Advanced Placement (AP) Biology Practice Exam

What type of vacuole is responsible for pumping excess water out of a cell?

1. Central vacuole

2. Contractile vacuole

3. Storage vacuole

4. Food vacuole

The correct answer is: Contractile vacuole

The contractile vacuole is responsible for pumping excess water out of a cell, particularly in certain protists such as paramecia and some freshwater organisms. This specialized structure functions as a regulatory mechanism to maintain osmotic balance and prevent the cell from bursting due to the influx of water. When water enters the cell through osmosis, the expanding contractile vacuole collects this excess water and periodically contracts to expel it from the cell, ensuring that the internal environment remains stable and that cellular functions can proceed without disruption. The other types of vacuoles mentioned do not perform this function. The central vacuole is primarily involved in storage and maintaining turgor pressure in plant cells, while storage vacuoles are utilized for storing nutrients and waste products. Food vacuoles are temporary structures used for the digestion of food particles within the cell. Each type of vacuole serves a distinct role that does not relate to the expulsion of excess water, highlighting the unique function of the contractile vacuole in cellular water management.