What is the definition of osmolarity?

• A. Concentration of permeant solutes
• B. Concentration of solute particles in a solution
• C. Pressure needed to stop osmosis
• D. Equal concentration of solute inside and outside the cell

Answer: (B)

Osmolarity is defined as the concentration of solute particles in a solution. It quantifies the total number of dissolved particles—such as ions, molecules, or compounds—in a liter of solution. This measurement is crucial in physiological contexts because it affects how water moves across cell membranes through osmosis. Understanding osmolarity helps in grasping how cells maintain their shape and function by regulating the internal environment and adapting to changes in external conditions.

The option regarding the concentration of permeant solutes does not represent osmolarity accurately, as it focuses specifically on solutes that can cross cell membranes rather than encompassing all dissolved particles. The pressure needed to stop osmosis pertains to osmotic pressure, which is a different concept related to the movement of water rather than the concentration of solute particles. The idea of equal concentration inside and outside the cell describes isotonic conditions but does not define osmolarity itself, which can vary even in isotonic solutions.