Molecular Biology ★☆☆ Easy

💧 Osmosis & Turgor Pressure

Place a cell into hypo-, iso-, or hypertonic solution and watch water molecules flow through the semipermeable membrane. Observe turgor pressure, plasmolysis, and lysis in real time. Adjust solute concentration and temperature.

Presets:
50 mOsm
280 mOsm
25 °C
1.0×
Isotonic
Net flow: 0.0 mol/s Cell vol.: 100% Turgor P: 0.0 atm ΔΨ: 0.0 mOsm

How Osmosis Works

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 — the osmotic equilibrium.

Key Concepts

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.

About this simulation

This simulation models a plant cell placed in solutions of varying solute concentration, and tracks water movement across its semipermeable membrane. The driving force is the concentration gradient between the fixed internal solute level and the external solute you set, corrected for the cell's own volume and any turgor pressure already built up. 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.

🔬 What it shows

A plant cell with a rigid cell wall, an inner vacuole, and a semipermeable membrane sitting in solution. Individual water dots cross the membrane probabilistically based on the net driving force, while solute dots stay fixed on their own side. As net water flow changes cell volume, a wall-resistance term generates turgor pressure once volume exceeds 100%, and the state badge switches between hypotonic, isotonic, hypertonic, plasmolysis, and lysis.

🎮 How to use

Drag External solute concentration (0–600 mOsm) against the fixed Cell solute concentration (50–500 mOsm) to set the direction and strength of osmotic flow. Temperature (0–50°C) speeds up molecular jitter, and Membrane permeability (0.1×–3×) scales how fast water actually crosses. Try the five presets — Pure Water, Hypotonic, Isotonic, Hypertonic, and Extreme Hyper — then press Play and watch the Cell Volume graph and stats bar (net flow, volume %, turgor, and ΔΨ) evolve toward equilibrium.

💡 Did you know?

Real plant cells rarely burst even in pure water because the cell wall pushes back once turgor pressure rises, exactly as modelled here (capped at 8 atm). That same turgor pressure is what keeps a healthy houseplant's stem rigid — when a plant wilts, its cells have lost enough water that turgor has dropped toward zero.

Frequently asked questions

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. In this simulation the driving force is calculated as the internal solute concentration (corrected for current cell volume) minus the external concentration you set, minus a turgor-pressure term that pushes back once the cell has swollen. When that number is positive, water flows in; when negative, water flows out; at zero the cell is at osmotic equilibrium.

What do hypotonic, isotonic, and hypertonic mean here?

Hypotonic means the external solute concentration is lower than the cell's, so water flows in and the cell swells — in this simulation that raises the volume percentage and eventually turgor pressure. Isotonic means the two concentrations roughly match, so there is little or no net flow and the volume stays near 100%. Hypertonic means the external concentration is higher, so water leaves the cell and it shrinks. The colored state badge automatically switches between these three labels as you adjust the sliders.

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; the simulation models it as increasing linearly above 100% volume, capped at 8 atmospheres to represent the wall's mechanical limit. Plasmolysis occurs when the cell shrinks below 55% of its normal volume in a strongly hypertonic solution, causing the membrane to pull away from the wall. Lysis occurs when volume exceeds 145% in a strongly hypotonic solution, representing the membrane bursting under unchecked swelling.

How do the sliders actually change the simulation?

External and Cell solute concentration set the two sides of the concentration gradient in milliosmoles, directly determining the driving force and its sign. Temperature increases the random thermal jitter speed of the animated water molecules, reflecting that diffusion is faster at higher temperature. Membrane permeability multiplies how quickly water molecules actually cross once they reach the membrane, so a low value slows the whole process down even with a large concentration difference, while a high value speeds equilibrium up.

Why does the cell volume graph level off instead of increasing forever?

As water enters a swelling cell, its internal solute becomes more dilute (the same fixed amount of solute spread through a larger volume), which shrinks the effective internal concentration term in the driving-force calculation. At the same time, turgor pressure rises and is subtracted from the driving force. Both effects reduce the net inward flow as volume increases, so the curve naturally flattens out at osmotic equilibrium rather than growing without bound — unless the imbalance is severe enough to trigger lysis first.