Final answer:
A vesicle is the small sac formed when the cell membrane surrounds a substance to bring it into the cell.
Step-by-step explanation:
The small sac that is formed when the cell membrane surrounds a substance to bring it into the cell is called a vesicle. Vesicles are small, membrane-bound sacs that function in cellular storage and transport. They are formed in the Golgi apparatus and the endoplasmic reticulum, and can be used to move molecules between locations inside the cell.