Here's why:
* Rough ER: This part of the ER is studded with ribosomes, which are responsible for protein synthesis. The rough ER receives instructions from the nucleus and manufactures proteins that will be used in the cell or exported outside.
* Smooth ER: This part of the ER lacks ribosomes. It plays a role in synthesizing lipids, steroids, and other molecules. It also detoxifies harmful substances and stores calcium ions.
Therefore, the ER acts as a manufacturing and processing center within the cell, taking in raw materials and transforming them into usable products for the cell's needs.
