Which term means the solution and the cell are equal?

• A. Law
• B. Constant
• C. Polarity
• D. Isotonic

Answer: (D)

Tonicity describes how the solute concentration outside a cell compares to the inside. When they are equal, the outside solution is isotonic to the cell. In that situation, water moves in and out at the same rate, so there is no net change in the cell’s size. This equal balance means the solution and the cell are effectively the same in terms of solute concentration. If the outside were less concentrated, water would enter the cell and it would swell; if the outside were more concentrated, water would leave and the cell would shrink. Terms like law, constant, or polarity don’t describe this balance of concentrations, so they don’t fit.