Diffusion and Osmosis
A live simulation, not a labeled diagram: watch actual particles move down their concentration gradient, and see exactly why water — not solute — is what crosses a cell membrane during osmosis. The other tabs dig into why water behaves the way it does, why the membrane itself is built the way it is, and a real example of filtration in the body.
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, driven by nothing more than random motion — given enough time, particles spread out until concentration is equal everywhere. The simulation below uses a concrete, familiar solute — dissolved NaCl (table salt, as Na⁺ and Cl⁻ ions) in water — rather than a generic unlabeled particle. Toggle the membrane to let NaCl pass freely, and watch the orange NaCl particles diffuse across until both sides settle near the same count.
Osmosis is a special case of diffusion: specifically, the movement of water across a selectively permeable membrane — one that lets water through but blocks the solute. When the solute can’t equalize itself, water moves instead, flowing toward whichever side has more solute, diluting it. That’s exactly what the simulation shows with the membrane toggle off: watch the water readout drift toward whichever side you set to have more starting solute.
This single mechanism explains real cell behavior. A solution with more solute than the cell’s interior is hypertonic — water leaves the cell, and it shrinks. A solution with less solute than the interior is hypotonic — water enters, and the cell swells (a plant cell gains turgor pressure and firms up; an animal cell can burst). A solution matching the cell’s interior is isotonic — no net water movement either way.
The Kidney Filtration tab shows a real place this happens in your own body — though, as that tab explains, the kidney’s version isn’t actually driven by concentration the way osmosis is.