What is the primary function of the endoplasmic reticulum?

The primary function of the endoplasmic reticulum (ER) revolves around its role in the synthesis and modification of proteins, which aligns with the choice regarding packaging and transporting proteins. The ER comes in two forms: rough and smooth. The rough endoplasmic reticulum is studded with ribosomes, which are sites for protein synthesis. Once proteins are synthesized, they enter the lumen of the rough ER, where they undergo post-translational modifications, such as folding and glycosylation.

Following these modifications, proteins are often packaged into vesicles and transported to their next destination, which may include the Golgi apparatus for further processing or to specific cellular locations. This transport aspect is crucial for the proper functioning of the cell, as it ensures that proteins are correctly processed and delivered to perform their specific roles.

While protein synthesis is an important step, it is just one part of the broader function of the endoplasmic reticulum. The processes of modifying, packaging, and transporting proteins capture the comprehensive role the ER plays in maintaining cellular function. The other options do not accurately represent the primary functions of the endoplasmic reticulum, as it is not directly involved in DNA replication or cell division.