However, the ER is a vast, interconnected network of membranes that extends throughout the cytoplasm of a cell. It's like a continuous highway system within the cell.

While there's only one continuous ER, it can be divided into two distinct regions:

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

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

So, while a cell has one ER, it can be functionally and structurally divided into these two subregions.