When the solute concentration of the cell and its surrounding medium are the same, the solution is said to be

1. Analyze the question The question asks what we call a situation where the solute concentration inside a cell equals the solute concentration of the surrounding solution. This is a topic in osmosis and cell transport.

2. Explain the concept The prefix tells you the comparison between the surrounding solution and the cell:

• Hypertonic: the surrounding solution has a higher solute concentration than the cell. Water leaves the cell, so it shrinks (an animal cell crenates, a plant cell becomes plasmolysed).
• Hypotonic: the surrounding solution has a lower solute concentration than the cell. Water enters the cell, so it swells (an animal cell may burst, a plant cell becomes turgid).
• Isotonic: the solute concentrations are equal. There is no net movement of water. Water still moves in both directions, but at the same rate.
• Acidic: this refers to pH (a high concentration of H⁺ ions), not to solute balance across a membrane.

3. Step-by-step solution

• Step 1: The question says the concentrations are the same inside and outside the cell.
• Step 2: "Iso-" means equal, and "tonic" refers to solute concentration or tension.
• Step 3: Hypertonic and hypotonic both describe unequal concentrations, so we can eliminate A and B.
• Step 4: Acidic has nothing to do with solute concentration balance, so we can eliminate D.
• Step 5: The only option that fits is isotonic.

4. Answer The correct option is C. isotonic.

5. Exam tip Remember that iso = equal (like "isosceles" triangles with equal sides). For hypertonic and hypotonic, think of the prefix as describing the outside solution: hyper = more solute outside, so water rushes out of the cell, and hypo = less solute outside, so water rushes in.