2.12: Tonicity

Tonicity, which is directly related to the osmolarity of a solution, affects osmosis by determining the direction of water flow.

Master this section and you'll be able to

• Define tonicity and describe its relevance to osmosis.
• Compare hypotonic, isotonic, and hypertonic solutions, and predict how each affects the volume and shape of a cell.

💧 Introduction to Tonicity

Tonicity describes how a solution outside the cell (the extracellular fluid) affects the movement of water into or out of the cell — and as a result, how it can change the cell’s volume.💡 Remember: The solutes cannot cross the membrane, but the water can. So the water moves to even things out.

Tonicity is closely tied to osmolarity, which is the total concentration of solutes in a solution:

• A solution with low solute concentration has low osmolarity, which means there are fewer dissolved particles (solute) and more water in the mixture.

• A solution with high solute concentration has high osmolarity, which means there are more dissolved particles (solute) and less free water (because there is less room for water molecules available).

If two solutions with different osmolarities are separated by a selectively permeable membrane (like a cell membrane) that lets water through but not the solutes, water will move from the side with more water (low osmolarity) to the side with less water (high osmolarity). This is osmosis in action!

 

🐟 Why Can Saltwater Fish Not Live in Freshwater?

Tonicity explains why some fish can only survive in either salt water or fresh water — not both. For example, a saltwater fish has cells that are used to living in a salty environment. Its cells are packed with solutes to match the high osmolarity of the ocean water.

But if you suddenly move that saltwater fish into freshwater (which has low osmolarity, or fewer solutes and more water), the water will rush into the fish’s cells. Why? Because water moves toward areas with more solutes, trying to “balance things out.” The fish’s cells swell, and eventually, they can burst. That is why saltwater fish cannot survive in fresh water — and vice versa.

Osmolarity and Cell Survival: The Tonicity Trio

Three key terms — hypotonic, isotonic, and hypertonic — describe how the osmolarity of the fluid surrounding a cell compares to the osmolarity inside the cell. Let’s break down how each type of solution affects the movement of water and the shape of the cell.

1) Hypotonic Solutions: Water Moves Into the Cell

In a hypotonic solution, the fluid outside the cell has a lower osmolarity than the fluid inside the cell. That means:

• The extracellular fluid has fewer solutes and more water compared to the inside of the cell.

• Water moves into the cell, following its concentration gradient.

Since living systems always use the cytoplasm (the fluid inside the cell) as the reference point, the prefix “hypo-” tells us that the extracellular fluid has a lower solute concentration (and thus a higher water concentration) than the cell.

As water enters the cell, the cell swells. If too much water enters, it may burst (a process called lysis).
 

2) Hypertonic Solutions: Water Moves Out of The Cell

In a hypertonic solution, the word “hyper-” tells us the extracellular fluid has more solutes than the fluid inside the cell — meaning it has less water.

Because the cell has more water than the surrounding fluid, water starts to leave the cell in an attempt to balance things out.

As the water exits, the cell begins to shrink and shrivel. The outcome is akin to a grape turning into a raisin. Not exactly ideal for cell survival!
 

3) Isotonic Solutions: No NET Water Movement

An isotonic solution is the Goldilocks zone for cells: not too salty, not too watery, but just right. ⚖️

In this case, the osmolarity of the extracellular fluid is exactly the same as the fluid inside the cell. That means the concentration of solutes (and water) is balanced on both sides of the membrane.

Water molecules still move in and out of the cell, but they do so at equal rates, so there is no net gain or loss of water. The cell stays the same size—happy and stable!

 

Review the image below: red blood cells are great visual examples of how tonicity affects cell shape.

• In a hypertonic solution, water leaves the cell, causing it to shrink and shrivel.

• In an isotonic solution, they keep their normal, round shape — everything is in balance.

• In a hypotonic solution, water rushes in, making the cell swell — and if the difference is too extreme, the cell can burst (a process called hemolysis).

Figure \(\PageIndex{1}\): Changes in Cell Shape Due to Dissolved Solutes. Osmotic pressure changes the shape of red blood cells in hypertonic, isotonic, and hypotonic solutions.

⚠️ One Final Note

Just because a solution looks “clear” or “cloudy” does not tell you how concentrated it is. Osmolarity depends on the number of dissolved particles, not how the solution looks. You could have a crystal-clear solution that has tons of dissolved solutes—and therefore a very high osmolarity!