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Diffusion is the spontaneous movement of molecules in a solution from regions of high concentration to regions of low concentration.

It’s spontaneous – happens by itself – we don’t need to add energy. How is that possible?

Think about molecules in a cup of water (or in an animal cell.)

There is always some heat (thermal energy) in the water.

So water molecules are always moving. They bump the particles in the water.

See the protein molecule (big white circle) moving – because water molecules (small yellow circles) keep bumping into it


Now let’s see what diffusion does to the red molecules:

We get a lot of motion, and we don’t need to add any extra energy. It is spontaneous.

Diffusion Maxwell Boltzmann Translational motion

from Wikipedia, Brownian_motion, Translational_motion.gif


Over time, we see that the dissolved particles reach a uniform concentration throughout the solvent.

That means that each part of the liquid will have the same concentration of particles,


Solvent = the liquid or gas that particles are dissolved in

Permeable = can pass through

Impermeable = can’t pass through

Semi-permeable = Some things can pass through but not all.


Diffusion through a cell membrane

Cell membranes are semi-permeable

Diffusion occurs across cell membranes when particles are small enough to fit through the membrane’s holes.

Let’s watch food coloring undergo diffusion in a bowl of water:

See how the molecules spontaneously spread out to fill the bowl?

Now let’s watch two different colors of food coloring undergo diffusion in the same bowl:

See how both sets of molecules spontaneously spread out to fill the bowl?

And they overlap with each other.


A simple model of cell diffusion

Here’s a box filled with water.

Across the middle is a semi-permeable membrane. That represents the cell membrane.

Left side = inside of the cell

Right side = outside of the cell

broken barrier = cell membrane (lipid bilayer)

Start (left pic): all molecules are inside the cell.

1. What happens as time goes by to the concentration of water molecules inside the cell?

2. What happens as time goes by to the concentration of water molecules outside the cell?

Diffusion 2

Diffusion and Osmosis <- document


the net movement is from high particle concentration to low concentration

once an equal number of particles are moving from one side to another, we’re at dynamic equilibrium

protein channel cell membrane diffusion

from the Virtual Cell Web Page


Diffusion in cells

In this beautiful image, we see that diffusion is moving particles.

Cell membrane lipid bilayer animation

Learning Standards

AAAS Benchmarks for Science Literacy

5C/H1a ( Grades: 9-12 ): Every cell is covered by a membrane that controls what can enter and leave the cell.

Next Generation Science Standards

MS-LS1.A.3 ( Middle School Life Sciences ): Within cells, special structures are responsible for particular functions, and the cell membrane forms the boundary that controls what enters and leaves the cell.

HS-LS1.A.1 ( High School Life Sciences ): Systems of specialized cells within organisms help them perform the essential functions of life.


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