Tonicity is a measure of effective osmolarity or effective osmolality in cell biology. Osmolality an osmolarity are properties of a particular solution, independent of any membrane. Tonicity is a property of a solution in reference to a particular membrane, and is equal to the sum of the concentrations of the solutes which have the capacity to exert an osmotic force across that membrane. Tonicity, also, depends on solute permeability (permeant solutes do not affect tonicity; impermeant solutes do affect tonicity). Tonicity is generally classified in three ranges; hypertonicity, hypotonicity and isotonicity. Hypertonic, isotonic and hypotonic solutions are defined in reference to a cell membrane by comparing the tonicity of the solution with the tonicity within the cell.