Advanced Placement (AP) Biology Practice Exam

What type of solution causes water to leave a plant cell, leading to wilting?

1. Isotonic solution

2. Hypertonic solution

3. Hypotonic solution

4. Milliosmolar solution

The correct answer is: Hypertonic solution

A hypertonic solution causes water to leave a plant cell, leading to wilting. In a hypertonic environment, the concentration of solutes outside the cell is greater than the concentration inside the cell. This osmotic gradient prompts water to move out of the cell in an effort to equalize solute concentrations on both sides of the cell membrane. When water leaves the plant cell, the vacuole, which stores water and helps maintain turgor pressure, begins to shrink. Turgor pressure is essential for keeping plant cells rigid and supporting the overall structure of the plant. As water continues to exit, the cell loses turgor pressure, resulting in wilting—where the cells become flaccid and the plant appears limp. In contrast, an isotonic solution has equal concentrations of solutes both inside and outside the cell, allowing for a state of equilibrium without net movement of water, thus preventing wilting. A hypotonic solution, on the other hand, has a lower concentration of solutes outside the cell, causing water to enter the cell and potentially leading to swelling, not wilting. The term "milliosmolar solution" does not specifically describe a hypertonic, isotonic, or hypotonic condition,