Hypotonic Solution — Definition & Examples (Cells)

Written by

Malcolm McKinsey

January 12, 2023

Edited by

Courtney Adamo

Fact-checked by

Paul Mazzola

What is a hypotonic solution?

A hypotonic solution is a solution with a lower concentration of solutes than another solution. In biology, hypotonic solutions carry across semipermeable membranes—plant cell walls and animal cells—to infuse the cells with fluids.

A hypotonic solution is also a solution with lower osmotic pressure than another solution. Osmotic pressure is the hydrostatic pressure between solutions separated by a semipermeable membrane, like a cell wall.

The word hypotonic is an adjective exclusively linked to solutions, and has two parts: hypo, meaning “less” or “less than,” and tonic, meaning “stretching.”

Hypotonic solution diagram

Here is a diagram showing molecules flowing from hypotonic solutions into cells:

Hypotonic solution examples

In medicine, hypotonic solutions include:

• Saline 0.45 percent (less than half the normal saline level found in blood)

• Saline 0.25 percent with or without dextrose (a quarter of normal saline level)

• 5 percent or 2.5 percent dextrose

• Pure distilled water

These hypotonic solutions are administered under careful supervision by medical professionals to restore normal fluid balance within the human body.

Water is an example of a hypotonic solution found in nature. The tap water, bottled spring water, and filtered water you drink every day are all hypotonic solutions.

What are hypotonic solutions used for?

Medical professionals administer hypotonic solutions intravenously to prevent dehydration by delivering large quantities of pure fluids with minimal solutes to the bloodstream.

Rainwater acts as a hypotonic solution to keep plants sufficiently hydrated. Rainwater has a lower concentration of solutes than the plant cell’s cytosolic fluid, so rainwater is hypotonic compared to the cytosolic fluid within plant cells.

The concentration difference, or concentration gradient, between the rainwater and the plant's cytosol causes a flow of water molecules through the cell membrane by diffusion.

What happens to a cell in hypotonic solution?

Both plant and animal cells are eukaryotic cells. They both absorb hypotonic solutions through the cell wall, causing the cell to swell. However, plant cells can regulate their intake; animal cells cannot.

Hypotonic solutions are always in comparison to solutions in proximity to them, and the most useful places to study comparative solutions are in cells: plant, animal, and human red blood cells.

A plant cell in hypotonic solution

In a hypotonic solution, water moves into the plant cells through the cell wall. The plasma membrane will swell without bursting the rigid cell wall and become turgid (stiff). This allows the cell to regulate water consumption and survive dry spells.

Both plants and fungi regulate water intake to hold as much water as possible in their cells.

An animal cell in hypotonic solution

When exposed to a hypotonic solution, animal cells will absorb the hypotonic solution (often water) and then burst. Animal cells do not have rigid cell walls like plants and the cell wall is unable to prevent permeation by the hypotonic solution.

A human red blood cell in hypotonic solution

Human blood cells are animal cells, so they can absorb too much hypotonic solution and burst.

Too much hypotonic solution can overwhelm cells and cause:

• Cerebral edema (swelling of the brain)

• Hypovolemia (lowered levels of extracellular fluid)

• Hypotension (low blood pressure)

• Lysis (cell death due to cell walls bursting)

Hypertonic vs. hypotonic solution

Hypertonic and hypotonic solutions are comparatives and the amount of solute in the solutions determines which is hypotonic and hypertonic. The solution with a lower solute concentration is hypotonic. The solution with a higher concentration of the solute is hypertonic. Two solutions with an equal amount of solute are isotonic.

• High concentration = hypertonic

• Low concentration = hypotonic

Hypertonic solutions in medicine and biology have higher concentrations of electrolytes than cytosolic fluids or blood plasma. Isotonic solutions have equal concentrations of electrolytes as cytosolic fluids or blood plasma.

Both hypertonic and hypotonic solutions are comparatives, so any solution identified as hypotonic

Hypotonic quiz

The concept of hypotonic solutions can be confusing since they are always comparatives. See if you have a basic understanding by answering these four questions.

1. In your own words, what is a hypotonic solution?

2. What happens when you expose a plant cell and an animal cell (like a human blood cell) to a hypotonic solution?

3. Do hypotonic solutions affect plant life?

4. Please highlight the differences between hypertonic, isotonic, and hypotonic solutions in the medical field.

If you had problems answering these, please review the article and do further investigations on your own, then try again.

1. You may have said a hypotonic solution is any solution with a lower concentration of solutes compared to a surrounding or nearby solution.

2. A plant cell in a hypotonic solution will swell up and become turgid, while an animal cell will swell up and burst (lyse).

3. Hypotonic solutions keep all plant life sufficiently hydrated, since rainwater, irrigation water, and all precipitation is hypotonic compared to the fluids inside plant cells.

4. In medicine, hypertonic solutions have higher concentrations of solutes (electrolytes) than normal blood plasma, isotonic solutions have concentrations equal to normal blood fluids, and hypotonic solutions have lower concentrations of electrolytes than normal blood fluids.