Mitochondria (sing. mitochondrion) are eukaryotic organelles that produce adenosine triphosphate (ATP), the primary energy molecule used in biological systems, by oxidizing organic macromolecules. The presence of oxygen at the end of the processes carried out in the mitochondria allows for aerobic cellular respiration, which produces sixteen to eighteen times more ATP than fermentation, the anaerobic counterpart to cellular respiration. Carbon dioxide and water are byproducts of the cellular respiration that produces ATP.
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KREBS (CITRIC ACID/TRICARBOXYLIC ACID) CYCLE
Before moving into the mitochondria, the simple sugar glucose is broken down into two molecules of pyruvate in a process known as glycolysis, which takes place in the cell’s cytoplasm. The pyruvate molecules are then transported to the matrix, the fluid that fills the mitohchondria, and oxidized into molecules of acetyl CoA. Each acetyl CoA is then carried through a series of eight steps known as the Krebs cycle, which produces numerous NADH and FADH2 molecules that will ultimately be used to produce ATP.
After the Krebs cycle is complete, electrons from the NADH and FADH2 molecules are passed through a series of complexes situated on the cristae, or inner folded membranes of the mitochondria, that comprise the electron transport chain, of which oxygen is the final member. At the same time, hydrogen ions, once paired with those electrons, are pumped to one side of the cristae to form a proton gradient. These accumulated protons pass through an enzyme called ATP synthase, also situated on the cristae, and this chemiosmosis leads to inorganic phosphates being bound to molecules of adenosine diphosphate (ADP), thereby replenishing the supply of ATP. The hydrogen ions bind to the oxygen at the end of the electron transport chain to form the water molecules that are a byproduct of aerobic respiration.
Along with ubiquinone, cytochrome C is a transport molecule that carries electrons among the various complexes of the electron transport chain. Under normal respirative conditions, cytochrome C is anchored to the cristae via connections with cardiolipin molecules. As cells age, these connections deteriorate, and cytochrome C begins to leak out of the mitochondria. This leaking is one of the first signs of apoptosis, or programmed cell death.
Like chloroplasts, mitochondria have their own genome of DNA, sometimes abbreviated mtDNA, which is separate from the cell’s main complement of DNA housed in the nucleus. Some of the hundreds of proteins necessary for cellular respiration to take place are actually translated from the mitochondrial DNA itself. Due to the significantly larger size of and, therefore, far greater numbers of mitochondria housed in egg cells as opposed to sperm cells, it has been theorized that all of an organism’s mitochondrial DNA is inherited from its mother, though recent studies indicate that trace amounts of this DNA may be inherited from its father, due to the small numbers of still-active mitochondria present in sperm at the moment of fertilization. In humans, through her chance survival and subsequent reproduction, almost all mitochondrial DNA seems to have been inherited from a single female ancestor, often colloquially referred to as “mitochondrial Eve.”
Cellular biologists are confident that mitochondria, because they contain their own simple genome on a circular plasmid molecule of DNA were once free-living prokaryotic (i.e., bacterial) organisms that were particularly adept at harvesting ATP for their energetic needs. These proteobacteria engaged in a mutualistic symbiosis with eukaryotic cells, providing vast amounts of ATP to those eurkaryotes in exchange for the natural protection inherently provided by being engulfed by larger, heartier cells. The significant ATP synthesis that this symbiosis provides helps to explain why eukaryotic organisms have become so successful and evolved into such large, multicellular organisms. The same endosymbiotic theory also applies to chloroplasts, which contain their own genomes, and allow certain eukaryotic organisms to synthesize their own food molecules.
Quizbowl is about learning, not rote memorization, so we encourage you to use this as a springboard for further reading rather than as an endpoint. Here are a few things to check out:
* North Dakota State’s Virtual Cell Animation collection of videos provides easily digestible, yet detailed, accounts of all the steps in cellular respiration. See the “Cellular Energy Conversion” links on the lefthand menu:
* Geneticist Mark Stoneking dispels some of the misconceptions surrounding the theorized ancestral source of all mitochondrial DNA, a female who has come to be known as “mitochondrial Eve:”
(It should be noted, however, that very recent discoveries suggest that a small amount of mitochondrial DNA is likely inherited from fathers)
* Al Jazeera provides a basic primer on endosymbiotic theory and new therapies to combat the inheritance of mitochondrial diseases in humans:
* The Amoeba Sisters provide a clear, fun explanation of how all the steps in harvesting ATP work:
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