What is the primary function of the rough endoplasmic reticulum?

The primary function of the rough endoplasmic reticulum (RER) is indeed protein synthesis and temporary storage. The RER is characterized by the presence of ribosomes on its cytoplasmic surface, which gives it a "rough" appearance. These ribosomes are the sites where proteins are synthesized, particularly those that are to be secreted outside the cell, incorporated into the cell's plasma membrane, or sent to an organelle such as the lysosome.

Once synthesized in the RER, proteins often undergo initial folding and modification, before being packaged into vesicles for transport to other locations. This temporary storage aspect is crucial because it allows the cell to manage and regulate the processing and distribution of proteins. Thus, the RER plays a fundamental role in the overall protein production pathway within the cell, making it essential for maintaining proper cellular function, especially in cells that are heavily involved in secretion or require significant amounts of membrane-protein interaction.