apparatus. It is there that they coalesce to form new cisternae.
  2. Secretory vesicles: These vesicles are varied-sized membrane limited. They discharge from margins of cisternae of Golgi apparatus. Usually, they are located between the plasma membrane and the trans face of the Golgi apparatus.
  3. Clathrin-coated vesicles: These vesicles are spherical protuberances. Clathrin-coated vesicles play a role in the intracellular traffic of secretory products and of membranes. They have a rough surface and are about 50um in diameter. These vesicles are seen at the periphery of the Golgi body, mostly at the ends of single tubules.
  • Tubules

A complex arrangement of associated vesicles and anastomosing tubules surrounds the dictyosome and diverges from it. Therefore, the peripheral area of the dictyosome is lace-like in structure.

  • Cisternae

The cisterna is the simplest element of the Golgi apparatus. They are central flattened plate-like closed compartments. Cisternae are held in stacks with one above the other. However, they are separated in each stack by a space of 20-30 nm which may contain rod-like elements or fibers.

Each stack thereby forms a dictyosome that may contain 5-6 Golgi cisternae in animal cells. For plant cells, it may contain 20 or more cisternae. A smooth unit membrane about 7.5 nm thick bound each cisterna. The entire dictyosome of the Golgi apparatus takes on a bow-like appearance as the margin of each cisterna is gently curved. Moreso, the cis- face is the cisternae at the convex end of the dictyosome. Whereas, the cisternae at the concave end of the dictyosome comprise the trans-face.

Golgi bodies have 3 primary compartments. They are known generally as cis, medial and trans. The cis is the cisternae that are nearest to the endoplasmic reticulum. Then, the Medial is the central layer of cisternae. The trans is the cisternae that are farthest from the endoplasmic reticulum. Trans-Golgi network and Cis Golgi network comprise the outermost cisternae at the cis and trans faces. These 2 networks are responsible for the crucial task of sorting proteins and lipids that are received at the cis face or released at the trans face by the Golgi body.

In conclusion, the Golgi apparatus structure comprises approximately 4-8 cisternae. In some single-celled organisms, it may comprise as many as 60 cisternae. These cisternae, however, are held together by matrix proteins. The structure of the Golgi apparatus is actually supported by cytoplasmic microtubules.

Golgi apparatus functions

  1. Golgi vesicles have a major function of sorting many of the proteins and membrane constituents of the cell. It sorts and directs them to their appropriate destinations. Hence, these vesicles are usually referred to as traffic police of the cell.
  2. The Golgi apparatus function as the assembly factory of the cell as raw materials is sent to the Golgi body before being passed out from the cell.
  3. This organelle is involved in the formation of certain cellular organelles in the cell. Organelles like the lysosomes, plasma membrane, acrosome of spermatozoa, and cortical granules of a variety of oocytes.
  4. Golgi apparatus functions in the transportation of lipid molecules around the cell.
  5. Another function of the Golgi apparatus is to generate new vesicles from the existing membrane of the Golgi apparatus and put glycoproteins and other substances that are made in the Golgi network into these vesicles.
  6. In animal cells, the Golgi complex also has a role to play in the production of proteoglycans. Proteoglycans are molecules in the extracellular matrix of animal cells.
  7. Golgi apparatus serves as a major site for the synthesis of carbohydrates.
  8. The Golgi apparatus function in the sulfation process of certain molecules. Also, the process of phosphorylation of molecules by the Golgi complex needs the import of ATP into the lumen of the Golgi complex.
  9. In plants, the function of the Golgi apparatus is the secretion of materials of primary and secondary cell walls. The formation and export of lipids, glycoproteins, monomers, and pectins, for hemicellulose, cellulose, lignin, etc is an example.
  10. Together with the endoplasmic reticulum, the Golgi apparatus works to produce new cell membranes. Also, they recycle the cell membranes of vesicles by merging two membranes when vesicles are absorbed.
  11. In animals, the Golgi apparatus is responsible for the packaging and exocytosis of certain materials.
  12. In the cis and medial cisternae of the Golgi apparatus, the removal of mannose moieties occurs.
  13. The Cis and Trans Golgi network sort the proteins and lipids that are received by the Golgi apparatus at the cis face and released by the trans face.
  14. In the trans cisternae of the Golgi apparatus, the addition of galactose occurs.

