Tonicity: Definition and Biology Terms

Tonicity Definition

The concentration of a solution in comparison to another solution is known as tonicity. The quantity of solutes dissolved by a solution is referred to as concentration. Hypertonic refers to a solution that has a greater concentration of solutes (less water) than another. A hypotonic solution has fewer solutes and more water than a hypertonic solution. The concentration of solutes in isotonic solutions is the same. In biology, how water travels through the semipermeable membrane is determined by the tonicity of the environment in comparison to the cell. The graph below depicts the tonicity of various environments, as well as the direction in which water flows. Remember that water travels to balance the solute concentration gradient. It will transition from a high to a low concentration of solutes.

Because of its polar nature, water has the capacity to dissolve solutes. Water molecules with different electrical polarities create transient hydrogen bonds with other water molecules and solutes. This disperses solutes and transports water to new areas. As more solutes are added, the tonicity of the water increases, but labelling the solution with one of the particular parameters of tonicity can only be done when comparing it to another. Similarly, the quantity of water required to dissolve two distinct solutes is varied. One solute, such as silicon, may require several water molecules to dissolve whereas another, even at high concentrations, may dissolve entirely. The osmolarity of water, which is the total concentration of all dissolved solutes, may be used to indicate its overall tonicity.

Tonicity Related Biology Terms

  • Hypotonic: when one solution has a higher concentration of water and a lower concentration of solutes than another.
  • Hypertonic: a solution with a concentration greater than that of the solution being compared.
  • Isotonic: two solutions that have the same amount of solute.
  • Osmolarity: the concentration of a solution expressed as the ratio of the solute to the volume of water.


  1. A plant cell is placed in an aqueous solution. Water floods the cell, and the cell becomes rigid with pressure. What is the tonicity of the solution compared to the cell?
    1. Isotonic
    2. Hypertonic
    3. Hypotonic

Answer to Question #1

C is correct. The solution is hypotonic compared to the cell. The cell has a plasma membrane, which is semi-permeable. Only water can pass through the membrane, while the solutes suspended in the solution cannot. If water moves into the cell, it is because the concentration of solutes is higher on the inside of the cell. This means that the solution is weaker in concentration than the cell, otherwise known as hypotonic.

  1. Two identical solutions contained in narrow beakers are separated by a semi-permeable membrane. If salt is added to one beaker, increasing its tonicity, what will happen to the volume of water in the beaker?
    1.  It will increase
    2. It will decrease
    3. It will stay the same

Answer to Question #2

A is correct. The solutions start out identical, or isotonic compared to each other. When you add salt to one side of the system, the concentration of salt is higher in that side. The water molecules can travel freely between the beakers, and tend to balance the concentration gradient. To do this, more water must move into the beaker with salt added. This will decrease the amount of solute per unit volume of water, or decrease the tonicity. The other side will increase in tonicity as solutes present will be divided by less water.

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