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.
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?
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 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
Diffusion in cells
In this beautiful image, we see that diffusion is moving particles.
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.