Final answer:
When an onion cell is placed in saltwater, the central vacuole shrinks as water moves out of it and the cytoplasm. This can lead to the wilting of the cell.
Step-by-step explanation:
In an onion cell, the central vacuole serves multiple functions, including regulating the cell's concentration of water in changing environmental conditions and supporting cell expansion. When placed in saltwater, the concentration of solutes in the saltwater is higher than in the cell's cytoplasm. As a result, water moves out of the central vacuole and cytoplasm, causing the central vacuole to shrink. This loss of water from the central vacuole can lead to the wilting of the onion cell.