Osmosis
Osmosis is the movement of water molecules from a region of higher water concentration to a region of lower water concentration, through a partially permeable membrane.
Key terms
- Partially permeable membrane: lets small molecules like water through, but not larger solute molecules.
- Water potential: a measure of the tendency of water to move out of a solution. Pure water has the highest water potential.
- Water moves from a region of higher water potential to a region of lower water potential.
Osmosis in animal cells
- In a hypotonic solution (more dilute than the cell): water moves in, the cell swells and may burst (lysis).
- In a hypertonic solution (more concentrated than the cell): water moves out, the cell shrinks (crenation).
Osmosis in plant cells
- In a hypotonic solution, the cell wall resists the pressure instead of bursting. The cell becomes turgid.
- In a hypertonic solution, the cell membrane pulls away from the cell wall. This is plasmolysis.
Worked example
Question: A potato chip is placed in a concentrated sugar solution. Predict and explain what happens to its mass.
Answer: The sugar solution has a lower water potential than the cells inside the potato. Water moves out of the potato's cells, by osmosis, down the water potential gradient. The potato chip loses mass.
Common mistake: saying water moves from low concentration to high concentration. Water moves down its own concentration gradient, from high water concentration to low water concentration, the opposite direction to how the solute's concentration is usually described.
