Isotonic solution is defined as a solution with equal solute concentration to the cell interior.

• A. A solution with lower solute concentration
• B. A solution with no solutes
• C. A solution with equal solute concentration to the cell interior
• D. A solution with higher solute concentration

Answer: (C)

Tonicity is about how the outside solution compares to what's inside the cell. An isotonic solution has the same solute concentration as the cell interior, so there’s no net movement of water across the membrane; the cell stays the same size because water leaves and enters at the same rate.

If the outside has fewer solutes (lower concentration), it’s hypotonic to the cell, and water rushes into the cell, potentially causing swelling. If there are no solutes, that’s even more hypotonic, again driving water into the cell. If the outside has more solutes (higher concentration), it’s hypertonic, and water leaves the cell, causing it to shrink. So the equal concentration that defines isotonicity is the correct concept.