= Tonicity
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<Tonicity> describes how a surrounding <chemical solution> changes cellular <volume> through <osmosis>. It depends on which <solutes> remain effectively excluded on the timescale of interest, not simply on the amount of dissolved material. The following comparisons take a functional <cell membrane>, an initially unpressurized <biological cell>, and effectively excluded <solutes> as the reference; rising <turgor pressure> can oppose entry in a walled <biological cell>.
Operationally, <hypotonic solutions> cause sustained swelling, <hypertonic solutions> sustained shrinking, and <isotonic solutions> no sustained <volume> change in the specified passive comparison. State the reference <biological cell>, membrane properties and observation interval. A <permeating solute> can transiently change <water potential> while crossing; after its <chemical potential> equilibrates, its initial unequal <concentrations> cannot be treated as a permanently retained osmotic load. Thus equal <osmolarities> need not imply <isotonic solutions>. Conversely, a stationary walled <biological cell> may have unequal <osmolarities> balanced by <turgor pressure>.<Osmosis, Cell Membranes-Structure and Transport, BYU-Idaho>[Cell Membranes—Structure and Transport, Osmosis: paragraphs beginning Here is another way and final Note; permeating solutes and transient effects]{ref}
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