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Tonicity is the ability of an extracellular solution to change cell shape by causing the cell to gain or lose water. It depends on the concentration of impermeable solutes outside the cell relative to inside, which determines water movement direction. Changes in tonicity can cause cells to shrivel, swell, or maintain their original shape.

In a hypertonic solution, the solute concentration outside the cell is higher than inside, causing water to move out of the cell. This results in cell crenation, where the cell shrivels. The cell loses water because the extracellular fluid contains less water than the cell's interior.

In a hypotonic solution, the solute concentration outside the cell is lower than inside, so water moves into the cell. The cell swells as it absorbs water and may eventually rupture, a process called cell lysis. This occurs because water follows its concentration gradient into the cell.

An isotonic solution, such as 0.9 percent sodium chloride, has equal solute concentrations inside and outside the cell, resulting in no net water movement. This prevents red blood cells from crenating or lysing, maintaining cell volume and shape during intravenous administration to dehydrated patients.

Plant cells have cell walls that prevent lysis, so they do not burst in hypotonic solutions. Instead, water enters the cell, creating turgor pressure that stiffens the cell wall and supports the plant. In non-woody plants, this turgor pressure is essential for structural support and plant rigidity.

Osmolarity is the measure of the total amount of solutes dissolved in a specific amount of solution. Tonicity describes how a solution's osmolarity compares to a cell's osmolarity. The terms hypotonic, isotonic, and hypertonic relate the osmolarity of the extracellular fluid to the osmolarity inside the cell, similar to how the glycocalyx and its functions influence cellular interactions.

When plants lack adequate water, cells become hypertonic and water leaves the cell. This causes the plant to lose turgor pressure, which normally stiffens the cell walls. Without sufficient turgor pressure, non-woody plants cannot maintain their rigid structure and wilt.