A solution is isotonic with another solution if it neither to gains water from, nor loses water to, the second solution when separated from it by a semipermeable membrane. This is the case when the solution has the same osmotic concentration as the second solution, For that reason such solutions are sometimes called isosmotic. The membrane is assumed to be freely permeable to the solute (usually water) but impermeable, or nearly so, to substances in solution or suspension in the two fluids. Growth medium used to culture animal cells must be isotonic with the protoplasm of the cells to avoid damage. A solution of 0.9% sodium chloride, sometimes called "physiological saline," is isotonic to human cells.