Osmosis Simulator — Concentration Gradient
Adjust solute concentrations on each side of a semi-permeable membrane and watch water particles move down the water potential gradient in real time. Pause, adjust, and observe isotonic, hypertonic, and hypotonic scenarios. No signup, runs entirely in your browser.
How to Use the Osmosis Simulator
- 1Set the solute concentration on each side of the central semi-permeable membrane.
- 2Press play and watch water molecules cross while larger solute particles bounce off the membrane.
- 3Observe the net direction — water moves toward the higher-solute (lower water potential) side.
- 4Set both sides equal to see the isotonic case, where crossings balance and volume stays constant.
Worked Example: Why Salted Slug Shrivels and Fresh-Water Fish Bloat
Set the left side (inside a cell) to a moderate solute concentration and the right side (surroundings) much higher — a hypertonic environment, like salt poured on a slug. Run the simulation: water leaves the cell across the membrane because it moves down its water potential gradient toward the saltier side. The cell loses volume — in an animal cell this is crenation, in a plant cell plasmolysis. That is literally why salt dehydrates and preserves food: it makes the outside hypertonic and pulls water out of any microbial cells present.
Now reverse it — make the surroundings nearly pure water (hypotonic), the situation for a freshwater fish or a red blood cell dropped into distilled water. Water floods into the cell. An animal cell with no wall keeps swelling until it bursts (lysis); a plant cell is saved by its rigid wall, becoming firm and turgid instead. The simulator makes the key teaching point visible: osmosis has no “goal” — water simply crosses more often toward the concentrated side until the two water potentials equalize, which is why medical IV fluids must be isotonic to blood.
Osmosis Concepts to Know
Osmosis in plant cells
Plant cells have a cell wall that provides pressure resistance. A turgid cell has high turgor pressure — ideal. Plasmolysis occurs when the cell loses so much water that the cell membrane pulls away from the cell wall, causing wilting.
Osmosis in animal cells
Animal cells lack a cell wall. In a hypotonic solution (lower solute outside), animal cells swell and may burst (lysis). In a hypertonic solution, they shrink (crenation). This is why IV fluids must be isotonic to blood plasma.
Water potential
Water potential (ψ) is measured in pascals. Pure water has ψ = 0. Adding solutes lowers ψ (makes it more negative). Water always moves from higher to lower ψ — this formalises the direction of osmosis and includes pressure potential.
Dialysis tubing experiment
A classic lab uses dialysis tubing (a semi-permeable membrane) filled with sugar solution and placed in water. The bag swells as water enters by osmosis. Measuring mass change at different concentrations gives the solute potential of the contents.
Frequently Asked Questions
What is osmosis?
Osmosis is the net movement of water molecules across a semi-permeable membrane from a region of lower solute concentration (higher water potential) to a region of higher solute concentration (lower water potential), down the water potential gradient.
What is a semi-permeable membrane?
A semi-permeable (selectively permeable) membrane allows small water molecules to pass through but prevents larger solute molecules from crossing. Cell membranes are selectively permeable, which is why osmosis controls cell volume.
What do hypertonic, hypotonic, and isotonic mean?
Hypertonic: the solution has a higher solute concentration than the cell — water leaves the cell (crenation in animal cells, plasmolysis in plants). Hypotonic: lower solute concentration outside — water enters (swells). Isotonic: equal concentrations — no net water movement.
Why do solute particles not cross the membrane?
Solute molecules are too large to pass through the pores of a semi-permeable membrane. Only small water molecules can fit through. In this simulation, solute particles hard-bounce off the central membrane while water molecules can cross based on concentration differences.
What determines the rate of osmosis?
Rate of osmosis depends on: the concentration gradient (larger difference = faster net movement), membrane surface area (more area = higher rate), membrane thickness, and temperature (higher temperature increases kinetic energy of water molecules).
Is the simulation physically accurate?
The simulation uses probability-based crossing to model net water movement qualitatively. It correctly demonstrates the direction of net osmosis relative to concentration gradient, but does not model exact molecular speeds or real osmotic pressure values.