What is a membrane-bound sac called?

A membrane-bound sac is referred to as a vesicle, which is a small, membrane-enclosed structure found within cells. Vesicles are crucial for transporting materials within the cell and to the exterior environment. They can store substances, facilitate metabolic processes, and assist in communication between various parts of the cell.

In contrast, vacuoles are larger membrane-bound sacs that typically store water, nutrients, or waste products and are more prominent in plant cells. However, the term "vacuole" generally refers to these larger structures rather than the smaller and more versatile vesicles.

The cell membrane is the outer boundary of the cell, serving as a barrier that regulates what enters and leaves the cell; it is not a sac itself. An organelle refers to specialized structures within a cell that perform specific functions and can include vesicles, but it is a broader term and not specific to membrane-bound sacs. Thus, when identifying a term that specifically describes a small, membrane-bound compartment, the correct term is vesicle.