View Larger Version | Mitochondria are among the small structures called organelles that reside within a cell. Known as the cells’ powerhouses, mitochondria extract energy from fuels such as glucose in the presence of oxygen to produce a molecule called adenosine triphosphate, or ATP (green), which provides energy for the cell. In the process, mitochondria generate potentially dangerous free radicals (red). Credit: Illustration: Nicolle Rager Fuller