Structure, Function, and Protein Transport Mechanisms

Golgi Apparatus (AG) The Golgi apparatus is a cell organelle in the form of flat membrane sacs called cisternae which at opposite ends have different thicknesses and compositions. The membrane of each cistern in one stack will separate its internal space from the cytosol. The two poles of the AG stack consist of cis, intermediate (cisternae), and trans which consist of vesicles containing various secretory protein materials. The cis face is located close to the ER, where transport vesicles will move material from the ER to the AG. The transport vesicles with the secretory proteins that need to be perfected will combine to form a new structure called the Golgi apparatus (AG). Meanwhile, the trans face will produce vesicles containing protein content resulting from maturation in the AG which are ready to be transported out of the cell via an exocytosis mechanism, so that the transport vesicles will eventually fuse with the plasma membrane. A few minutes after the protein is synthesized in the rough ER, the results of the ER secretion will be stored in transport vesicles to be taken to other membrane-bound organelles, namely the Golgi apparatus. The ER has a related mechanism of action with the Golgi apparatus, but the membrane structure of the two is different which also shows differences in function, for example phospholipids are synthesized in the ER and sphingolipids are synthesized in the Golgi apparatus. Based on this research, it provides evidence that the proportion of spingomyelin in the total percentage of membrane lipid phosphorus appears to be six times greater in the Golgi than in the ER. Another case shows that membrane translocation to the cell membrane will be very selective with certain membranes only. Secretory proteins to be exocytosed are usually processed first in the ER and will be matured in the AG. The mechanism of the material secretion system is through exocytosis which is able to structure the plasma membrane. The products of ER are modified while in the cis and trans poles of AG, where membrane proteins and phospholipids can change their composition. For example, many enzymes are modified in AG in the oligosaccharide portion of glycoproteins. When produced by RE, proteins that tend to be identical in AG have some of their sugar monomers replaced with other molecules so that they produce various kinds of oligosaccharides with specific structures and functions. Differences in lipid composition also indicate special functions of membranes. For example: the plasma membrane of absorption epithelial cells in the intestine will be in two areas: the upper surface of the lumen, the basolateral surface which is the site of interaction with other epithelial cells and with extracellular structures. As a function of cell polarization, the ratio of sphingolipids to phosphoglycerides with cholesterol on the basolateral membrane is 0.5:1.5: 1. Conditions that usually appear on the plasma membrane as non-polarized cell types caused by being under stress.