Osmosis Calculator
Calculate osmotic pressure, water potential, and water movement direction across semipermeable membranes. Determine if a cell will swell, shrink, or remain unchanged in a given solution.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Osmosis Calculator | — | Ψ = Ψs + Ψp (water potential) | atm or MPa |
Step-by-Step Examples
Cell inside: 300 mM. Solution outside: 150 mM.
- Inside osmotic pressure = 300/1000 x 0.08206 x 310 = 7.63 atm
- Outside pressure = 150/1000 x 0.08206 x 310 = 3.81 atm
- Net: water moves INTO cell (cell swells)
- In animal cells: lysis risk. In plant cells: turgor pressure builds
Cell: 300 mM inside. Saline: 600 mM outside.
- Higher outside osmotic pressure
- Water flows OUT of cell
- Cell shrinks (crenation in animal cells, plasmolysis in plants)
Cell: 300 mM inside. Solution: 300 mM outside.
- Osmotic pressures equal
- No net water movement
- Cell maintains normal size and shape
- Normal saline (0.9% NaCl = 308 mM) is approximately isotonic
Real-World Applications
Common Mistakes to Avoid
Water moves from low solute (high water concentration) to high solute (low water concentration). No energy required.
Osmolarity: total solute concentration. Tonicity: effective osmolarity (only non-penetrating solutes). Urea penetrates cells and does not contribute to effective tonicity.
Cell walls prevent lysis in hypotonic solutions. Turgor pressure builds up as water enters. At full turgor: no more net water entry. Animal cells lack wall: lysis in hypotonic.