Crossing Membranes are substances need to move around the body of an organism. To do this they must cross Biological Membranes. There are a number of ways in which this may happen. This lecture briefly present on what is diffusion. Diffusion is the movement of molecules (or ions) from a region of high concentration to a region of lower concentration until they are spread out evenly. Finally focus on what affects the rate of diffusion. The greater the difference in the concentration of a substance in two areas, the faster the rate of diffusion.