The Golgi apparatus is an organelle found in most eukaryotic cells. According to Keaton et al. (1993) he noted that the Golgi apparatus is prominent in cells involved in cell secretions of various chemical products. This is due to the level of secretory activity of these cells changes corresponding changes occur in their morphology of the organelle. Its main function is to process and package macromolecules such as proteins and lipids after their synthesis. According to Solomon et al. (2008) the Golgi apparatus which is also known as the Golgi complex was first described in 1898 by the Italian microscopist Camillo Golgi. Mr. Camillo found a way to specifically stain this organelle. However in further study Solomon et al.(2008,pg 91) stated that “many investigators thought the Golgi complex was an artifact until cells were later studied with the electron microscope in the 1950s.”
The Golgi apparatus is usually located near the cell nucleus, and in animal cells it is often close to the centrosome, a small structure near the cell centre (Albert et al. 2004).This organelle consists of a collection of flattened membrane-enclosed sacs known as cisternae which are piled up like a stack of plates. Solomon et al. (2008) notes that some cisternae may be distended because they are filled with cell products. Each of the flattened sacs has an internal lumen. The Golgi complex contains a number of separate compartments, as well as some that are interconnected. The stack of Golgi has three different regions which are the an entry- the cis, medial and exit-the trans. Albert et al.(2004) established that the cis face is adjacent to the endoplasmic reticulum and the trans points towards the plasma membrane.
Steven (1998) explained that the soluble and properly folded proteins enter the cis Golgi network via transport vesicles. They are transported from the endoplasmic reticulum to the Golgi complex. If a protein has an endoplasmic reticulum retention signal reaches the cis Golgi network Steven (1998) noted that it binds to a specific receptor and is repackaged into vesicles that return it to the endoplasmic reticulum. Freeman (2003) further described that the newly synthesized proteins and lipids have been modified within the cis-cisternae 50 nm diameter coated vesicles bud of the dilated ends of this compartment and fuse with the medial cisternae. Within each region there are different enzymes that transform proteins to be secreted and membrane proteins differently. Lodish et al. (2003) discussed that this process depends on their structures and destination. After the enzymes of the medial cisternae acts on the proteins and lipids, the process continues in an assembly line fashion by coated vesicles budding from the terminal dilations and fusing with the Trans cisternae. From the Trans (exit) cisternae the proteins and lipids will exit the Golgi and then are passed into the Trans Golgi network. Lodish et al. (2003) distinguished that after this process the macromolecules are sorted into different transport vesicles destined for the lyosomes, plasma membrane or secretion. These Golgi compartments have their own specific functions.
Secretory proteins are released from the cell by exocytosis. Albert et al. (2004) clarified that in all eukaryotic cells there is a steady stream of vesicles that grow from the Trans Golgi network and combine with the plasma membrane. Goodman (1998) suggested that this exocytosis pathway performs continuously and provides newly made lipids and proteins to plasma membrane. Proteins, fats and polysaccharides are carried from the Golgi apparatus to their ultimate destination via the secretory pathway, Cooper et al.( 2009).This entails the sorting of proteins into diverse kinds of carrying vesicles which sprout from the trans Golgi network and distribute their contents to the appropriate cellular locations.Moroever Cooper et al.(2009) noted that proteins that purpose within the Golgi apparatus must be maintained within that organelle rather than being conveyed along the secretory pathway. Transportation from the Golgi apparatus to the cell surface can occur by at least three routes. The simplest is the direct transport from the Trans Golgi network to the plasma membrane. Henceforth this leads to the arrangement of innovative proteins and lipids into the plasma membrane and the constant secretion of proteins from the cell. In addition to that Cooper et al. (2009) described that proteins can also be transported from the Golgi to the plasma membrane via an intermediate of recycling endosomes. Moreover these pathways which lead to continual unregulated protein secretion some cells obtain a distinct synchronized secretory pathway in which particular proteins are secreted in response to body signals. For instance regulated secretion entails the liberation of hormone from endocrine cells, the discharge of neurotransmitters from neurons and lastly the liberation of digestive enzymes from the pancreatic cells.
Furthermore proteins are arranged into the synchronized secretory track in the Trans Golgi network where they are packaged into specialized secretory vesicles. This sorting appears to be mediated by cargo receptors that recognize signal patches shared by various proteins that go through the pathway, Cooper et al. (2009). Then the receptor cargo complexes selectively aggregate in cisternae of the Trans Golgi network and are then discharged by budding as immature secretory vesicles. Goodman(1998) noted that these vesicles which are bigger than transport vesicles further process their protein contents and often combine with each other to form mature secretory vesicles. Therefore the mature secretory vesicle then stores their contents until specific signals express their synthesis with the plasma membrane. For instance, the digestive enzymes manufactured by pancreatic cells are stored in mature secretory vesicles in anticipation of the existence of food in the stomach and( ileum) small intestine which activate their secretion. This method is known as vesicular transport.
Last but not least it can be concluded that these following processes are involved in the Golgi apparatus. They are known as cisternal maturation and vesicular transport. First of all the polypeptides are synthesized on ribosomes.Keaton et al.(1993) described that the protein is then assembled and carbohydrate component is added in the lumen of the rough endoplasmic reticulum. Moreover transport vesicles move the glycoprotein to Golgi Cist face. The Golgi apparatus receives proteins from endoplasmic reticulum and starts to chemically modifying them. In Trans face glycoprotein are packaged in transport vesicles.Glycoproteins are then transported to plasma membrane and finally released from cell.
Last accessed Sunday 01.11.09
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