Diffusion is the net movement of molecules or ions from a region of high concentration to a region of low concentration until evenly distributed.Diffusion is described as passive transport.The energy for diffusion comes from the kinetic energy of the particles.Particles are always moving randomly and colliding with other particles and their container.There will be more frequent collisions where there is a higher concentration than where there is a lower concentration so more movement from a higher to a lower concentration than from a lower to higher concentration.This net movement will eventually lead to an even distribution of particles.Molecules such as oxygen and carbon dioxide diffuse across phospholipid membranes because they are small and non-polar.The rate of diffusion increases with surface area and concentration gradient.