Mitochondrion
A mitochondrion is the organelle where cellular respiration releases energy from glucose and stores it as ATP; the plural is mitochondria.
Definition
A mitochondrion is the part of a cell that makes usable energy, taking in food molecules and oxygen and releasing the energy the cell needs to work; the plural is mitochondria. It is a double-membraned organelle found in nearly all eukaryotic cells that carries out cellular respiration, breaking down glucose and other fuels with oxygen, capturing the energy in ATP, and giving off carbon dioxide and water, with the inner membrane folded into cristae to increase surface area. Mitochondria are of bacterial endosymbiotic origin, with an outer membrane, an inner membrane, an intermembrane space, and a matrix that holds circular mitochondrial DNA, ribosomes, and the enzymes of the citric acid cycle and fatty-acid oxidation. Electron transport chain complexes in the inner membrane pump protons into the intermembrane space, and ATP synthase uses the resulting proton-motive force to phosphorylate ADP. Mitochondria also regulate calcium, generate heat in brown fat, and initiate apoptosis.
Example
Muscle cells are packed with mitochondria because muscles need so much energy, which is why an athlete's leg muscles have more mitochondria than those of someone who rarely exercises. Breaking down one glucose molecule in mitochondria yields about $30$ to $32$ ATP, compared with only $2$ ATP from glycolysis alone in the cytoplasm, and a heart muscle cell devotes about a third of its volume to mitochondria. Human mitochondrial DNA is a circle of $16{,}569$ base pairs encoding $37$ genes, inherited almost exclusively from the mother, and mitochondrial ribosomes resemble bacterial ribosomes, which is why some antibiotics have mitochondrial side effects.
Key Insight
Mitochondria are often called the powerhouse of the cell, but they do not make energy from nothing; they release energy that was already stored in food. Both plants and animals have them, since plants make sugar in chloroplasts but still need mitochondria to release the energy from that sugar. Chemiosmosis, the coupling of a proton gradient to ATP synthesis, was Peter Mitchell's Nobel-winning insight, and the same principle drives ATP production in chloroplasts and bacterial membranes, uniting energy metabolism across all life.