D2.3.3—Osmosis into/out of cells
Osmosis into or out of cells depends on relative osmolarity, with isotonic conditions causing no net water movement across membranes.
- Syllabus
- First assessment 2025
- Objective
- D2.3.3
- Level
- HL
Use Tonicity to Predict Osmosis in Cells
Osmosis is the net movement of water across a partially permeable membrane, and the cell's environment determines its direction.
| External environment | Relative external solute concentration | Net water movement |
|---|---|---|
| Hypotonic | Lower than inside | Into the cell |
| Hypertonic | Higher than inside | Out of the cell |
| Isotonic | Equal effective concentration | No net movement |
In an isotonic environment, water molecules continue crossing in both directions at equal rates. This is dynamic equilibrium, not an absence of molecular movement.
A cell placed in hypertonic solution loses water and decreases in volume because more water leaves than enters.
Always state the solution relative to the cell. 'Hypotonic' or 'hypertonic' without a comparison has incomplete meaning.
Osmosis into/out of cells
Assessment in practice
3 marksHow it is assessed
This objective is assessed through structured response, commonly using Explain.
Command terms
Explain
What earns marks
Build the answer around this relationship: Hypotonic external solutions cause net water entry into cells.
Representative question
Question 1
[Maximum number: 3]
Explain the reason that animal cells and tissues under investigation must be maintained in solutions with the same osmolarity.
| a | to prevent osmosis; |
| b | cells placed in the incorrect osmolarity might swell/burst/shrink; |
| c | hypotonic solution would cause water to enter cells/tissues; |
| d | hypertonic solutions would cause water to leave cells/tissues; |
| e | water loss would hinder (metabolic) reactions in cell cytoplasm OR distort appearance of the cells (for the investigation); |
Core Osmosis Effects
Water forms hydration shells around ions and polar solutes; hydrogen bonding and charge attraction reduce free water movement. Water moves by osmosis across partially permeable membranes from hypotonic/lower solute solutions toward hypertonic/higher solute solutions. Osmosis direction depends on internal and external solute concentration; isotonic conditions have dynamic water movement but no net osmosis. Plant tissue changes mass or length in sucrose solutions; percentage change graphs estimate isotonic or osmotic concentration. Animal cells can lyse in hypotonic solutions and crenate in hypertonic solutions; freshwater protists use contractile vacuoles to expel excess water. Plant cells become turgid in hypotonic solutions as vacuoles swell; hypertonic solutions cause flaccidity and plasmolysis from water loss. Isotonic saline prevents harmful water gain or loss in body cells; IV fluids and transplant organ baths must match tissue osmotic concentration.
Concept essentials
- Hypotonic external solutions cause net water entry into cells.
- Hypertonic external solutions cause net water loss from cells.
- Isotonic conditions produce no net osmosis.
- Animal tissues are maintained in matching osmolarity to preserve cell structure.