Here's why:
* Rough ER: This portion of the ER is studded with ribosomes, which are responsible for protein synthesis. Many of the proteins produced by the rough ER are destined for the cell membrane or other organelles. As these proteins are synthesized, they are inserted into the membrane of the ER, expanding its surface area.
* Smooth ER: This portion of the ER lacks ribosomes and is primarily involved in lipid synthesis. Lipids, including phospholipids, are essential components of cell membranes. The smooth ER produces these lipids, which are then incorporated into the ER membrane and subsequently transported to other parts of the cell, including the plasma membrane.
Therefore, both the rough and smooth ER contribute to the production of new membrane for the cell.
