Summary
Describes how an external solution affects cell volume: hypotonic causes swelling, hypertonic causes shrinking.
In depth
In an isotonic solution there is no net water movement. In hypotonic surroundings water enters: animal cells swell and may lyse, plant cells become turgid. In hypertonic surroundings water leaves: animal cells shrivel (crenation) and plant cells undergo plasmolysis.
This explains why intravenous fluids are made isotonic and why drinking seawater dehydrates you.
Tonicity depends only on solutes that cannot cross the membrane, unlike osmolarity, which counts all dissolved particles. Urea has high osmolarity but is hypotonic or isotonic to cells because it crosses readily. Saline of 0.9 percent NaCl is isotonic with human blood, and 5 percent glucose is initially isotonic but becomes effectively hypotonic once cells metabolise the sugar.
Plant cells in hypotonic solution do not burst because the wall resists, and cells in hypertonic soil can lose water, which is why over-fertilising or salting land damages plants.
Example: marine fish drink seawater and excrete salt through gills, whereas freshwater fish take in water and excrete dilute urine.
Frequently asked questions
What is Tonicity?
Describes how an external solution affects cell volume: hypotonic causes swelling, hypertonic causes shrinking.
Which area of biology covers Tonicity, and how advanced is it?
Tonicity is one of the 52 topics in Cell Biology. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include Osmosis, Vacuole, Simple Diffusion.
Can you give an example of Tonicity?
Paramecium uses a contractile vacuole to survive in a hypotonic pond.
What should I learn before Tonicity?
Work through the Beginner topics in Cell Biology first, such as Cell Theory, Prokaryotic Cell, Eukaryotic Cell, then return to Tonicity. Intermediate topics assume the Beginner vocabulary.
What topics are related to Tonicity?
The closest topics to Tonicity are Osmosis, Vacuole, Simple Diffusion, Facilitated Diffusion. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Tonicity?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Tonicity. To practise the whole of Cell Biology, use the Cell Biology quiz.
How is Tonicity different from Osmosis?
Tonicity: Describes how an external solution affects cell volume: hypotonic causes swelling, hypertonic causes shrinking. Osmosis: The diffusion of water across a selectively permeable membrane from lower to higher solute concentration. Read both side by side and note what each one explains.
How should I study Tonicity step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Osmosis and explain the difference aloud. Finally, take the Cell Biology quiz and come back to this page for any question you miss.