The medium has a lower water concentration than the cell.
Explanation
Osmosis is the diffusion of water across a semi-permeable membrane. Water moves from a region of higher water concentration (lower solute concentration) to a region of lower water concentration (higher solute concentration). The terms hypotonic, hypertonic, and isotonic describe the relative concentration of solutes in the external solution compared to the cell's interior.
Option Analysis:
- The medium has a higher water concentration than the cell. is incorrect: If the medium has a higher water concentration than the cell, it implies a lower solute concentration. This is defined as a hypotonic solution. In this case, water enters the cell, causing it to swell.
- The medium has a lower water concentration than the cell. is correct: A hypertonic solution contains a higher concentration of solutes compared to the cell. Consequently, it has a lower water concentration. Due to the concentration gradient, water moves out of the cell into the external medium (exosmosis), causing the cell to shrink.
- The medium has the same water concentration as the cell. is incorrect: If the medium has the same water concentration (and solute concentration) as the cell, it is an isotonic solution. Water moves across the membrane in both directions at an equal rate, resulting in no net change in cell volume.
- The medium contains no solutes. is incorrect: A medium containing no solutes represents pure water, which has the highest possible water concentration. This creates a strongly hypotonic environment relative to the cell.
Key Takeaway:
A hypertonic solution has a lower water concentration (higher solute concentration) than the cell, leading to the net movement of water out of the cell.