process by which an organism exchanges gases with its environment. The term now refers to the overall process by which oxygen is abstracted from air and is transported to the cells for the oxidation of organic molecules while carbon dioxide (CO 2 ) and water, the products of oxidation, are returned to the environment. In single-celled organisms, gas exchange occurs directly between cell and environment, i.e., at the cell membrane. In plants, gas exchange with the environment occurs in special organs, the stomates, found mostly in the leaves (see leaf ; transpiration ). Organisms that utilize respiration to obtain energy are aerobic, or oxygen-dependent. Some organisms can live in the absence of oxygen and obtain energy from fuel molecules solely by fermentation or glycolysis ; these anaerobic processes are much less efficient, since the fuel molecules are merely converted to end products such as lactic acid and ethanol, with relatively little energy-rich ATP produced during these…