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Cell Biology
The World of Cells
Endoplasmic Reticulum
Endoplasmic Reticulum The part of the cell involved in protein synthesis is the endoplasmic reticulum (the "inside cell network") sometimes simply called the ER. The ER is a finely divided system of membrane-enclosed compartments with an interconnecting network of tubules. The membrane-enclosed compartments are called cisternae and the network a reticulum. Spreading throughout some cells, the ER becomes flattened into thin pancakes called lamellae, which are very prominent in cells carrying out a lot of protein synthesis.
Two types

a ribosome (blue)
with mRNA (brown)
and a polypeptide (red)
Under the electron microscope, it is possible to distinguish two types of endoplasmic reticulum: rough and smooth. Rough endoplasmic reticulum, (RER), has large numbers of ribosomes attached to the outer surfaces of the lamella membranes. These ribosome-rich areas play an important role in the production, storage and export of proteins. Proteins assembled on these ribosomes are passed directly through the lamellar membrane into the interior of the cisternae to be processed and modified. For example, within the cisternae, carbohydrates are often added to the proteins, forming glycoproteins (protein + carbohydrate). Special enzymes in the ER carry out this process.
Rough ER The modified proteins move slowly along the inside of the RER until finally they are packaged into small vesicles of ER membrane at the tips of the tubules. Completed packages of protein (transition vesicles) are moved by elements of the cytoskeleton either to the Golgi apparatus or to the plasma membrane. The membranes around the vesicles fuse with the plasma membrane, liberating the contents of the vesicles to the outside of the cell by exocytosis.
Smooth ER Smooth endoplasmic reticulum, ( SER), has been found involved in steroid synthesis. SER does not have ribosomes on its surface, however, and thus does not participate in the making of proteins. It does, however, have other functions. For example, in liver cells, the SER contains many enzymes and other proteins that have the ability to detoxify a wide range of drugs and poisons.
Figure legend: Endoplasmic reticulum, mostly showing RER, cisternae and some vesicles.

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© 2001, Professor John Blamire