Final answer:
Upon synthesis by ribosomes, proteins are transported to the ER lumen in the Rough Endoplasmic Reticulum where they undergo modifications such as folding. These proteins can then be incorporated into cellular membranes or secreted from the cell, and phospholipids are also produced in the RER.
Step-by-step explanation:
The proteins that are transported to the ER lumen upon synthesis by ribosomes undergo several modifications in the Rough Endoplasmic Reticulum (RER). These modifications include, but are not limited to, folding and acquiring side chains. This process facilitates the proteins' incorporation into cellular membranes or their secretion from the cell in the form of hormones, enzymes, and other types of proteins.
Furthermore, the RER is also responsible for the synthesis of phospholipids which are crucial components of cellular membranes. It's worth noting that these critical functions of the RER make it abundant in cells that have a high rate of protein secretion, such as liver cells. When these modified proteins do not remain in the RER, they move in transport vesicles, which bud from the RER and typically head towards the Golgi apparatus for further processing and eventual sorting for their final destination.