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What Are Ribosomes?
Ribosomes that are found in eukaryotic cells are tiny, round organelles that generate proteins from mRNA. Ribosomes are composed of protein and ribosomal RNA. The structure of the ribosome is composed of two subunits. One of the subunits is larger than the other. There are four strands of rRNA that hold the structure together.
Ribosomes are produced in the nucleolus of the cell. The proteins that make up the ribosome are brought into the nucleolus and assembled into subunits. Afterward, the subunits are transported to the cytoplasm of the cell. The subunits of a ribosome join when it beings to build proteins.
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Location of Free and Bound Ribosomes
The place where the ribosome is located is one of the differences between free and bound ribosomes. Free ribosomes are located in the cytoplasm of the cell. They are not attached to any structure, but they may group together with other ribosomes to form polysomes. In the cytoplasm, ribosomes are free floating. They can move all around the cell.
Bound ribosomes are located on the surface of the endoplasmic reticulum. The endoplasmic reticulum that contains ribosomes is described as the rough endoplasmic reticulum because of the bumpy surface. Bound ribosomes can not move to other areas of the cell. They are attached to the cytosolic side of the endoplasmic reticulum.
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Types of Proteins Synthesized by Free and Bound Ribosomes
The types of proteins produced by the ribosomes is another one of the differences between free and bound ribosomes. Free ribosomes produce proteins that are used by the cell. This includes proteins that are used for the metabolism of food. The free ribosomes produce enzymes involved in the metabolism of glucose.
Bound ribosomes produce proteins that are transported out of the cell. This includes proteins that are required for a specific function, such as digestive enzymes. Bound ribosomes also produce polypeptide hormones. A few proteins produced by bound ribosomes are used in the cell membrane and on the outer surface of the cell. Surface receptors and cell signaling proteins are produced by bound ribosomes.
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Both free and bound ribosomes produce proteins by translating the genetic code of mRNA into a string of amino acids. Ribosomes require tRNA to produce proteins. tRNA brings the amino acids that correspond with the genetic code in mRNA to the ribosome. The tRNA molecule binds to the ribosome at three places. There is a site for the portion of the tRNA that binds to the next amino acid and a site for the growing amino acid chain. The third area is the exit site which releases the tRNA after the protein is completed. Bound ribosomes send most proteins into the endoplasmic reticulum, while most of the proteins produced by the free ribosomes head toward the Golgi apparatus. In both cases, the proteins are eventually modified and transported to their destination.
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1. Davidson, Michael W. "Ribosomes." Molecular Expressions Cell Biology Florida State University. http://micro.magnet.fsu.edu/cells/ribosomes/ribosomes.html
2. Kimball, John W. "Ribosomes." Kimball's Biology Pages. http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/R/Ribosomes.html