Final answer:
The extensive system of membrane-enclosed cavities within the cytoplasm of a cell is called the endomembrane system, which includes the endoplasmic reticulum, Golgi apparatus, and vesicles.
Step-by-step explanation:
The extensive system of membrane-enclosed cavities within the cytoplasm of a cell is known as the endomembrane system. This system includes several organelles such as the endoplasmic reticulum (ER), Golgi apparatus, and vesicles. The ER functions as a series of channels or passages throughout much of the cell for transporting, synthesizing, and storing materials, and it's contiguous with the nuclear envelope. The Golgi apparatus works in concert with the ER to modify, sort, and package proteins and lipids for storage or transport out of the cell. Various types of vesicles are involved in transport and storage within the cell, as well as in the metabolism and enzyme storage. The cytoplasm, composed of all these components and the gel-like substance called cytosol, plays a crucial role in maintaining cell structures and providing a site for the biochemical reactions inside the cell.