Golgi bodies functions in animal cell

The Golgi apparatus has several functions in the animal cell although all its functions are associated with the movement of molecules from the endoplasmic reticulum to their various end destination. It has a major function in the animal cell where it sorts many of the proteins and membrane constituents of the cell. This organelle sorts and directs them to their appropriate destinations. Hence, the Golgi apparatus serves as the assembly factory of the cell as raw materials are sent to the Golgi body before being passed out from the cell.

This organelle is involved in the formation of certain cellular organelles in the animal cell. Organelles like the lysosomes, plasma membrane, acrosome of spermatozoa, and cortical granules of a variety of oocytes. Golgi bodies transport lipid molecules around the animal cell. It also has a role to play in the production of proteoglycans. These proteoglycans are molecules in the extracellular matrix of animal cells.

Golgi apparatus has multiple sacs that serve as chambers for different chemical reactions. As products move through the Golgi body from the endoplasmic reticulum, they are transferred continuously into new environments. Hence, different reactions take place. In this process, a product can be modified or multiple products can come together to form large macromolecules. So, the many sacs and folds that the Golgi apparatus possess allow many reactions to occur at the same time. Hence, increasing the speed at which an organism can produce products.

As the endoplasmic reticulum makes most of the products and bases used in the animal cell, the function of the Golgi apparatus is to take charge of the final presentation and assembly of the products. To obtain certain end products, the environment must be slightly different from the surroundings in the endoplasmic reticulum. However, the many sacs of the Golgi body function as they provide several different areas for reactions to take place in the most favorable conditions.

Proteins and lipids are received at the cis face of the Golgi apparatus. They arrive in clusters of fused vesicles. These fused vesicles move along microtubules through the vesicular-tubular cluster. The vesicular tubular cluster lies between the Golgi apparatus and the endoplasmic reticulum. The products are delivered into the lumen of the cis face cistern when a vesicle cluster combines together with the cis membrane. However, as proteins and lipids move from the cis face to the trans face, they are modified. The proteins and lipids are modified into functional molecules and tagged for delivery to specific destinations. Their final destination could be intracellular or extracellular.

Some modifications done on these products may involve cleavage of oligosaccharide side chains followed by attaching different sugar moieties in place of the side chain. Others may involve adding fatty acids or phosphate groups (phosphorylation) or removing monosaccharides. However, the various enzyme-driven modification reactions are particular to the compartments of the Golgi apparatus. For instance, the removal of mannose moieties occurs mainly in the cis and medial cisternae. While the addition of galactose or sulfate occurs mainly in the trans cisternae.

Modified proteins and lipids are sorted in the trans-Golgi network at the final process of transportation within the Golgi complex. They are then packaged into vesicles at the trans face. These vesicles thereby deliver the molecules to their target locations such as the cell membrane or lysosomes. Some molecules as well as some soluble proteins and secretory proteins are carried in vesicles to the cell membrane for exocytosis. Exocytosis refers to release into the extracellular environment. However, the exocytosis of secretory proteins may be regulated as a ligand must bind to a receptor to set off vesicle fusion and protein secretion.

Golgi body function in plant cell

The Golgi apparatus can also be seen in plant cells and they may contain up to hundreds of Golgi bodies. The Golgi apparatus within the plant cell provides an additional function of synthesizing the main polysaccharide molecules that aid in the formation of the cell wall. In order to achieve this, plants usually possess more Golgi apparatus than animal cells. Moreso, plants do not possess lysosomes. As a result, the central vacuole replaces it and serves as a large lysosome and an organelle for the storage of water. Thereby, several vesicles from the Golgi apparatus of the plants move to the vacuole and fuse their content with the large vacuole.

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