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

The primary function of rough endoplasmic reticulum (ER) is to synthesize and transport proteins. This organelle is called "rough" due to the presence of ribosomes attached to its cytoplasmic surface, which are the sites of protein synthesis. During translation, ribosomes on the rough ER synthesize polypeptide chains that are either secreted from the cell, incorporated into the cell's plasma membrane, or sent to an organelle within the cell, such as the lysosomes.

Additionally, the rough ER is involved in protein folding and post-translational modifications, which are essential for the functionality of the proteins synthesized. Once proteins are synthesized in the rough ER, they are then packaged into vesicles that transport them to the Golgi apparatus for further modification, sorting, and eventual distribution to their destinations.

In contrast, other options relate to different cellular functions: lipid synthesis primarily occurs in the smooth ER, energy production is associated with mitochondria, and detoxification of chemicals is also primarily a function of the smooth ER. Thus, the emphasis on protein synthesis and transport clearly identifies rough ER's crucial role in cellular biology.