The Endoplasmic Reticulum and Ribosomes
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Endoplasmic Reticulum:
The endoplasmic reticulum is an interconnecting network consisting of vesicles, sacs, and tubules (called cisternae) all working together to serve specialized functions in the cell. These functions can include protein synthesis, sequestration of calcium, the production of steroids and glycogen, the storage of glycogen, and the insertion of membrane proteins. The endoplasmic reticulum (ER) is such an extensive organelle, it accounts for more than half of the membrane of eukaryotic cells, and the amount of RER and SER does vary from cell to cell (depending on the cell's functions). Endoplasmic reticulum is either smooth or rough, both kinds of which, connect to each other structurally.



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Overall, the endoplasmic reticulum acts as a packaging system for the cell. However, it does interact with the Golgi apparatus, ribosomes, RNA, mRNA, and tRNA. Within its name, endoplasmic means "within" the cytoplasm and reticulum means is derived from a word meaning "network".








Smooth Endoplasmic Reticulum: ser.gif

The Smooth Endoplasmic Reticulum, otherwise known as SER, is the portion of the ER that lacks attached ribosomes and is typically tubular rather than disc-like. SER functions act as a storage organelle and helps in the synthesis of lipids. The smooth endoplasmic reticulum also assists in the process of carbohydrate metabolism and detoxification of drugs and poisons (by adding hydroxyl groups to drugs). Its acts as a storage organelle, and is important in the creation and storage of steriods, and in addition, it stores ions that the cell may need for other purposes at different times. Enzymes of the smooth ER produce oils, phospholipids, and steroids, (ex: sex hormones). Cells specialized for the synthesis or metabolism of lipids (like cells that produce steroid hormones) have large amounts of smooth endoplasmic riticulum.



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Rough Endoplasmic Reticulum:
The Rough Endoplasmic Reticulum (RER) is the portion of the ER studded with ribosomes giving it a rough look (hence its name) and is typically arranged as interconnecting stacks of disc-like sacs. In most cells, the RER is the predominant form of ER present within the cell. It is important to note that although the RER is deemed such, because of its rough surface (due to the ribosomes embedded on its surface), the surface of the RER is not always rough, because ribosomes only bind to the endoplasmic reticulum once it begins to synthesize a protein destined for sorting.

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The rER goes through something called protein synthesis. First, the cell receives a message to make a particular protein, and it is the rER's job to respond to that need. A portion of a DNA molecule unwinds, revealing the gene responsible for the particular protein the cell is asking for. Nucleotides, along with enzymes, move along the single strand of DNA to form a molecule of mRNA (messenger RNA). Mutiple copies of the mRNA are made in the process depending on what the cell calls for in terms of the need of the type of protein. It then leaves the nucleus and enters the cytoplasm through the nuclear pores. From here, the mRNA binds with the ribosome so the ribosome can decode the mRNA. The message is read in portions called a triple code or a codon (three nucleotide bases). Each codon has a specific amino acid that corresponds with its arragement, and the matching amino acid will then be activated by the enzyme. This is where the tRNA comes in, which has two different kinds of ends. One end contains the codon and the other end is the anticodon which is what matches the tripe code. This tRNA molecule is then attached to the activated amino acid that is being processed and read by the ribosome. Anticodon bases pair with codons in order to bring the specific amino acid to the correct place. This process is repeated for a second activated amino acid in the ribosome. The first tRNA then releases its amino acid and connects itself to the second tRNA. The two connected tRNAs now form a peptide bond, using ATP as its source for energy. The previous tRNA continues to attach itself to the next tRNA connecting through peptide bonds. Once the entire chain of mRNA is processed, this makes up a polypeptide chain. After the whole chain is finished being read, the polypeptide chain will fold itself into its final confirmation, completing the process and leaves the site.

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Ribosome: external image ribosomeAssembly.jpg
Since cells need to produce protein, possibly serving as enzymes or support for the cell, the presence of ribosomes in a cell is of great importance . A ribosome is a cell organelle constructed in the nucleolus, functioning as the site of protein synthesis in the cytoplasm; consists of rRNA and protein molecules, which make up two subunits (Campbell 111). Ribosomes are the sites where amino acids are assempled into proteins; they are either protein builders or protein synthesizers for the cell. Ribosomes are a necessity for protein synthesis, thus, cells with a high rate of protein synthesis, have a higher ribosome count, meaning the more protein a cell produces, the more ribosomes it holds.

There are two kinds of ribosomes, both are identical structurally, however what becomes of these ribosomes differs. Free ribosomes are suspended in a structure of cell's called the cytosol, while bound ribosomes are attached to the outside of the RER. The proteins that bound ribosomes produce are assigned to various locations including the cell membrane, specific organelles, and sometimes are even exported outside the cell. The jobs of the free roaming ribosomes as opposed to the bounded ribosomes, differ. The ribosomes in the cytoplasm allow its protein to roam around freely as itself, while the ribosomes on the rER transport their protein into the ER.

subunits.gifEvery ribosome consists of two subunits linking together to form one single ribosome. Scientists gave the names 60-S (large) and 40-S (small). When the two are put together, it makes a whole ribosome unit. This 60 to 40 ratio works for the eukaryotic cell. The prokaryotic cell however has a 50-S and 30-S subunit ratio. The ribosomes' subunits are synthesized by the nucleolus, and join together when the ribosome attaches to mRNA (messenger RNA) to produce a protein the cytoplasm.

There are thousands of ribosomes in each cell, and makes up about 25% of the cell's dry weight. Its diameter is about 20 nanometers, only capable of being seen with a microscope.



A video to understand the process of protein synthesis:


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Wicked Cool Links:


Protein Synthesis
Diagram of Protein During Protein Synthesis

An Interactive Cell Model
Cell Anatomy & Organelle Functions


external image 365px-Nucleus_ER_golgi.svg.png
1. nucleus, 2. nuclear pore, 3. rough endoplasmic reticulum (RER), 4. smooth endoplasmic reticulum (SER), 5. ribosomes, 6. proteins, 7. transport vesicle, 8. golgi apparatus, 10. cis face of the golgi apparatus, 11. cisternae of the golgi apparatus

Citations:

Campbell, Neil A., Jane B. Reece, and Lawrence G. Mitchell. Biology. Fifth ed. Menlo Park: Wesley Longman, Inc., 1999. Print.

Campbell, Neil, and Jame Reece. Biology. Sixth ed. Menlo Park: Wesley Longman, Inc., 2004. Print.

"Cell Structure." Rader's Biology4kid.com. Web. 27 Oct 2009. <www.biology4kids.com/files/cell_er.html>.

Curtis, Helena, and N.Sue Barnes. Invitation to Bology. Fifth. New York, New York: Worth, 1994. Print.


Davidson, Michael. "The Endoplasmic Reticulum." Molecular Expressions. 13 dec 2004. Web. 28 Oct 2009. Florida Sate University. <http://www.microscopy.fsu.edu/cells/endoplasmicreticulum/endoplasmicreticulum.html>.


"The Endoplasmic Reticulum." 22 Sept 2007. Web. 28 Oct 2009. <http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/ER.html>.