Osmosis Simulator — Visualise Osmosis and Water Potential
Osmosis is the net movement of water molecules through a semipermeable membrane from a region of lower solute concentration (higher water potential) to a region of higher solute concentration (lower water potential). It is a passive process — no energy is required — and is fundamental to how cells maintain water balance, how plants absorb water from soil, and how kidneys regulate body fluid concentration. The osmosis simulator makes this invisible process visible.
Hypotonic, Isotonic, and Hypertonic Solutions
| Solution type | Solute concentration | Net water movement | Effect on cell |
|---|---|---|---|
| Hypotonic solution | Lower than cell | Water enters the cell by osmosis | Animal cell: swells and may lyse (burst). Plant cell: becomes turgid (firm — desired state) |
| Isotonic solution | Equal to cell | No net water movement (equal in both directions) | Animal cell: normal/optimal state. Plant cell: flaccid (not turgid — wilting may occur) |
| Hypertonic solution | Higher than cell | Water leaves the cell by osmosis | Animal cell: crenates (shrivels). Plant cell: plasmolysis (membrane pulls away from wall) |
How to Use the Osmosis Simulator
- Open the osmosis simulator.
- Set the solute concentration on each side of the semipermeable membrane using the sliders (measured in arbitrary units or molarity).
- Click Start to watch water molecules move across the membrane. The side with fewer solutes (higher water potential) loses water; the side with more solutes (lower water potential) gains water.
- Observe how net water flow slows as the concentrations equalise (osmotic equilibrium).
- Try the osmotic pressure mode: apply external pressure to the high-solute side to observe how pressure can prevent or reverse net osmosis (reverse osmosis).
Real-World Applications of Osmosis
| Application | How osmosis is involved | Why it matters |
|---|---|---|
| Kidney function | Nephrons reabsorb water from filtrate by osmosis. The loop of Henle creates a concentration gradient enabling variable urine concentration. | Dialysis works on the same principle — waste diffuses across a membrane along concentration gradients |
| IV fluids in medicine | Saline solutions for IV drips must be isotonic (0.9% NaCl) to avoid damaging red blood cells by osmosis. | Hypotonic IV fluids would cause red blood cells to swell and lyse; hypertonic would cause crenation |
| Food preservation (salt/sugar) | Salting meat or pickling draws water out of bacteria and food cells by osmosis, preventing microbial growth. | High solute concentration outside bacteria → water leaves bacteria → bacteria cannot function |
| Plant water uptake | Root hair cells absorb water from soil by osmosis — soil water is typically hypotonic relative to root cell contents. | Wilting = insufficient turgor pressure from inadequate osmosis; watering restores water potential gradient |
| Reverse osmosis (water purification) | Pressure forces water across a membrane from high solute concentration to low, the reverse of natural osmosis. | Used for desalination of seawater, water purification, removing contaminants |
| Tears and contact lenses | Contact lens solution must be isotonic (~0.9% saline) to avoid painful osmotic effects on the sensitive corneal cells. | Using tap water with contacts causes discomfort and risk of infection — salinity mismatch creates osmotic effects |
Water Potential
Water potential (Ψ) is a measure of the tendency of water to move from one place to another. Water always moves from higher water potential to lower water potential — the same direction as water moving from high pressure to low pressure.
Ψ = Ψs + Ψp
Where:
- Ψs = solute potential (osmotic potential) — always negative (solutes lower water potential)
- Ψp = pressure potential — can be positive (turgor pressure in plant cells) or zero
Pure water has a water potential of 0 (by definition). Adding solutes makes water potential negative. A plant cell with turgor pressure has a positive pressure potential component, which raises its water potential above the pure solute potential.
Diffusion vs. Osmosis
Both diffusion and osmosis involve net movement from high concentration to low concentration. The key difference:
- Diffusion refers to the movement of solute molecules from high concentration to low concentration. Any molecule can diffuse through media that does not block it.
