Molecules that readily cross the plasma membrane enter or leave cells by diffusion. Diffusion is the result of random molecular motion. Molecules move in straight lines until they encounter obstacles. There are more obstacles where molecules are highly concentrated, so a substance will tend to move from an area of high to an area of low concentration. It is said to diffuse down its concentration gradient. This is why, given enough time, two different types of molecules will end up evenly mixed among one another. In the example here, a crystal of dye is placed in a beaker of water.