Hypotonic Solutions: A Basic Explanation and List of Examples

If you put a cell into a hypotonic solution, it will swell and bloat. But why? Hypotonic solutions have lower solute concentrations, such as salt and electrolytes, than the cells inside of them. Water from the solution crosses the cell membrane and enters the cell. You'll find hypotonic solutions in the lab, in the hospital and in your everyday life. Learn more about how they work with these hypotonic solution examples.

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Plant Environments

Plants are built to maintain a hypotonic solution in the soil and environment around them. When you water a garden, water flows from the wet soil into the plant cells through the roots. Plant cells become turgid (swollen) but don't burst, thanks to their strong cell walls. This allows plants to survive periods without water and still maintain an ideal structure for photosynthesis.

Fungal Environments

Environments where species of fungi live are also hypotonic. Like plant cells, fungus cells have strong walls that keep them from bursting after they take in water. If you put a mushroom in a cup of water, for example, it would absorb the water around it and become mushy and swollen. That's how it works in the soil as well — fungi cells take in the water from the soil, ensuring that the environment is always hypotonic.

Extracellular Fluid in a Hydrated Body

When you take a big drink of water, that water has to go somewhere. And it does — into your cells! Humans, like all animals, become dehydrated if their extracellular (outside the cell) fluid has more solutes than their cells do. Cells take in the water from extracellular fluid and become saturated. Human and animal cells don't have structured cell wells like plants do, so too much water can cause them to burst. That's why a well-hydrated body has slightly more water in the extracellular fluid than in the cells themselves.

IV Drips and Injections

Doctors and nurses use intravenous (IV) drips and injections to rehydrate their patients all the time. Take a look at these common hypotonic solutions that are meant to replenish water in cells.

Half-Normal Saline

Half-normal saline, or 0.45% sodium chloride (NaCl) dissolved in sterile water, is a hypotonic solution used in IV drips. It has half the saline of normal saline (0.9% NaCl), which is used as a maintenance fluid because its composition is similar to plasma in the body. It's effective for patients who are mildly dehydrated, malnourished, burned, or experiencing diabetic ketoacidosis. Half-normal saline provides dehydrated cells with necessary fluid.

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Quarter-Normal Saline

Quarter-normal saline includes 0.22% sodium chloride and 5% dextrose. It has half the amount of sodium chloride as half-normal saline, making it the most hypotonic form of saline available. Quarter-normal saline is typically used in neonatal intensive care units and pediatric cases.

5% Dextrose in Water (D5W)

Medical professionals may also use 5% dextrose in water, known as D5W or D5. The hypotonic solution dilutes extracellular fluid, providing free water for the kidneys in renal patients. Once the body's cells have absorbed the sugar (dextrose), the remaining water can remain in the extracellular fluid to maintain balance.

What's the Difference Between Hypotonic and Hypertonic?

It can be easy to mix up the terms hypotonic and hypertonic since they sound so similar. But these solutions work in opposite ways.

You now know that water flows into a cell when it is immersed in a hypotonic solution, but in a hypertonic solution, there are more solutes in the solution than in the cell. Water flows from the cell into the solution in an effort to achieve homeostasis. Isotonic solutions have the same amount of solutes as the cells inside of them.

If you put a cell into each solution, it would:

  • hypotonic solution - bloat, swell and/or burst (water flows in)
  • hypertonic solution - shrivel and dehydrate (water flows out)
  • isotonic solution - remain the same (water level is equal)

Of course, tonicity depends on your perspective. When you put a mushroom into a solution and its cells bloat, you know the solution is hypotonic. But it's only hypotonic compared to the mushroom, which has more solutes than the solution. If you put a cell with fewer solutes into the solution, the solution is now hypertonic.

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Maintaining Balance All Around Us

Our bodies work all day and night to keep our extracellular and cellular fluids balanced. But there are many biological and chemical factors at play that keep us healthy and active. Learn the difference between osmosis and diffusion in biology for more examples of transport processes at work. You can also identify the various ways plants and animals receive their fluids with these examples of passive and active transports.