Osmosis & Diffusion
Molecules never stop moving — and that restlessness drives all of chemistry and life. Adjust the sliders to see how concentration gradients push molecules toward equilibrium, and what happens to a cell when you change its surroundings.
Left solute
7/10
Gradient
-4
Right solute
3/10
Net flow: Right → Left (left is hypertonic — less water, pulls water in)
Drag the sliders — watch molecules respond in real time
Diffusion — molecules seek equilibrium
All molecules are in constant random motion. When more molecules occupy one region than another, random collisions statistically produce a net flow from the crowded side to the sparse side. This is diffusion: net movement down a concentration gradient, requiring no energy input.
- Driven entirely by kinetic energy of molecules — no ATP required
- Continues until concentrations equalize (dynamic equilibrium)
- Rate scales with gradient steepness and temperature
- Applies to gases, liquids, and dissolved solutes
Three things to keep straight
Equilibrium drives everything
Neither diffusion nor osmosis requires a motor protein or ATP. The random collisions of molecules statistically push the system toward equal concentrations — thermodynamics does the work.
Solute can't cross
A semipermeable membrane is the key constraint. Solute molecules are too large to fit through aquaporin channels, so only water crosses — magnifying the osmotic effect.
Water follows solute
Counter-intuitive but true: water moves toward higher solute concentration. High solute means low water concentration — water diffuses down its own gradient, not the solute's.
Diffusion vs. Osmosis
| Property | Diffusion | Osmosis |
|---|---|---|
| What moves | Any molecule | Water only |
| Membrane needed | No | Yes (semipermeable) |
| Energy cost | None (passive) | None (passive) |
| Direction | High → low conc. | Low solute → high solute |
| Stops when | Concentrations equal | Osmotic pressure balances |
When osmosis goes wrong
IV fluids must be isotonic (~0.9% NaCl) or cells suffer immediately. A hypotonic drip swells red blood cells until they burst (hemolysis). A hypertonic drip shrinks them — dangerously reducing their flexibility. Cholera toxin forces chloride channels open, pulling water out of intestinal cells by osmosis, causing the catastrophic dehydration that kills within hours. Cryopreservation of cells uses cryoprotectants that match the internal osmolarity so cells don't shatter when frozen.
Quick recap
- 1Diffusion — Net movement of molecules from high to low concentration — driven by random molecular motion, no energy needed.
- 2Osmosis — Diffusion of water across a semipermeable membrane toward the side with higher solute concentration.
- 3Aquaporins — Protein channels that allow water to cross the membrane ~1 billion molecules per second — much faster than simple diffusion.
- 4Tonicity — Describes a solution relative to a cell: hypotonic (cell swells), isotonic (no change), hypertonic (cell shrinks).
- 5Osmotic pressure — The hydrostatic pressure that exactly counteracts osmosis — higher solute concentration = higher osmotic pressure.