Difference Between Tonicity and Osmolarity

Tonicity vs Osmolarity

Osmolarity is the measure of osmotic pressure of a solution. In simpler terms, it is roughly the measure of the amount of solute in the solution but as dissociation of solute affects its osmolarity. Osmolarity is not a direct measure of molar concentration. Osmolarity is therefore the movement of water from inside to the outside of a cell (and it’s reverse also). Tonicity on the other hand refers to relative concentration of solute particles inside a cell with respect to concentration outside the cell. Thus both tonicity and osmolarity seem to be similar concepts but there are differences that will be talked about in this article.

A solution may contain both penetrating as well as non penetrating solutes. With respect to the terms called tonicity and osmolarity, tonicity is a state created by non penetrating solute only (the molecules that don’t move across the membrane freely). When talking of osmolarity, both penetrating as well as non penetrating solute molecules are taken into consideration. Thus tonicity is effective osmotic pressure while Osmolarity is merely another way of expressing the concentration of a solution.

Two solutions are said to be isosmotic if they have same osmolarity (in other terms, osmotic pressure). If you place two isosmotic solutions side by side in a semi permeable membrane, you will find that since osmotic pressure on both sides is same, there is no net flow of water across the membrane. Tonicity refers to response of cells to the solution in which they are immersed. In hypertonic solution, there is net movement of water out of the cell causing the cell to shrivel. On the contrary, in a hypotonic solution, net flow of water is into the cell which sometimes may result in bursting of cell. Tonicity is a concept that is pertinent only in the context of a cell or a tissue.

Tonicity vs Osmolarity

• Osmolarity and tonicity are similar concepts having subtle differences

• Osmolarity is a measure of solute concentration while tonicity is comparison of osmolarities between a cell and the solution to which it is exposed.