Here's why:

* Stack of membranous sacs: The Golgi apparatus is indeed composed of flattened, membrane-bound sacs called cisternae, which are stacked upon each other.

* Sorting proteins: The Golgi apparatus plays a crucial role in modifying, sorting, and packaging proteins that are synthesized in the endoplasmic reticulum (ER). It acts like a "post office" for the cell, directing proteins to their correct destinations, whether it be within the cell or outside.

Let me know if you have any other questions!