Isotonic solution is defined as

• A. Equal concentration outside and inside the cell
• B. Concentrated solutes outside cell
• C. Dilute solutes outside cell
• D. No solutes outside cell

Answer: (A)

Isotonic means equal solute concentration on both sides of the cell membrane, so there is no net movement of water by osmosis. In this case, water flows in and out at the same rate, and the cell’s volume stays stable. For many cells, a practical isotonic solution is about 0.9% salt (NaCl), which matches the osmolarity inside the cells.

Why the other situations aren’t isotonic: if the outside solution has more solute, water moves out of the cell to balance the concentration, causing the cell to shrink (hypertonic). If the outside has fewer solutes, water moves in, making the cell swell and potentially burst (hypotonic). If there are no solutes outside, the outside is effectively pure water, which creates a strong inward water flow.