Many molecules in and around cells exist in concentration gradients across the cell membrane, meaning that the molecules are not always evenly distributed inside and outside of the cell. Isotonic solutions have the same molecular concentrations inside and outside of the cell.So if the solution has the same solute concentration, and thus the same water concentration, as the cells do, it is isotonic to the cells. Diffusion drives molecules to move from areas where they are in high concentration to areas where they are in a lower concentration. As a result, there would not be a concentration gradient since a gradient by definition involves a difference. There would thus be no net flow of water between the cell and the solution. This does not mean that water would not move between them, just that the rate of exiting and entering the cell are equal. There is no net change in the cell's appearance (there is no change in size of the cell Or in other words, the cell retains their normal shape).