Final answer:
The Endoplasmic Reticulum (ER) is the extensive system of membrane-enclosed cavities within the cytoplasm of a cell and is part of the endomembrane system along with the nuclear envelope, Golgi apparatus, lysosomes, and vesicles.
Step-by-step explanation:
The extensive system of membrane-enclosed cavities within the cytoplasm of a cell is known as the Endoplasmic Reticulum (ER). The ER is composed of the same lipid bilayer material as the nuclear membrane and can be thought of as a series of winding thoroughfares similar to the waterway canals in Venice.
It provides passages throughout much of the cell that function in transporting, synthesizing, and storing materials. The endomembrane system, which includes elements such as the nuclear envelope, ER, Golgi apparatus, lysosomes, and vesicles, work collectively to modify, package, tag, and transport proteins and lipids that form the membranes.