Final answer:
Isotonicity and Isosmolarity are not equivalent. Isotonic solutions prevent net water movement across a cell membrane due to equal solute concentration, maintaining cell size. Isosmolarity refers to the total solute concentration in a solution.
Step-by-step explanation:
Isotonicity and Isosmolarity are not equivalent. Although related, these terms describe different aspects of a cell's environment. An isotonic solution means the extracellular fluid has the same osmolarity as the cell's intracellular fluid, resulting in no net movement of water across the cell membrane. Water molecules still move in and out, but at an equal rate, maintaining the cell's volume. Isosmolarity specifically refers to solutions having the same total solute concentration.
In an isotonic condition, solute and solvent concentrations are equal across the cell membrane, meaning that there is no net water movement and no change in the size of the cell. However, osmolarity is broader and can describe solutions regardless of their effects on specific cells. Thus, isotonicity is a special case among isosmolar solutions, where the balance specifically prevents cells from swelling or shrinking.