What is the main function of the rough endoplasmic reticulum?

The rough endoplasmic reticulum (RER) is primarily involved in the synthesis of proteins. It is characterized by its ribosomes on the cytoplasmic surface, giving it a "rough" appearance. These ribosomes are the sites of translation, where messenger RNA (mRNA) is read to assemble amino acids into polypeptide chains, forming proteins.

The proteins synthesized in the RER often have specific destinations: some will be secreted from the cell, while others will be incorporated into the plasma membrane or sent to lysosomes. Protein synthesis is a critical function of the RER, as it plays a key role in producing the proteins necessary for various cellular functions and maintaining homeostasis.

In contrast, other organelles are responsible for the functions mentioned in the other options. For example, lipid synthesis occurs primarily in smooth endoplasmic reticulum, detoxification processes are mainly handled by the liver's smooth ER and peroxisomes, and calcium storage is typically associated with the sarcoplasmic reticulum in muscle cells as well as the smooth endoplasmic reticulum in non-muscle cells. Thus, the rough endoplasmic reticulum is uniquely suited to its role in protein synthesis.