Smooth endoplasmic Reticulum

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OVERVIEW

The endoplasmic reticulum (reticulum= little net), or ER, is an organelle found in all eukaryotic cells. Usually the ER is located near the nucleus. It is a vast network of membrane bound vesicles and tubules that is a complex network of interconnected vesticles, called cisternae, and membranous channels. A cisterna uses a single membrane that surrounds an enclosed space named the ER lumen. The ER is part of the endomembrane system and occurs in both rough and smooth forms, each with specialized structure and function. The smooth ER membranes do not have ribosomes attached to their surfaces and therefore do not play a role in protein systhesis. They do, however, contribute to lipid sythesis. This is it makes up a greater proportions than Rough ER in cells that synthesize lipids or beak down toxins and and makes up a lesser proportion in cells that are highly active in making proteins.


Image:Smoothrough.jpg

STRUCTURE

ER is composed of a complex network of cisternae, which the cytoskeleton holds together. The phospholipid membrane envolopes either the cisternal or lumen space and separates it from the cytosol. Smooth ER connects to the nuclear envelope. Smooth ER lacks the protein producing ribosomes which line Rough ER and give it its name, however some cells contain areas that are dilated like the sacs of rough ER. Smooth ER contains the enzyme Glucose-6-phosphatase, and its branch like network creates greater surface area for the storage and reaction of enzymes and their products. The complex network of tubes is also flexible, and although the overall size of the ER remains constant, compartments and membranes change their shapes continuously, determined by the forces of membrane fusion and membrane fragmentation.

Image:Endoplasmicreticulumfigure1.jpg

FUNCTIONS

The smooth ER serves a number of different purposes. It is responsible for protein translation by aiding in the folding and the transporting of proteins to be used in the cell membrane. The smooth ER also helps in the production and storage of glycogen, triglycerides, steroids, and other marcomoleucles. Inside the smooth ER are a number of enzymes that are important to detoxification. As a result, the smooth ER turns drugs, poisons, and toxic by-products of cellular metabolism into substances that are suitable for the body. Furthermore, the smooth ER is important to muscle cells. It stores and regulates the calcium ions in the muscle cells,as well as facilitating muscle contraction; the smooth ER contracts impulses from surface membranes of the cell into the depths of muscle cells. Lastly, the smooth ER synthesizes membrane phospholipids which go inward from the endoplasmic reticulum to the nuclear membrane to replace it. Sometimes, though, the membrane phospholipids go outward to form the Golgi complex, lysosome, and the cell membrane. Most of the membrane phospholipids go to replacing the endoplasmic reticulum membrane itself.


Image:Smooth_er_em.gif (An electron micrograph of the smooth endoplasmic reticulum)


More information on Smooth Endoplasmic Reticulum can be found at: http://sun.menloschool.org/~cweaver/cells/e/smooth_er/ and http://micro.magnet.fsu.edu/cells/endoplasmicreticulum/endoplasmicreticulum.html and http://www.springerlink.com/content/l4k6814557122n60/?p=e088a610f041427db4a5fe909efb1b0c&pi=1 and http://en.wikipedia.org/wiki/Endoplasmic_reticulum and at your local library.

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