AEMCA Practice Exam

The rough endoplasmic reticulum (RER) plays a crucial role in the cell's protein synthesis process due to its association with ribosomes. These ribosomes give the RER its "rough" appearance and are the sites where translation of messenger RNA (mRNA) into polypeptide chains occurs. Once proteins are synthesized by the ribosomes on the RER, they can be modified, folded, and prepared for transport to other locations within the cell or for secretion outside the cell. This process is vital for the production of proteins that are embedded in cell membranes, secreted from the cell, or sent to lysosomes.

The other options describe functions associated with different cellular structures. The detoxification of harmful substances is primarily handled by the smooth endoplasmic reticulum or peroxisomes, while lipid synthesis is also a function of the smooth endoplasmic reticulum. Transporting materials outside the cell typically involves vesicles and the Golgi apparatus, rather than the rough endoplasmic reticulum directly. Therefore, the main and defining function of the rough endoplasmic reticulum is indeed linked to the production of protein synthesis facilitated by the attached ribosomes.