Diffusion and Osmosis (Interactive Cell Transport)
Diffusion, osmosis, facilitated diffusion and active transport compared side by side, with a live membrane that pulls water to the saltier side, spreads solute out, and spends ATP on a pump.
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.
Diffusion vs osmosis: the short answer
Osmosis is diffusion. It is the name for one particular case of it: the diffusion of
water across a selectively permeable membrane. So most of what is true of one is
true of the other, and the differences are narrow and specific.
Same random motion, same downhill direction, both free. The only thing that
changed is whether the solute can get through, and that decides which
substance ends up moving.
Diffusion and osmosis compared on seven questions. Four of the answers are the
same for both and span the table; three differ.
Diffusion
Osmosis
What moves
Any particle that can cross the membrane
Water, and only water
What kind of membrane
Any, or none at all: diffusion happens in open air and open water too
A selectively permeable one, or the solute would simply move instead
Which direction
Down the gradient of whatever is doing the moving
Down water's gradient, which points toward the saltier side
Passive or active
Same for both: Passive
Does it need energy
Same for both: No. Neither one costs the cell anything
What drives it
Same for both: The random motion of particles, nothing more
When does it stop
Same for both: It does not. At equilibrium particles still cross both ways, in equal numbers, so there is no further net change
Four of those seven answers are identical, which is the point. Both are passive, both are
free, both are driven by nothing but random motion, and neither ever really stops. What
separates them is only this: in diffusion the solute can cross, so the solute is what
moves; in osmosis it cannot, so the water moves instead, and it goes toward the side
with more solute.
If the sentence you need is a definition rather than a comparison:
Diffusion is the net movement of particles from a region of higher concentration to
a region of lower concentration.
Osmosis is the diffusion of water across a selectively permeable membrane.
Net movement is what is left once you subtract the traffic going the other way.
Both are examples of passive transport, and diffusion continues until
equilibrium.
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:
The three tonicities of a solution surrounding a cell, with which way water
moves and what happens to the cell.
Surroundings
Solute compared to the cell
Water moves
The cell
Hypertonic
More solute outside than in
Out of the cell
Shrinks
Hypotonic
Less solute outside than in
Into the cell
Swells, and may burst
Isotonic
Equal solute inside and out
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.
All five ways across a membrane
Diffusion, osmosis and active transport are three of five. The other two come up as soon
as a question asks how a cell moves something that will not fit through a gap.
The five ways a substance crosses a cell membrane, with what moves, which way it
goes relative to the concentration gradient, whether it needs a protein or
energy, and an example of each.
Type
Passive or active
What moves
Which way
Needs a protein?
Needs energy?
Example
Simple diffusion
passive
Small uncharged particles
Down its gradient, crowded to sparse
No
No
Oxygen into a cell from the blood
Facilitated diffusion
passive
Ions and larger polar molecules
Down its gradient, crowded to sparse
Yes
No
Glucose through a transporter
Osmosis
passive
Water, and only water
Down water's gradient, toward MORE solute
No
No
Water out of a cell in salty water
Active transport
active
Whatever the pump is built for
AGAINST its gradient, sparse to crowded
Yes
Yes
The sodium-potassium pump
Bulk transport (endocytosis and exocytosis)
active
Whole packets and particles
Either way, wrapped in a vesicle
Yes
Yes
A white blood cell engulfing a bacterium
Two columns do all the work. Needs energy separates passive from active, and it is not
a coincidence: needing energy is what “active” means. Needs a protein separates simple
from facilitated diffusion, which are otherwise the same process, both passive and both
downhill; the protein is a doorway, not a pump.
Notice that only one row goes against the gradient. Nothing free ever does, which is
why active transport is the only line in the table with a Yes in the energy column.
The five definitions
Simple diffusion
The net movement of particles from a region of higher concentration to a region of lower concentration, straight through the membrane.
Facilitated diffusion
Diffusion through a channel or carrier protein, for particles that cannot cross the membrane on their own. Still downhill, still free.
Osmosis
The diffusion of water across a selectively permeable membrane, from the more dilute side to the more concentrated side.
Active transport
The movement of a substance against its concentration gradient, using a pump protein and a supply of energy.
Bulk transport (endocytosis and exocytosis)
Moving material in or out inside a vesicle, by folding the membrane around it, rather than passing it through the membrane at all.
Diffusion never actually stops
When the counts on both sides even out, it is tempting to say diffusion has finished. It
has not. Particles keep crossing in both directions; there are simply now equal numbers
going each way, so the net movement is zero while the movement itself carries on.
That state is called a dynamic equilibrium, and the word net is doing all the work in
the definition: if 100 particles cross to the right and 60 to the left, the net movement
is 40 to the right, and when the two flows match, the net is nothing at all.
The sandbox shows this plainly. Let a diffusion run settle and the readouts stop changing
while the dots keep moving exactly as before. Nothing switched off.
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.
Are diffusion and osmosis examples of passive or active transport?
Passive. Both run on the random motion of particles alone and cost the cell nothing, which is what passive transport means. Facilitated diffusion is passive too, even though it needs a protein, because the protein is only a doorway and does not push. Active transport is the one that costs energy, and it is the only one that can move a substance against its concentration gradient.
What is the difference between diffusion, osmosis and active transport?
Diffusion is the net movement of any particle from where it is crowded to where it is sparse. Osmosis is that same process applied to water across a selectively permeable membrane, so the water ends up moving toward the side with more solute. Active transport is the opposite in every respect: it moves a substance against its gradient, from sparse to crowded, and it needs a pump protein and a supply of energy to do so. The first two are free and downhill; the third is neither.
What is facilitated diffusion?
Diffusion through a channel or carrier protein in the membrane, for particles that cannot cross the lipid layer on their own, such as ions and glucose. It is still passive and still downhill: the protein is a doorway, not a pump, and no energy is spent. The only difference from simple diffusion is that a protein is involved.
Do molecules stop moving when diffusion stops?
No. Diffusion never really stops; what stops is the NET movement. At equilibrium particles are still crossing in both directions, just in equal numbers, so the counts on each side hold steady while the individual particles keep wandering. This is called a dynamic equilibrium, and you can watch it in the sandbox above: the readouts settle down while the dots carry on moving.
Diffusion continues until what?
Until the concentration is even on both sides, that is, until equilibrium. From that point there is no further net movement, because as many particles cross one way as the other. Nothing is directing them; it is simply that when both sides are equally crowded, equal numbers happen to wander each way.
What does net movement mean?
The movement that is left over once you subtract the traffic going the other way. Particles are always moving in both directions, so what matters is the difference: if 100 particles cross to the right and 60 cross to the left, the net movement is 40 to the right. When the two flows are equal the net movement is zero, even though nothing has stopped moving.
Does osmosis use ATP?
No. Osmosis is passive, so it costs the cell nothing. Water crosses because of the random motion of its own molecules, and it goes downhill from where water is plentiful to where water is scarcer. Only active transport spends ATP, and that is because it is pushing something uphill against its gradient.
Can diffusion and osmosis happen at the same time?
Yes, and in a real cell they usually do. Whatever solutes can cross the membrane will be diffusing across it while water is moving by osmosis in whichever direction the remaining solute gradient dictates. The sandbox above separates them into two modes to make each one visible on its own, but a living membrane is doing both at once.
What happens when there is the same amount of water inside a cell as outside?
The solution is isotonic, and there is no net movement of water. Water still crosses the membrane in both directions, but equally, so the cell neither swells nor shrinks. This is the state your body works hard to keep its fluids in, which is why an intravenous drip uses a saline solution rather than pure water.