There is no structure called a "sac" in an animal cell. It's likely you're thinking of one of these structures:

* Saccule: This is part of the vestibular system in the inner ear, responsible for detecting linear acceleration and gravity.

* Vacuole: These are membrane-bound organelles that are found in both plant and animal cells. They are responsible for storing various substances like water, food, and waste products. Animal cells generally have smaller and more numerous vacuoles compared to plant cells.

Please clarify what you mean by "sac" so I can give you a more accurate answer!