Which term describes a solution with the same solute concentration as inside the cell?

• A. Hypertonic solution
• B. Isotonic solution
• C. Hypotonic solution
• D. Osmotic solution

Answer: (B)

Tonicity describes how the solute concentration outside a cell compares to the inside and how water moves by osmosis. When the outside solution has the same solute concentration as the inside of the cell, the situation is isotonic. In this case, water moves in and out at equal rates, so there is no net water flow and the cell maintains its normal size and function. For example, physiological saline around 0.9% NaCl is isotonic to many human cells, keeping them stable.

If the outside solution has more solute (hypertonic), water leaves the cell and it shrinks. If the outside has less solute (hypotonic), water enters the cell and it may swell or burst. The term “osmotic solution” isn’t a standard way to describe this balance, since it refers to osmosis in general rather than a specific condition of equal concentration.