Particles move about randomly, and after a bit they end up evenly spaced.
Diffusion is the gradual movement of particles from places where there are lots of them to a place where there are fewer of them.
It is just the natural tendency for stuff to spread out.
The fancy way of saying all of this is:
Diffusion is the spreading out of particles from an area of high concentration to an area of low concentration.
Diffusion happens in both solutions and gases, this is because the particle in the substances are free to move about randomly.
The simplest type is when different gases diffuse through each other, an example of this is when the smell of perfume diffuses through the air in the room.
The bigger the difference in concentration, the faster the rate of diffusion.
Cell membranes and diffusion.
The cell membrane is clever because they hold the cell together, BUT they let stuff in and out of the cell.
Dissolved substances can move in and out of the cell by diffusion.
Only very small molecules can diffuse through the cell membrane:
- Things like oxygen, glucose, amino acids and water.
Big molecules like starch and proteins can't fit through the membrane.
Just like with diffusion in air, particles flow through the cell membrane from where there's high concentration ( a lot of them) to where there's a low concentration ( not many of them).
They're moving about randomly, so the particles go both ways, but if there are a lot more particles on one side of the membrane , there's a net (overall) movement from that side.