- Osmosis specifically refers to the movement of water (the solvent) through a semipermeable membrane that allows water molecules to pass but restricts solute molecules.
Because the membrane blocks solute molecules, the water behaves as if it is moving toward the high solute concentration — it moves to dilute the more concentrated solution. The driving force is the difference in water potential, not the concentration of water molecules directly (though the two are related).
Plant Cells and Osmosis
Turgidity and plasmolysis
Plant cells have a rigid cell wall surrounding the plasma membrane. When a plant cell gains water by osmosis, the cell contents press against the cell wall — creating turgor pressure. This pressure supports non-woody plant structures (leaves, young stems) in the same way air pressure supports a balloon.
When a plant cell loses water (placed in a hypertonic solution), the plasma membrane pulls away from the cell wall — plasmolysis. The cell becomes flaccid. If the whole plant loses too much water, it wilts.
Tissue experiment (potato osmosis)
A classic A-level biology experiment: potato cylinders are placed in sugar solutions of varying concentrations. In hypotonic solutions, the potato gains mass (water enters cells). In hypertonic solutions, it loses mass (water leaves cells). At the isotonic point, no net water movement occurs and mass stays constant. The isotonic concentration approximates the water potential of potato cells (~0.3 mol/L sucrose).
Animal Cells and Osmosis
Animal cells have no cell wall — they rely on internal mechanisms to regulate water content:
- Red blood cells in hypotonic solutions: Swell and eventually burst (haemolysis)
- Red blood cells in hypertonic solutions: Crenate (shrivel, becoming spiky)
- Normal blood plasma: Maintained at osmolality of ~285–295 mOsm/kg — red blood cells in blood are in an effectively isotonic environment
Freshwater fish face a constant osmosis challenge: their body fluids are more concentrated than the surrounding fresh water, so water constantly enters by osmosis. They must actively pump out excess water through their kidneys. Marine fish face the opposite problem — seawater is more concentrated than their body fluids, so they must drink seawater and excrete concentrated urine.
Osmotic Pressure
Osmotic pressure is the pressure that must be applied to a solution to prevent water from entering by osmosis. It quantifies how strongly a solution draws water toward itself. Van't Hoff's Law:
π = iCRT
Where π = osmotic pressure (atm), i = van't Hoff factor (number of solute particles per formula unit), C = molar concentration, R = gas constant, T = temperature (K).
Human blood has an osmotic pressure of approximately 7.8 atm at 37°C — meaning blood plasma draws water toward itself with this pressure. Reverse osmosis desalination plants must apply pressures exceeding seawater's osmotic pressure (~27 atm) to force water across the membrane against the natural osmotic gradient.
Common Questions
Is osmosis active or passive transport?
Osmosis is passive transport — it requires no cellular energy (no ATP). Water moves down its water potential gradient spontaneously through channel proteins called aquaporins in biological membranes. Active transport (like sodium-potassium pumps) uses ATP to move molecules against their concentration gradient; osmosis works with the gradient.
Why does osmosis stop before concentrations fully equalise?
In a closed system, osmosis increases the volume and hydrostatic pressure on the high-solute side. This increased pressure raises the water potential of the high-solute side until it equals that of the other side — at which point net osmosis stops, even though concentrations may not be equal. The pressure differential exactly counterbalances the solute concentration difference. This is the osmotic pressure in action.
What happens to cells in pure water?
Pure water is hypotonic relative to virtually all living cells. Animal cells placed in pure water swell and burst (osmotic lysis). Plant cells swell but resist bursting due to the rigid cell wall creating counter-pressure. This is why you cannot use pure water for IV fluids or contact lens solution — 0.9% saline (isotonic with body cells) is required.
Simulate Osmosis
Set solute concentrations on each side of a membrane and watch water molecules move — an interactive visualisation of osmosis and water potential.
Open Osmosis Simulator