Final answer:
An isotonic solution is one in which the extracellular fluid has the same osmolarity as the cell's interior, preventing any net water movement across the cell membrane and maintaining cell shape and function.
Step-by-step explanation:
An isotonic situation occurs when the extracellular fluid has the same osmolarity as the fluid inside the cell. This equilibrium results in no net movement of water into or out of the cell, although individual water molecules will continue to move across the cell membrane. Osmolarity refers to the total amount of substances dissolved in a specific amount of solution, and in this case, is balanced both inside and outside of the cell.
In biological contexts, maintaining an isotonic environment is essential for cells to preserve their normal shape and function. If the solutes' concentration is equal on both sides of a cell membrane, the environment is said to be isotonic. This is critically important in medical applications such as when administering intravenous fluids to ensure that cells do not shrink (as in a hypertonic solution) or swell and possibly burst (as in a hypotonic solution).