1. If the water concentration of the cell�s cytoplasm is lower then that of the medium (i.e. the medium is a hypotonic solution) surrounding the cell then osmosis will result in the cell gaining water. The water molecules are free to pass across the cell membrane in both directions, but more water molecules will enter the cell than will diffuse out with the result that water enters the cell, which will then swell up and could possibly burst.

2. If the water concentration inside the cell is the same as that in the surrounding medium (i.e. the medium is a isotonic solution) there will exist a dynamic equilibrium between the number of molecules of water entering and leaving the cell and so the cell will retain its original size.

For example, the red blood cell in the blood plasma retains its shape because of the isotonic nature of the plasma.