A live membrane splits a beaker of water and solute particles. Watch osmosis pull water to the saltier side, diffusion spread solute out, tonicity swell or shrink a cell, and a pump spend ATP.
Water SoluteMembrane: lets water through, blocks solute
The outside is hypertonic to the cell, so water moves out of the cell and the cell shrinks (its water level drops).
Water (in / out)
80 / 80
Solute (in / out)
6 / 24
Net water flow
out of cell
ATP spent (pump)
0
The blue level shows how much water is on each side, and it sits higher on the saltier side. Molecules keep moving even after the levels settle.
Outside is hypertonic to the cell. Net water flow is out of the cell, so the cell shrinks.
Osmosis vs diffusion: molecules on the move
Cells constantly move materials in and out. Two of the ways need no energy at all,
because they run on the random motion of particles. The sandbox above is a beaker split
by a membrane, with water (blue) and solute (purple) particles doing a
random walk. Nothing is scripted: any net movement you see emerges from chance, the
same idea you can watch at the pure-physics level in the
Particle Box.
Diffusion: spreading out down a gradient
Diffusion is the net movement of particles from where they are crowded to where
they are sparse, until they are spread evenly. Each particle wanders randomly, but
because there are more of them on the crowded side, more happen to wander outward than
inward, so the net flow is down the concentration gradient.
Osmosis: when only the water can cross
Cell membranes are selectively permeable: small water molecules slip through, but
larger solutes often cannot. When solute is stuck on one side, it is the water that
moves instead. Osmosis is the diffusion of water across a selectively permeable
membrane, and water always moves toward the side with more solute (the side where
water itself is comparatively scarce).
Tonicity: will the cell swell, shrink, or hold?
Tonicity compares the surroundings with the inside of the cell, and it predicts what
happens to the cell:
Surroundings
Solute compared to cell
Water moves
The cell
Hypertonic
More solute outside
Out of the cell
Shrinks
Hypotonic
Less solute outside
Into the cell
Swells (may burst)
Isotonic
Equal solute
No net movement
Stays the same
This is where the plant cell has an advantage: in a hypotonic
setting its rigid cell wall stops it bursting, and the water pressure just makes it
pleasantly firm (turgid). An animal cell has no wall, so in the wrong solution it can
shrivel or pop, which is why your body works hard to keep its fluids isotonic.
Active transport: going the other way, for a price
Diffusion and osmosis are passive: they always run down the gradient, for free.
But cells often need to move something up its gradient, from sparse to crowded. That
takes a protein pump and costs energy as ATP. This is active transport.
Using this with a class
Give students a scenario (“a red blood cell in fresh water”, “a cucumber in salt brine”)
and have them set the sliders to match, predict the outcome, then run it and check the
tonicity readout. It pairs naturally with the plant cell turgor
slider. This sandbox is free to embed on your own site or LMS.
Frequently asked questions
What is the difference between diffusion and osmosis?
Diffusion is the net movement of any particles from where they are crowded to where they are sparse, down a concentration gradient, until they are evenly spread. Osmosis is a special case: the diffusion of water across a selectively permeable membrane. In osmosis the solute cannot cross, so instead the water moves, always toward the side with more solute (the side where water is comparatively scarcer). Both are passive and need no energy.
Which way does water move in osmosis?
Water moves toward the side with the higher solute concentration, that is, from the more watery (dilute) side to the saltier (concentrated) side. It looks like the water is chasing the salt, but really the water is just diffusing down its own concentration gradient. A common mistake is to say water moves toward the more dilute side; it is the opposite.
What are hypertonic, hypotonic, and isotonic solutions?
These describe the surroundings compared with a cell. In a hypertonic solution (more solute outside than in the cell) water leaves the cell and it shrinks. In a hypotonic solution (less solute outside) water enters and the cell swells, and an animal cell can burst. In an isotonic solution the solute is equal on both sides, so there is no net water movement and the cell keeps its size.
What is the difference between passive and active transport?
Passive transport (diffusion, osmosis, and facilitated diffusion) moves particles down their concentration gradient, from crowded to sparse, and needs no energy. Active transport moves particles the other way, up the gradient from sparse to crowded, and must spend energy in the form of ATP, using protein pumps. The sodium-potassium pump in your nerve cells is a classic example.
Does movement stop when diffusion reaches equilibrium?
No. At equilibrium the particles are spread evenly, but they never stop moving. They keep crossing back and forth in equal numbers, so there is no further NET change. Confusing 'no net movement' with 'no movement' is one of the most common misconceptions about diffusion and osmosis.