Ribosomes, Endoplasmic Reticulum, and Golgi Bodies

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MISC

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Biology

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Nov 24, 2024

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Section Objective: • Students will be able to describe how the functions of ribosomes, endoplasmic reticulum, and Golgi bodies are related. We know that DNA is stored in the nucleus of eukaryotic cells, that DNA contains the genetic information for all proteins, structural or enzymatic, and that the nucleus contains a mechanism for transporting matter into and out of the nucleus through the nuclear pore complexes. The finishing work necessary to convert a nucleic acid sequence into a poly - peptide and a polypeptide into a working protein takes place outside of the nucleus. Immediately outside the nucleus, peptides are transported into a system of membrane bound organelles, which complete protein synthesis, including any modifications or cova - lent attachments, and transport it to its final destination (e.g., activity or storage). Just outside the nucleus, and contiguous with it, is the Rough Endoplasmic Reticulum (RER). This structure is called rough because in micrographs, it appears as a membrane complex with very small dark spots on the ER membrane. We now know the dark areas to be structures called ribosomes . Ribosomes are very important structures to all known life forms. They are the cellular mechanism by which a nucleic acid polymer is converted through a process called translation into a polypeptide polymer. Prokaryotes also have ribosomes. However, they are not bound to a membrane. The number of ribosomes present in a cell can indicate how busy the cell is with protein production. Liver cells, for example, have tremendous numbers of ribosomes. The ER forms a continuous network of membranes throughout the cell, from the nucleus to the cell membrane. Ribosomes are located near the nucleus; as you gain distance from the nucleus, ribosomes are no longer seen on the ER. This portion is known as the smooth ER (SER). The ribosomes on the RER synthesize the proteins and insert water soluble poly - peptides into the lumen of the RER for further processing and proper folding. Of course, the RER synthesizes the proteins that are needed by the RER as well. Lipid soluble pro - teins, such as transmembrane proteins, are inserted into the membrane, and can be trans - ported to the surface.
The SER is contiguous with the RER. The SER is responsible for lipid biosynthesis and has some other specialized functions. The SER is involved in building carbohydrates and detoxification of toxins. Consequently, the SER is very large in liver cells, the organ where detoxification takes place and is very large in the brain, which is very dependent upon car - bohydrate metabolism. SER is also involved in calcium metabolism and because calcium release is related is muscle activity, muscle cells also have extensive networks of SER. Golgi bodies are membrane bound organelles closely associated with the ER. They appear as stacks of sacks or compressed balloons in micrographs. One side of the Golgi complex, cis -Golgi, faces the ER. Vesicles from the ER containing synthesized proteins break off and fuse with the cis-Golgi . The protein then passes through the stack of the Golgi com - plex, and further modifications of carbohydrate and lipid are added. The Golgi bodies add many signals that indicate where in the cell the protein will ultimately end up. On the op - posite side of the Golgi, the trans-Golgi , the product is either released into the cell or is again incased in a vesicle. The vesicle may fuse with the cell membrane for release outside the cell. Some vesicles remain in the cell, forming lysosomes , which will be discussed in the next lesson. The ribosomes, the endoplasmic reticulum and the Golgi bodies together make up the protein manufacturing apparatus of the cell. From the ribosome, where the translation into polypeptide takes place, to the Golgi bodies, where they are finalized, fully modified and folded protein is directed to its final destination, this is assembly line of the cell for proteins. Concept Reinforcement 1. Describe the role of ribosomes in protein synthesis. 2. Describe the structural components of the ER and the Golgi. 3. How do the Golgi bodies contribute to protein synthesis?
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