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We're learning about osmoregulation in AP Biology and the terms Tonicity and Osmolarity are really confusing me. I watched this video on Khanacademy to try to understand what the difference is, and from my understanding Tonicity refers to strictly two solutions (ex: you can't take a cup with water and salt in it and claim it's hyper, hypo, or iso- tonic because there's nothing to compare it to), whereas Osmolarity refers to the composition of the single solution itself.

But then my Campbell's biology textbook completely confuses me. It has the following picture and text:

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...osmosis, a special case of diffusion, is the movement of water across a selectively permeable membrane. It occurs whenever two solutions separated by the membrane differ in osmotic pressure, or osmolarity (total solute concentration expressed as molarity, that is, moles of solute per liter of solution).....If two solutions separated by a selectively permeable membrane have the same osmolarity, they are said to be isoosmatic. Water molecules continually cross the membrane, but under these conditions they do so at equal rates in both directions....

And then as if to clarify, they claim that tonicity refers to "solutions of known solute concentrations"...but how does this differ from osmolarity?

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You're correct that tonicity needs two solutions to define.

Osmolarity (or osmotic concentration) is the measure of solute concentration, defined as the number of osmoles of solute per litre (L) of solution (Osm/L).

Tonicity, on the other hand, refers to the relative concentration of two solutions separated by a semipermeable membrane.

The difference is based what is considered for osmosis and tonicity. In case of osmosis, it is the water (or solvent) which moves across the membrane, while tonicity depends on the solutes which cannot move across the membrane. From Wikipedia:

... osmolarity takes into account the total concentration of penetrating solutes and non-penetrating solutes, whereas tonicity takes into account the total concentration of only non-penetrating solutes.

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