Biology-2-diffusion.

Diffusion is the spreading out of particles from an area of high concentration to an area of low concentration.

For example: 

When perfume is sprayed, the smell of perfume diffuses through the air in the room.

Cell membranes hold the cell together but they let stuff in and out too. 

Only very small stuff can diffuse through cell membranes- things like oxygen, glucose, amino acids and water. Big molecules like starch and proteins cant fit through the membrane.

If there are a lot more particles on one side of the membrane, there is a net(overall) movement from that side.

The rate of diffusion is affected by the difference in concentration of the particles- the bigger the difference in concentration, the faster the diffusion rate.