The expression for osmotic concentration relative to a reference is called?

• A. Tonicity
• B. Osmolarity
• C. Osmolality
• D. Molarity

Answer: (A)

Tonicity is the relative osmotic concentration with respect to a reference, typically the cell’s interior. It describes how a solution will affect cell volume by drawing water across the cell membrane. While osmolarity and osmolality measure the total concentration of osmotically active particles in a solution, they are absolute and don’t account for whether those solutes can cross the membrane. Molarity is simply moles per liter and likewise doesn’t address osmotic effects. Because tonicity depends on which solutes are impermeant and thus contribute to water movement, it’s the term that expresses osmotic concentration relative to a reference.