What is the primary function of the rough endoplasmic reticulum (ER)?

The rough endoplasmic reticulum (ER) plays a crucial role in the synthesis and processing of proteins. It is characterized by the presence of ribosomes attached to its cytoplasmic surface, which give it a "rough" appearance. These ribosomes are the sites of protein translation, where polypeptide chains are synthesized based on the genetic code from messenger RNA (mRNA).

Once the proteins are synthesized, they enter the lumen of the rough ER, where they can undergo folding and modifications, such as glycosylation (adding carbohydrate groups) or forming disulfide bonds. This processing is essential for the proteins to achieve their functional conformations. Additionally, the rough ER is involved in the initial steps of protein sorting, directing proteins either for secretion outside the cell, delivery to the cell membrane, or localization to specific organelles.

In contrast, the other listed functions do not appropriately align with the primary role of the rough ER. While it does not primarily store calcium ions, produce lipids and carbohydrates, or transport genetic material, its specialized function in protein synthesis and processing sets it apart in cellular activities.