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Passive transport is the movement of molecules from an area of high concentration to an area of low concentration (in other words, down their concentration gradient) until equilibrium is reached. This is done without the use of ATP energy. For example, osmosis is a form of passive transport. When water moves through a semi-permeable membrane to an area of low concentration no cellular energy is used. Diffusion and facilitated diffusion are also examples of passive transport. Molecules are moving on their own down their concentration gradient and thus the cell does not need to expend energy to assist them. A concentration gradient is basically the change in concentration of a substance over a distance.