Campbell 1996
Episode 3 - The Endoplasmic Reticulum
The Endoplasmic Reticulum (ER) is composed of two parts: Smooth ER and Rough ER.
Structure Functions of the Rough ER
The Rough ER is a series of connecting stacked, flat sacs with ribosomes attached to it. The Rough ER is connected to the nucleus, and curves its way through the cytoplasm. Its membrane is continuous with the nuclear envelope, and thus the space between the nuclear envelope membranes is continuous with the ER lumen.
Campbell 1996
The basic function of the Rough ER is to further modify secretory proteins. The Rough ER takes the new polypeptide chains created by the ribosomes, allows the proteins to form its 3-D shape, and attaches “side-chains” to them, to further modify them into specialized proteins. The Rough ER may then chose to secret the substance as a finished product, or may then send it to the Golgi body for further modifications. The Rough ER also produces membrane phospholipids for the cell. As its membrane grows, vesicles bud out and carry the membrane to other parts of the endomembrane system. Cells specialized for protein synthesis, like plasma cells (differentiated B lymphocytes that secrete many antibodies) are particularly rich in rough ER and Golgi.
Structure and Functions of the Smooth ER
The Smooth Endoplasmic Reticulum does not have any ribosomes attached to its membranes, and consists of a long, interconnected series of tubes that curves through the cytoplasm.
For most cells, the main function of the Smooth ER is to store enzymes that synthesize lipids (like oils, phospholipids, and steroids) The Smooth ER synthesizes steroid hormones like aldosterone, testosterone, and estrogen. Thus cells of areas like the testes, ovaries, and adrenal cortex are rich in smooth ER. The Smooth ER is also especially developed in cells of seeds, where there is an abundance of lipids, and still more lipids need to be synthesized. The lipids synthesized in the Smooth ER can also be transported directly by vesicles to anywhere in the cell or out of the cell, or can be sent by vesicles to the Golgi body, which further refines the lipids and then packages them off to be sent out of the cell.
The Smooth ER also inactivates most harmful by-products of cells’ metabolic activities and inactivates certain drugs (like in liver cells). Smooth ER enzymes add hydroxyl groups to drugs to make them more soluble in water and thus easier to be expelled from the body via urine. Smooth ER in the liver cells convert toxic ammonia into the less toxic urea (or uric acid).
In many specialized types of cells, a modified Smooth ER called the Sarcoplasmic reticulum stores calcium ions and releases them when stimulated to trigger different responses (ex. Muscle contraction).
(a)Electron micrograph endoplasmic reticulum.The rough ER is studded with ribosomes(TEM)
(b)Electron micrograph of smooth endoplasmic recticulum(TEM)
(Source: Lodish H, et al., 2003)
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