Which statement about isotonic solutions is true?

• A. They cause water to move into the cell more than out.
• B. They have the same solute concentration as inside the cell.
• C. They cause water to move out of the cell more than in.
• D. They are hypertonic relative to cell fluid.

Answer: (B)

Water movement across a cell membrane is driven by solute differences; isotonic means the outside has the same solute concentration as the inside. Because the concentrations are equal, there’s no net water flow—water moves in and out at the same rate, so the cell stays the same size. That’s why describing isotonic solutions as having the same solute concentration as inside the cell is true. If the outside were hypotonic, water would rush into the cell and it would swell; if the outside were hypertonic, water would leave the cell and it would shrink. An isotonic solution is not hypertonic relative to the cell.