Diffusion always takes place from high concentration to low concentration. When you look at the cell below, simply count the molecules of a substance inside and outside the cell. If the molecules are greater on one side, the molecules will diffuse to the other, until they are evened out. If molecules are equal on both side, then for every one that goes out, one goes in, and the concentration remains the same. Nitrogen is the only molecule that will change concentration in the diagram below. It will diffuse into the cell until its concentration is the same both inside and outside.