What best describes an isotonic solution?

• A. Higher concentration of impermeant solutes outside the cell
• B. Same concentration of impermeant solutes outside the cell as inside
• C. Lower concentration of solute particles outside the cell
• D. Higher osmotic pressure than inside the cell

Answer: (B)

An isotonic solution is characterized by having the same concentration of impermeant solutes outside the cell as there is inside the cell. This balance of solute concentration means that there is no net movement of water across the cell membrane, as the osmotic pressure inside and outside the cell is equal. In this scenario, the cell's shape and function remain stable since it is neither losing water to the external environment nor taking in excessive water that could lead to swelling and potential lysis.

In contrast, a solution with a higher concentration of impermeant solutes outside the cell would create a hypertonic environment, leading to water moving out of the cell. Conversely, a lower concentration of solute particles outside the cell represents a hypotonic solution, where water would move into the cell. Lastly, the option suggesting a higher osmotic pressure than inside the cell indicates a hypertonic condition, which would disturb the cell's integrity. The isotonic solution’s balance is crucial for maintaining cellular homeostasis and proper physiological function.