Brownian movement is related to the cell membrane in that it is the driving force behind passive movement and transport of particles across the membrane. The Brownian movement causes small ions and molecules in the fluid surrounding the cell to move randomly, resulting in these particles colliding with the cell membrane. If the ion or molecule concentration outside the cell is higher than the concentration inside the cell, these random collisions draw more ions and molecules into the cell, resulting in passive diffusion. Diffusion is one of the main ways substances cross the cell membrane from areas of higher concentration to areas of lower concentration.