Diffusion
Definition: Movement of particles from a high concentration to a low concentration until equilibrium is reached.
What does this actually mean?
Imagine a solid air freshener. Inside the air freshener, there is a very high concentration of the molecule that makes the smell. When the air freshener opens it allows the particles out. Gradually in their random movement they begin to spread out. The smell moves into the air where there was previously no smell. The particles move from areas where there were lots of them to areas where there were few. This will continue until the number of particles is the same everywhere. This is equilibrium.
When the particles are evenly distributed they don't stop moving, but the overall movement of the particles is in no particular direction.
So when the blood delivers food and oxygen the cells they are able to enter through the cell membrane by diffusion as they concentration of food and oxygen is higher in the blood than it is in the cell. Carbon dioxide produced by respiration will diffuse from the cell in the blood.
There are many factors that affect the rate of diffusion. Particles in a liquid diffuse slower than they do in a gaseous form.
Particles move faster at higher temperatures and so the rate of diffusion increases as the temperature increases.
The greater the difference between the concentrations the faster the rate of diffusion.
The size of the particles also affects the rate of diffusion. The larger the particles the slower the rate of diffusion.
