Tonicity describes the amount of solute in a solution. The measure of the tonicity of a solution, or the total amount of solutes dissolved in a specific amount of solution, is called its osmolarity. Three terms—hypotonic, isotonic, and hypertonic—are used to relate the osmolarity of a cell to the osmolarity of the extracellular fluid that contains the cells. In a hypotonic solution, such as tap water, the extracellular fluid has a lower concentration of solutes than the fluid inside the cell, and water enters the cell. It also means that the extracellular fluid has a higher water concentration than the cell. In this situation, water will follow its concentration gradient and enter the cell. It may cause an animal cell to burst or lyse.

In a hypertonic solution, the extracellular fluid contains less water than the cell, such as seawater. Because the cell has a lower concentration of solutes, the water will leave the cell. In effect, the solute is drawing the water out of the cell. This may cause an animal cell to shrivel or crenate.

In an isotonic solution, the extracellular fluid has the same osmolarity as the cell. If the concentration of solutes in the cell matches that of the extracellular fluid, there will be no net movement of water into or out of the cell. Blood cells in hypertonic, isotonic, and hypotonic solutions take on characteristic appearances.

Some organisms, such as plants, fungi, bacteria, and protists, have cell walls that surround the plasma membrane and prevent cell lysis. The plasma membrane can only expand to the limit of the cell wall, so the cell will not lyse. In fact, the cytoplasm in plants is always slightly hypertonic compared to the cellular environment, and water will always enter a cell if water is available. This influx of water produces turgor pressure, which stiffens the plant's cell walls. In non-woody plants, turgor pressure supports the plant. If the plant cells become hypertonic, as in a drought, or if a plant is not watered adequately, water will leave the cell. Plants lose turgor pressure in this condition and wilt.

This text is adapted from Openstax, Concepts of Biology, Section 3.5: Passive Movement.