Which term refers to the ability of a surrounding solution to affect the water balance of a cell?

The ability of a surrounding solution to affect the water balance of a cell is indeed referred to as tonicity. Tonicity specifically describes the comparative concentration of solutes between two solutions — typically one inside the cell and one outside in the surrounding environment. It helps determine the direction and extent of water movement across the plasma membrane due to osmosis, which is the diffusion of water through a selectively permeable membrane.

When a cell is placed in different tonic environments—whether isotonic, hypertonic, or hypotonic—the water movement can lead to alterations in cell volume and shape. For example, in a hypertonic solution, the concentration of solutes outside the cell is higher than inside, causing water to move out of the cell and potentially leading to cell shrinkage. In contrast, in a hypotonic solution, the external concentration of solutes is lower, which can lead to water moving into the cell, possibly resulting in swelling or even lysis.

Osmoregulation, while related, focuses more broadly on the mechanisms that organisms use to manage water and solute balance. Homeostasis refers to the overall balance within an organism's internal environment, including temperature, pH, and solute concentrations. A concentration gradient describes a difference in the