hypotonic

"Hypotonic" comes from Greek roots meaning "under tension." While that sounds dramatic, it simply refers to the relative concentration of solutes (like salt or sugar) in a solution—usually compared to another solution, like the fluid inside a cell. A hypotonic solution has a lower solute concentration than the liquid it is being compared to.

This difference in concentration creates what is known as osmotic pressure. Water naturally moves from areas of high water concentration (low solute) to areas of low water concentration (high solute) to achieve equilibrium—a process called osmosis. Therefore, if you place a cell in a hypotonic solution, water will move into the cell.

This is hugely important in biology! Plant cells actually need a hypotonic environment; the incoming water creates "turgor pressure," which helps the plant stay rigid and stand upright. Animal cells, however, are more delicate. Because they lack a sturdy cell wall, placing them in a strongly hypotonic solution can cause them to swell like balloons and burst, a process called lysis.

You might also hear "hypotonic" used when discussing intravenous (IV) fluids given to patients. Doctors carefully choose the tonicity of the fluid to ensure it helps the body without damaging blood cells. For instance, a hypotonic IV fluid may be used in some cases when water needs to shift back into cells, though this has to be managed carefully.

There is also another medical meaning. When doctors describe muscles, posture, or body tissue as hypotonic, they mean it has low tone. A hypotonic infant, for example, may seem floppy because the muscles are not resisting movement as firmly as expected. In this sense, the word is not about salt levels in a liquid but about reduced muscular tension or firmness.

So, depending on the field, "hypotonic" usually points to one of two ideas: a solution with lower solute concentration than another solution, or muscles and tissue with lower than normal tone. The context makes the meaning clear.