The series of folded membranes that move materials around in the cell is called the endoplasmic reticulum (ER).

Here's why:

* Folded Membranes: The ER is a network of interconnected membranes that form flattened sacs (cisternae) and tubules.

* Material Transport: The ER serves as a transportation system within the cell, moving materials from one part of the cell to another.

* Two Types: There are two main types of ER:

* Rough ER: Studded with ribosomes, responsible for protein synthesis and modification.

* Smooth ER: Lacks ribosomes, involved in lipid synthesis, detoxification, and calcium storage.

Let me know if you'd like to know more about the specific functions of the ER or its different parts!