The Endoplasmic Reticulum (ER) organelle is a network of disk-like tubules, sacks and vesicles found in eukaryotic cells. Its main function is to operate as a transport system. Endoplasmic reticulum is chiefly made up of a phospholipid membrane containing an internal fluid-filled space called the cisternal space or the lumen. The ER is folded and stacked layer upon layer within the cell and is connected to the cell's nuclear membrane. Under a microscope, the endoplasmic reticulum is seen as a highly folded structure surrounding the cell nucleus. There are two types of ER, Rough and Smooth.
Rough Endoplasmic Reticulum
Rough ER contains ribosomes on its surface, small circular structures that control protein synthesis, making it look bumpy under a microscope. Rough endoplasmic reticulum branches out and expands as protein synthesis increases, providing more surface area for ribosomes to spread out and create more proteins.
During protein synthesis, the ribosomes on the rough ER create new proteins and the ER then folds them properly and sorts them according to function and destination.
The rough ER carries out its protein folding activity deep within the cisternal space. There, newly made proteins are twisted and folded into the configurations necessary for them to carry out their function in the cell.
For sorting, the rough ER works in conjunction with the Golgi apparatus, another cell organelle, to target the newly synthesized proteins to their proper locations. Most proteins produced by ribosomes of the rough endoplasmic reticulum are destined for secretion out of the cell.
Smooth Endoplasmic Reticulum
Smooth ER has a few different functions in the cell, and its functions can vary with cell type. In general, the function of smooth endoplasmic reticulum is to provide surface area for the action of enzymes and to provide storage space for these important enzymes.
In muscle cells, for example, smooth ER stores calcium, releasing it during muscle contraction, triggering the movement of the muscle. In the liver, smooth ER is where detoxification takes place.
Smooth ER also synthesizes lipids (fats) and other components of the cell membrane.
Another function of smooth endoplasmic reticulum is to control the movement of newly synthesized proteins to their proper location in the cell or to the membrane to be sent outside the cell. This is done by a process called budding, where small vesicles of smooth ER are pinched off to carry the proteins to their new location.
The many functions of endoplasmic reticulum make it an important organelle for maintaining normal cell functioning. Defective or stressed endoplasmic reticulum has been implicated in diseases ranging from rare genetic conditions to well known conditions such as Alzheimer's disease, diabetes and heart disease.
Sources:
Alberts et al, Molecular Biology of the Cell, 3rd edition, Garland Publishing, 1994.
Benedetti, A., et al. Endoplasmic reticulum: a metabolic compartment. IOS Press, 2005
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