There is no net movement of water, and the cell stays the same size.
Explanation
Cellular transport involves the movement of substances across the plasma membrane. Osmosis is the specific diffusion of water across a semi-permeable membrane. The direction of net water movement is determined by the tonicity of the surrounding solution relative to the cell's cytoplasm.
Option Analysis
- The cell swells up. is incorrect: A cell swells when placed in a hypotonic solution. In this case, the external solution has a lower solute concentration (higher water concentration) than the cell interior, leading to the net inflow of water (endosmosis).
- The cell shrinks. is incorrect: A cell shrinks when placed in a hypertonic solution. Here, the external solution has a higher solute concentration (lower water concentration) than the cell interior, causing the net outflow of water (exosmosis).
- There is no net movement of water, and the cell stays the same size. is correct: An isotonic solution has the same solute concentration and osmotic pressure as the cytoplasm. Although water molecules continue to cross the membrane, the rate of water entering the cell equals the rate of water leaving it. Therefore, there is no net movement of water, and the cell retains its original size.
- The cell wall dissolves. is incorrect: The cell wall is a rigid structural layer found in plants, fungi, and bacteria. It does not dissolve due to the tonicity of the solution; rather, it prevents the cell from bursting during endosmosis.
Key Takeaway: In an isotonic environment, a cell exists in a state of dynamic equilibrium, where water flows in and out at equal rates, resulting in zero net change in cell volume.