Why does water move toward the side with more solute?

Water crosses the membrane in both directions all the time. On the side with more solute, a smaller share of the molecules at a pore are water, so fewer water molecules cross from there per second. The difference between the two one-way flows is the net flow, and it always points toward the higher solute concentration.

When does the flow stop?

Never completely — particles keep crossing in both directions. The net flow drops to zero when the solute concentration is the same on both sides (dynamic equilibrium). With the default 2 and 6 settings the inside water count settles near 75 and the outside near 225, so both sides reach about 27 solute per 100 water.

Why does a cell burst in pure water but a plant cell does not?

With no solute outside the concentration can never equalise, so water keeps entering and an animal cell swells until the membrane fails (lysis); the cartoon shows this as a dashed outline at 135 % volume. A plant cell has a rigid wall that builds up turgor pressure and stops the inflow — that pressure term is not included in this model.

What changes with large pores or no membrane?

Large pores let the solute diffuse too, as in a dialysis bag. The solute spreads until both sides hold about the same number, the concentrations become equal, and any water that moved osmotically in the first seconds flows back. Without a membrane both species simply spread evenly — pure diffusion with no osmosis at all.

Osmosis & Diffusion Through a Membrane

Water and solute particles in a box split by a membrane: pore size, tonicity presets, a cell that swells or shrinks, live counts and flow.

Membrane & solutions

Solute inside / left (×10 particles)2

Solute outside / right (×10 particles)6

Pore size

Temperature (speed factor)1×

Membrane present (off = pure diffusion)

Blue = water (small, fits every pore). Orange = solute (large, fits only large pores). The left compartment is the inside of a cell; the cartoon cell below grows or shrinks with its water count. Changing solute, pores or the membrane restarts the run; temperature changes speed live.

Shortcuts

Space / Enter — play / pause
R — reset particles

Measured values

Water inside / outside150 / 150

Solute inside / outside20 / 60

Concentration inside / outside13.3 / 40.0per 100 water

Net water flowbalanced

Solution outside isHypertonic (water leaves cell)

Cell volume100%

Time0.0s

About this model

The box holds 300 water particles (blue, small) and up to 100 solute particles (orange, large) moving with random Brownian-like velocities and bouncing elastically off the walls. A membrane with ten pores divides it: small pores let only water through, large pores let both species through, and the membrane can be removed entirely for plain diffusion. The left compartment stands for the inside of a cell and the right for the surrounding solution; the cartoon cell below the box scales its radius with the square root of the water count inside, clamped between 60 % and 135 % of the starting radius. Osmosis is implemented as a crossing rule: a water particle that reaches a pore passes with a probability that decreases with the solute concentration on its own side (solute per water molecule, c = s/w), normalised so the crossing rate does not depend on how many water particles happen to be on that side — liquid water has a fixed density, unlike the rarefied gas drawn on screen. The two one-way flows therefore balance exactly when the concentrations are equal, which is the textbook equilibrium condition. The model omits hydrostatic or turgor pressure, particle–particle collisions, charged solutes and real molecular sizes; numbers are particle counts, not moles per litre, and the ±5-particle wobble around equilibrium is statistical noise, not a physical effect.

Who it's for: Middle- and high-school biology and chemistry (grades 7–11) introducing diffusion, osmosis, tonicity and dialysis.

Key terms

Osmosis
Diffusion
Semipermeable membrane
Hypotonic / isotonic / hypertonic
Concentration gradient
Dynamic equilibrium
Dialysis

How it works

Particles move randomly in a box split by a membrane. Water (small) fits through every pore; solute (large) fits only through large pores. A water molecule that reaches a pore crosses with a probability that falls with the solute concentration on its own side, so more water crosses from the dilute side than from the concentrated side: that imbalance is osmosis. Net flow stops when the concentrations are equal. With large pores or no membrane the solute spreads out too — plain diffusion — and the osmotic flow disappears as the concentrations equalise.

Key equations

Osmosis: net water flow low solute → high solute

Net flow ∝ (c_outside − c_inside), c = solute / water

Equilibrium: c_inside = c_outside → flows balance

Diffusion: particles spread from high to low concentration

Frequently asked questions

Why does water move toward the side with more solute?: Water crosses the membrane in both directions all the time. On the side with more solute, a smaller share of the molecules at a pore are water, so fewer water molecules cross from there per second. The difference between the two one-way flows is the net flow, and it always points toward the higher solute concentration.
When does the flow stop?: Never completely — particles keep crossing in both directions. The net flow drops to zero when the solute concentration is the same on both sides (dynamic equilibrium). With the default 2 and 6 settings the inside water count settles near 75 and the outside near 225, so both sides reach about 27 solute per 100 water.
Why does a cell burst in pure water but a plant cell does not?: With no solute outside the concentration can never equalise, so water keeps entering and an animal cell swells until the membrane fails (lysis); the cartoon shows this as a dashed outline at 135 % volume. A plant cell has a rigid wall that builds up turgor pressure and stops the inflow — that pressure term is not included in this model.
What changes with large pores or no membrane?: Large pores let the solute diffuse too, as in a dialysis bag. The solute spreads until both sides hold about the same number, the concentrations become equal, and any water that moved osmotically in the first seconds flows back. Without a membrane both species simply spread evenly — pure diffusion with no osmosis at all.

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Osmosis & Diffusion Through a Membrane

About this model

How it works

Key equations

Frequently asked questions

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Osmosis & Diffusion Through a Membrane

About this model

How it works

Key equations

Frequently asked questions