2010-11+Smooth-Rough+Endoplasmic+Reitculum,+Ribosome

=__**Smooth and Rough Endoplasmic Reticulum, Ribosomes**__= The cell is an endlessly complex organism that requires many processes to stay alive. Some of the arguably most important processes are performed by the Endoplasmic reticula, and ribosomes. The ER, both smooth and rough play a major role in the synthesis of lipids and steroids, not only that, but the Rough Endoplasmic Reticulum also participates in the packaging and transportation of macromolecular structures. The ribosome, while similar to the ER in that it synthesizes macromolecules operates very differently. It is completely dependent on mRNA and tRNA for the process of transcription to occur. The ribosome is the primary medium of synthesis for proteins.

Endoplasmic Reticulum
The endoplasmic reticulum is a relatively large organelle found in eukaryotic cells. The ER forms a network that includes tubules and vesicles, and is primarily composed of [|cisternae], and supported by the cytoskeleton. The smooth endoplasmic reticulum synthesizes a variety of components of the cell, while the rough endoplasmic reticulum solely synthesizes proteins. If the demands of a cell change, the cell can quickly switch between rough and smooth ER. The inside of the cisternae combined all form the lumen. The lumen is therefore closed from the rest of the cell, although the membrane of the ER will let certain things pass.

Rough
The RER is covered by ribosomes on its surface. This makes it appear "rough", giving it its name. The RER is the site of protein synthesis for proteins that are leaving the cell. The ribosomes attached to the RER are not part of the stable structure of the organelle. Ribosomes wait in the cytosol of a cell, and attach to the RER when a [|system recognition particle] detects a certain amino acid. The ribosomes may be repeatedly bound to the membrane and released back into the cytosol.

Proteins are synthesized in the RER through the process of [|translation](see ribosomes) in the ribosomes. After the protein has been synthesized, the RER works with the Golgi Apparatus through vesicles in order to direct the newly made proteins to their target location. Some proteins do not leave the ER, and remain to help carry out essential functions. These are known as residential proteins.



Smooth
The SER is named for its lack of ribosomes that give the rough appearance. The SER has multiple functions. The SER synthesizes [|lipids] and steroids that are used in the cell. The SER serves as a location to detoxify harmful drugs and substances in the cell. The SER is very involved in the creation of building blocks for the metabolism of carbohydrates and steroids. SER is much less extensive in the cell than RER, although in cells that require a great amount of lipid synthesis (such as [|pancreatic] ), or detoxification (liver cells), the amount of SER will exceed that of RER.

Sarcoplasmic Reticulum
The Sarcoplasmic Reticulum is a

special type of SER found in muscles. The SR pumps calcium ions instead of synthesizing molecules. The pumping of calcium ions is crucial in the contracting aspect of muscle.

Ribosomes
Ribosomes have a highly specialized role in the cell, and consequentially are one the most vital structures to the well-being of the cell. This specialized role is the creation of [|proteins]. Ribosomes recieve the code to create proteins from messenger RNA. The mRNA recieves its code from the nucleus of a cell, and then goes to the ribosome. It then translates its code to transfer RNA. The ribosome then reads the tRNA, and builds amino acids in a [|polypeptide] chain accordingly to create proteins.

Ribosomes may either be located on the Rough Endoplasmic Reticulum, or suspended in the cytoplasm of the cell. Ribosomes are split into two different hemispheres, the large subunit, and the small subunit. The code of mRNA is read when it is threaded through the center of the ribosome, between the two subunits. As the mRNA enters the ribosome between the two subunits, tRNA enters the ribosome through the "E Site" of the large subunit. Codons, sections of three nucleic acids of an mRNA strand, are then read by anticodons on the tRNA in a process known as translation. The translation begins when the start codon, UAG is read. The tRNA then transfers the code to the ribosome, which creates amino acids at the "P Site". These are added to a polypeptide chain until a stop codon is reached. The pieces of tRNA exit the ribosome at the "A Site". The ribosome is constantly performing translation, it does not pause between protein chains. The location and type of the ribosome drastically affect the type of protein that will be created. Ribosomes on the rough endoplasmic reticulum will create proteins that are packaged and then transported somewhere outside of the cell. Ribosomes in the cytoplasm will often create proteins suspended in the cytoplasm as well, such as [|enzymes].

[[image:Ribosome.gif width="323" height="262" align="left" caption="A model of a ribosome"]]
media type="custom" key="7330615"media type="custom" key="7330833"