What does it mean for a solution to be isotonic?

An isotonic solution is characterized by having the same concentration of solutes as the cytoplasm of the cells it surrounds. This balance means that there is no net movement of water into or out of the cells; thus, the volume of water within the cells remains stable. In an isotonic environment, water molecules continue to move across the cell membrane in both directions, but because the concentration of solutes is equal on both sides of the membrane, these movements offset each other. As a result, the cells neither swell due to water influx nor shrink due to water loss, leading to no net change in water volume. This equilibrium is crucial for maintaining cell health and function, as any significant change in water volume can lead to cellular stress or damage.