Here's why:

* Series of tubes: The ER is a network of interconnected membranes that extend throughout the cytoplasm of eukaryotic cells.

* Transport proteins: The ER plays a crucial role in protein synthesis and transport. Ribosomes attached to the ER synthesize proteins, which are then folded and modified within the ER lumen.

* Other things: The ER is also involved in lipid synthesis, detoxification, and calcium storage.

Let me know if you'd like to learn more about the ER!