Isotonic conditions are those in which the solution inside and outside the cell are:

Isotonic conditions are those in which the solution inside and outside the cell are:

• A. Equal, causing no net water movement
• B. Higher outside, water leaves
• C. Lower outside, water enters
• D. Unchanging regardless of solutes

Answer: (A)

Isotonic conditions mean the solution outside the cell has the same osmolarity as the cell’s interior, so water moves across the membrane at equal rates in both directions and the cell’s volume stays the same. This balance prevents net water gain or loss, which is why cells maintain their shape in isotonic environments. In practice, this is what keeps cells from shrinking in isotonic saline and from swelling or bursting due to osmotic pressure. If the outside solution is hypertonic (more solutes outside), water leaves the cell to try to balance the concentrations, causing the cell to shrink. If the outside is hypotonic (fewer solutes outside), water enters the cell, leading to swelling and potential lysis. The idea isn’t that solutes can’t change, but that equal osmolarity across the membrane results in no net water movement.

Isotonic conditions mean the solution outside the cell has the same osmolarity as the cell’s interior, so water moves across the membrane at equal rates in both directions and the cell’s volume stays the same. This balance prevents net water gain or loss, which is why cells maintain their shape in isotonic environments. In practice, this is what keeps cells from shrinking in isotonic saline and from swelling or bursting due to osmotic pressure.

If the outside solution is hypertonic (more solutes outside), water leaves the cell to try to balance the concentrations, causing the cell to shrink. If the outside is hypotonic (fewer solutes outside), water enters the cell, leading to swelling and potential lysis. The idea isn’t that solutes can’t change, but that equal osmolarity across the membrane results in no net water movement.