Endoplasmic Reticulum

The endoplasmic reticulum is a multifold membranous structure within eukaryotic cells which plays a major role in the synthesis of the complex molecules required by the cell and the organism as a whole. Often the membranes of these structures are lined with ribosomes on their outer surfaces, giving them a rough appearance. These parts are called the rough endoplasmic reticulum to contrast them with the smooth endoplasmic reticulum where there are no attached ribosomes.

The ribosomes on the rough endoplasmic reticulum manufacture proteins which enter the channels of the endoplasmic reticulum and move to places where they can create pockets. These pockets can then break off as vesicles to transport their protein cargo to the Golgi complex for distribution.

Examples of protein synthesis by the rough endoplasmic reticulum are the proteins produced in secretory cells. These include the digestive enzymes produced in the stomach and the protein hormones like insulin produced in the pancreas. Organ systems which produce many proteins have cells with a large amount of rough endoplasmic reticulum.

The smooth endoplasmic reticulum plays a major role in synthesizing lipids by means of enzymes embedded in these smooth membranes. It produces the phospholipids and cholesterol used in membrane formation, and along with the membrane proteins produced by the rough ER it can synthesize more membrane for itself, for the Golgi complex, the cell membrane, lysosomes, and others.

In liver cells the smooth ER contains enzymes for the detoxification of harmful drugs and metabolic by-products. In the reproductive organs, smooth ER in the cells produces the steroid hormones testosterone and estrogen.