diffusion is the movement of molecules or stuff through the cell membrane
osmosis is the movement of water
water move from high contration to lower concentration
the there is salt outside of the cells the water will move out of the cell membrane and if there is salt inside the cells the water will move into the cell
Volume? Is that what you mean?
The word isotonic is used to refer to the solutions (two solutions) which have the same osmotic pressure in both sides of the semipermiable membrane. The condition allows the free movement of water across the membrane. However, this does not change the concentration of the solution across the membrane.
Hence, osmosis may not occur in this condition.