either of the two saclike respiratory organs in the thorax of humans and the higher vertebrates. 2. an analogous organ in certain invertebrates, as arachnids or terrestrial gastropods. Idioms. 3. at the top of one's lungs, as loudly as possible; with full voice: The baby cried at the top of his lungs.
Mitochondria are the energy factories of the cells. The energy currency for the work that animals must do is the energy-rich molecule adenosine triphosphate (ATP). The ATP is produced in the mitochondria using energy stored in food. Just as the chloroplasts in plants act as sugar factories for the supply of ordered molecules to the plant, the mitochondria in animals and plants act to produce the ordered ATP molecules as the energy supply for the processes of life.
The majority of ATP in aerobic, eukaryotic cells is produced by the mitochondria. In the breakdown of sugar (glucose) to carbon dioxide and water, approximately* 36 ATP are produced, in three main processes: Glycolysis, in the cytoplasm, provides a net of 2 ATP (it produces 4, but uses 2), Krebs Cycle (or Citric Acid Cycle), in the mitochondrion, also produces 2 ATP, as well as electron carriers NADH and FADH2. The NADH and FADH2 are fed into the Electron Transport Chain in the inner membrane of the mitochondrion, which generates another 32 ATP.