An isotonic solution can be described as a solution that has a balance of solutes and solvents inside and outside a cell. As a result, no concentration gradient is developed. Hence, there is no transfer of solutes or solvents in an isotonic solution.
But when solutes will be added into the solution a concentration gradient will develop. There will be more molecules of solutes outside the cell as compared to the inside of the cell. As result, the solvent will move outside the cell to maintain a balance. Hence, the cells will shrink.
