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Tonicity is the effective osmotic pressure gradient created by non-penetrating solutes across a semipermeable membrane, which influences water movement across it. This results in three possible scenarios that maintain or change the cell's volume.

When the solute concentration inside and outside the cell is equal, the solution is isotonic. There is no net movement of water because water enters and leaves the cell at equal rates, so the cell maintains its normal shape.

But, if the fluid outside the cell has fewer solutes and more water than the inside, the condition is hypotonic. Water moves into the cell, causing it to swell and possibly burst, a process called cell lysis.

On the other hand, in hypertonic conditions, the extracellular fluid contains more solutes and less water than the inside of the cell. Water moves out of the cell, causing it to shrink.