Breathing, the process whereby an organism (living thing) obtains oxygen from its surroundings, to take part in internal chemical reactions which break down food to release energy. Breathing also usually gives out waste carbon dioxide as an unwanted by-product of these reactions. Breathing is one aspect of respiration, which is a term that includes both physically obtaining oxygen, and also using it in chemical pathways inside cells to release energy—the process of cellular respiration Many aquatic animals obtain oxygen dissolved in the water by muscle-powered movements, such as when the forward motion of a fish or the pumping actions of its mouth and throat force a flow of water over its gills ( See also Fish). This is underwater breathing. Tiny organisms “breathe” by the process of passive seepage or diffusion of oxygen through their body surfaces. Plants also obtain oxygen by diffusion from the air or water around them, usually into the leaves.