Osmolarity measures the total concentration of dissolved particles that contribute to osmotic behavior. One osmole (Osm) corresponds ideally to one mole of dissolved particles, regardless of their chemical identity.

A solute that dissociates into several particles contributes more osmotic particles per mole than a solute that remains intact.

For an idealized dilute solution:

• $0.10,\mathrm{mol/L}$ glucose contributes about $0.10,\mathrm{Osm/L}$ because glucose remains as intact molecules;
• $0.10,\mathrm{mol/L}$ NaCl contributes close to $0.20,\mathrm{Osm/L}$ because it dissociates into approximately one $\mathrm{Na^+}$ and one $\mathrm{Cl^-}$ per formula unit.

Real solutions can deviate from this ideal particle counting, especially at higher concentrations.

Tonicity is different. It describes how a solution changes the volume of a cell or other membrane-bounded compartment over the relevant time scale. Tonicity depends mainly on solutes that do not readily cross the membrane.

Relative to a cell:

• an isotonic solution causes no sustained net change in cell volume;
• a hypertonic solution draws water out, shrinking the cell;
• a hypotonic solution drives water in, swelling the cell.

Why equal osmolarity need not mean equal tonicity

Suppose a cell membrane is highly permeable to urea but not to a large intracellular solute. An external urea solution could initially have the same osmolarity as the cytosol. Urea can nevertheless enter the cell, so it does not maintain the same long-term osmotic difference as an impermeant extracellular solute. Water can follow, changing cell volume.

Thus osmolarity counts particles, whereas tonicity asks which particles remain effectively confined and therefore sustain water movement.

Cells with rigid walls respond differently from animal cells. Water entry into a plant cell generates pressure against the cell wall, producing turgor pressure rather than unrestricted swelling. An animal cell in a sufficiently hypotonic solution can swell severely and may rupture.

Predicting cell-volume change therefore requires both solution composition and membrane permeability; comparing total solute concentration alone is not enough.