tonicity (hypotonic, hypertonic, isotonic)
/ toh-NIH-sih-tee /
If you have ever wondered why a fish-tank shop sells different salts for freshwater and marine fish, you have already met tonicity. Tonicity is a simple way of describing whether a surrounding fluid will make a cell swell, shrink, or stay the same — that is, which way water will flow by osmosis when you drop a cell into it. It compares the outside solution to the inside of the cell.
There are three words to learn. A hypotonic solution is more watery (fewer dissolved particles) than the cell, so water rushes in and the cell swells — and an animal cell can burst. A hypertonic solution is saltier (more particles) than the cell, so water rushes out and the cell shrivels. An isotonic solution has the same effective particle concentration as the cell, so water moves in and out equally and the cell keeps its size. The prefixes are just Greek for under, over, and equal.
Getting this right is a matter of life and death in medicine. Fluids dripped into a patient's vein are usually made isotonic with blood, so red blood cells neither burst nor shrink. Pure water given by IV would be hypotonic and could pop blood cells; very concentrated salt would be hypertonic and shrivel them. The same logic explains why salting meat or sugaring jam preserves it: the hypertonic coating sucks water out of any bacteria, leaving them too dry to grow.
A red blood cell dropped into pure water (hypotonic) swells and bursts; the same cell in strong brine (hypertonic) shrinks and crinkles; in saline matched to blood (isotonic) it stays perfectly normal.
Swell, shrink, or stay — tonicity decides which.
Plant cells behave differently: their stiff cell wall stops them bursting in hypotonic water, so they go firm and turgid rather than popping — which is exactly why wilted lettuce crisps up in cold water.