2.8: Tonicity and Osmoregulation

Cells are complex organisms. They have tons of small components within them, which each serve different and interconnected purposes. However, each of these components (and the cell as a whole) operate best in slightly different conditions. Topic 2.8 is about the effects of having different environmental conditions inside and outside the cell.

Vocab List

• Osmosis
• Isotonic
  • Normal
  • Flaccid
• Hypertonic
  • Shriveled
  • Plasmolyzed
• Hypotonic
  • Lysed
  • Turgid
• Water potential
• Solute potential
• Contractile vacuoles
• Aquaporins

Written Explanation

Osmosis:

In previous lessons, diffusion has been the movement of a solute through water (the solvent). In most cells though, many molecules can't freely enter or exit a cell. So what happens if the solute can't move? Does nothing happen? Does the water move instead? As it turns out, the water does move in a process called osmosis.

Oppositely to diffusion however, osmosis moves water from an area of low solute concentration to an area of high solute concentration. However, this still has the same overall effect, resulting in the overall concentration gradient smoothing out and reaching equilibrium.

Tonicity:

Tonicity describes the relationship between two solutions. These solutions could be the inside and the outside of a cell, the cell interior and mitochondrial matrix, or even tap water and ocean water.

When both solutions have the same concentration of solute, it is said that they are isotonic. When the solutions have different solute concentrations, the one with more solute is said to be hypertonic to the other. The one with less solute is said to be hypotonic to the other. Shown below is a chart with the effects of different tonicity on both plant and animal cells.

	Animal Cell	Plant Cell
Hypotonic	Shriveled	Plasmolyzed
Isotonic	Normal	Flaccid
Hypertonic	Lysed (Popped)	Turgid

It is important to remember which solution is being referred to, because saying that a cell is hypertonic means the same thing as saying the cell's environment is hypotonic. Just remember, water flows from low to high solute concentration for osmosis, and solute flows from high to low concentration in diffusion. Low to high, high to low. Low to high, high to low. Low to high, high to low…Yes, it's confusing.

Water potential:

Water potential is a measure of the potential energy water has to move. A high value of water potential might mean that there is something pushing the water around or a large imbalance of solute. Solute potential (also known as osmotic potential or tonicity) is a measure of how the solute concentration of a cell causes water to move. Here is another tricky place to remember high and low. Water flows from high to low areas of water potential and solute potential. Don't forget, a “large” negative number is less than a “small” negative number!

For more information on how to calculate solute potential (which you do need to know for the AP test), reference our 2.8 video lesson and kindly ask your teacher for help.

Ways to maintain solute potential/osmolarity/osmotic pressure/tonicity:

Because cells need a specialized cytosol which is generally hypertonic to its environment, cells need ways to move water out of their interior. This is why some protists and unicellular algae have contractile vacuoles. Contractile vacuoles are just pumps that constantly work to expel water from the cell.

Some cells in multicellular organisms (like ourselves) live in a carefully curated isotonic environment. For example, red blood cells are generally isotonic to the blood plasma which they are surrounded in. In these cases, cells may have aquaporins, which are integral membrane proteins that allow free water movement across the cell membrane. Increasing water flow allows cells to more rapidly adjust to the tonicity of their environment.