What is the main role of rough endoplasmic reticulum?

The rough endoplasmic reticulum (RER) primarily functions in the synthesis and packaging of proteins. It is characterized by the presence of ribosomes on its cytoplasmic surface, which gives it a "rough" appearance. These ribosomes are the sites of protein translation, where messenger RNA (mRNA) is decoded to synthesize proteins based on the genetic code.

Once proteins are synthesized in the RER, they undergo initial modifications and are then packaged into vesicles for transport to other parts of the cell, such as the Golgi apparatus or to the cell membrane for secretion. This process is crucial for the production of proteins that are either secreted from the cell, incorporated into the cell membrane, or sent to lysosomes for various cellular functions.

In contrast, the other options involve different cellular functions. Lipid synthesis and storage occur primarily in the smooth endoplasmic reticulum, while photosynthesis is a function of chloroplasts in plant cells. Storage of nutrients generally refers to functions related to vacuoles or other storage organelles, not the rough endoplasmic reticulum.