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.
⏱ 7 min read · Complete guide below
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.
What Osmosis Actually Is
Osmosis is the net movement of water across a semi-permeable membrane — a barrier that lets small water molecules through but blocks larger dissolved solute particles. Water moves from the side with more water (lower solute concentration) to the side with less water (higher solute concentration), which biologists describe as moving down the water potential gradient. The crucial subtlety the simulator makes visible is that this is not purposeful: individual water molecules move in both directions at random, but because there is more free water on the dilute side, more of them cross from it than back, producing a net flow toward the concentrated side. The process continues until the two water potentials are equal, at which point crossings balance and there is no further net movement.
Hypertonic, Hypotonic, and Isotonic
Three terms describe a cell's surroundings relative to its interior, and they determine what happens to the cell. A hypertonic solution has a higher solute concentration outside the cell, so water leaves and the cell shrinks — crenation in an animal cell, plasmolysis in a plant cell, and the reason salt dehydrates and preserves food. A hypotonic solution has lower solute concentration outside, so water floods in; an animal cell may swell until it bursts (lysis), while a plant cell's rigid wall lets it become firm and turgid instead. An isotonic solution has equal concentrations inside and out, so there is no net water movement and the cell holds its volume — which is precisely why intravenous fluids are formulated to be isotonic with blood. Setting the two sides of the simulator to these relationships lets you watch each outcome directly.
Why Osmosis Matters in Living Things
Far from being an abstract lab curiosity, osmosis governs how every cell manages its water balance. Plant turgor pressure — the firmness that keeps leaves and stems upright — comes from water drawn in by osmosis pressing the cell contents against the wall; lose that water and the plant wilts. Your kidneys rely on osmosis to reabsorb water and concentrate urine, and red blood cells depend on the blood plasma around them being isotonic, which is why saline drips are carefully matched to blood. Osmosis also explains everyday observations: why a slug shrivels when salted, why fingertips prune in fresh water, and why salting or sugaring food preserves it by drawing water out of any microbes present. Because the simulator strips the process down to particles crossing a membrane, it makes the single underlying rule behind all of these phenomena easy to see.
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.
What is the difference between osmosis and diffusion?
Diffusion is the general movement of any particles from a region of higher concentration to lower concentration until evenly spread. Osmosis is a specific case: the movement of water across a semi-permeable membrane, driven by differences in solute concentration on each side. In osmosis the solute cannot cross the membrane, so it is the water that moves to balance the concentrations. All osmosis is a form of diffusion, but of water specifically across a selective barrier.
Which way does water move in osmosis?
Water moves from the side with a lower solute concentration (more free water, higher water potential) to the side with a higher solute concentration (less free water, lower water potential). It is easy to remember as "water follows solute." The movement is not deliberate — water molecules cross randomly in both directions, but more cross from the dilute side, producing a net flow toward the concentrated side until the two water potentials are equal.
What happens to a cell in a hypertonic vs hypotonic solution?
In a hypertonic solution (more solute outside the cell), water leaves the cell and it shrinks — crenation in animal cells, plasmolysis in plant cells. In a hypotonic solution (less solute outside), water enters; an animal cell may swell and burst (lysis), while a plant cell becomes firm and turgid, protected by its wall. In an isotonic solution the concentrations match, so there is no net water movement and the cell keeps its normal volume.
Why must intravenous (IV) fluids be isotonic with blood?
Because red blood cells sit directly in the fluid being infused. If the IV fluid were hypotonic, water would rush into the cells and they could burst; if it were hypertonic, water would leave and the cells would shrivel. An isotonic fluid — matching the solute concentration of blood plasma — causes no net osmotic movement, so the red blood cells keep their shape and function. This is why saline drips are carefully formulated to the concentration of blood.