In biology, an isotonic solution is one that has the same solute concentration compared to the intracellular solute concentration. When a cell is placed in an isotonic solution, there is no net movement of water, so the size of the cell remains the same. This is because the concentration of solutes is the same inside and outside the cell, so water molecules are exchanged through the cell membrane without causing any change in the cells volume. In contrast, a hypertonic solution has a higher solute concentration compared to the intracellular solute concentration, causing water to move out of the cell and resulting in the cell shrinking. A hypotonic solution has a lower solute concentration compared to the intracellular solute concentration, causing water to move into the cell and resulting in the cell swelling.