The presence of ribosomes attached to the cytoplasmic side of this membranous network gives it a rough appearance, thus the name. Its main function is protein synthesis.
Initially, the rough ER does not have any ribosomes yet. Synthesis actually starts in ribosomes that are freely floating in the cytoplasm. However, once the initial sequence of amino acids called the signal peptide, are formed, the ribosomes then attach to the ER and continue with the synthesis. Since different kinds of proteins are being synthesized regularly, a continuous stream of ribosomes attach to the ER thus making them practically integral parts of the ER.
Protein synthesis is a very systematic and closely monitored process. Proteins which are destined for use within the cell are processed separately from proteins that are “exported” out of the cell. Markers are added along the way as the synthesis of proteins progresses. The markers ensure that proteins reach their proper destinations.
The rough ER works closely with the Golgi apparatus. Between the two of them, there is a regular flow of proteins in vesicles. There is even a whole process of vesicular movement between the ER, the Golgi, and the cell membrane that is called “membrane trafficking”. Yes, there is traffic flow in cells, but unlike the chaotic traffic in our society, this cellular traffic is very well coordinated and there are no traffic snarls along the way. Maybe our traffic coordinators could learn a thing or two by observing how cells do their trafficking.