Cell membrane — water molecules crossing under the osmotic gradient
Cell volume over time
Water moves across a semipermeable membrane from a region of low solute concentration (high water potential Ψ) to high solute concentration (low Ψ). The driving force is the water-potential difference ΔΨ = −iCRT, where C is solute concentration, R the gas constant, and T absolute temperature. Net flow stops when turgor pressure exactly balances the osmotic gradient — osmotic equilibrium.
This simulation models a plant cell placed in solutions of varying solute concentration, and tracks water movement across its semipermeable membrane. Water molecules are animated crossing the membrane individually, cell volume and turgor pressure update every frame, and a live graph plots volume over time as the system heads toward osmotic equilibrium.
Hypotonic solution: external conc. < internal conc. Water enters the cell — the cell swells. In animal cells this causes lysis; in plant cells the rigid cell wall builds turgor pressure.
Hypertonic solution: external conc. > internal conc. Water leaves the cell — the cell shrinks. Plant cells undergo plasmolysis (membrane detaches from wall). Animal cells undergo crenation.
Isotonic solution: equal concentrations — no net flow. Red blood cells are in osmotic equilibrium with blood plasma at ~280–310 mOsm/L.
What is osmosis and what drives it in this simulation?
Osmosis is the net movement of water across a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration. The driving force is the internal solute concentration (corrected for current cell volume) minus the external concentration, minus a turgor-pressure term that pushes back once the cell has swollen.
What do hypotonic, isotonic, and hypertonic mean here?
Hypotonic means the external concentration is lower than the cell's, so water flows in and the cell swells. Isotonic means the concentrations roughly match, so there is little net flow. Hypertonic means the external concentration is higher, so water leaves the cell and it shrinks.
What are turgor pressure, plasmolysis, and lysis?
Turgor pressure is the outward push of the cytoplasm against the rigid cell wall once the cell swells past its normal volume, capped at 8 atmospheres. Plasmolysis occurs below 55% of normal volume; lysis occurs above 145% volume.
How do the sliders actually change the simulation?
External and Cell solute concentration set the concentration gradient in milliosmoles. Temperature increases thermal jitter speed. Membrane permeability multiplies how quickly water molecules cross once they reach the membrane.
Why does the cell volume graph level off instead of increasing forever?
As water enters, the internal solute becomes more dilute and turgor pressure rises, both reducing the net inward flow, so the curve flattens at osmotic equilibrium unless the imbalance triggers lysis first.