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Mitochondrion

In cell biology, a mitochondrion (plural mitochondria) (from Greek mitos thread + khondrion granule) is an organelle found in most eukaryotic cells. Mitochondria are sometimes described as "cellular power plants," because their primary function is to convert organic materials into energy in the form of ATP via the process of oxidative phosphorylation. Usually a cell has hundreds or thousands of mitochondria, which can occupy up to 25% of the cell's cytoplasm. Mitochondria have their own DNA, and, according to the generally accepted Endosymbiotic theory, they were originally derived from external organisms.

Mitochondrion structure

A mitochondrion contains outer and inner membranes composed of phospholipid bilayers studded with proteins, much like a typical cell membrane. The two membranes, however, have very different properties.

The outer mitochondrial membrane, which encloses the entire organelle, contains numerous integral proteins called porins, which contain a relatively large internal channel (about 2-3 nm) that is permeable to all molecules of 5000 daltons or less [Alberts, 1994]. Larger molecules can only tranverse the outer membrane by active transport. The outer mitochondrial membrane is composed of about 50% phospholipids by weight and contains a variety of enzymes involved in such diverse activities as the the elongation of fatty acids, oxidation of epinephrine (adrenaline), and the degradation of tryptophan.

The inner membrane contains proteins with three types of functions [Alberts, 1994]:

those that carry out the oxidation reactions of the respiratory chain
ATP synthase, which makes ATP in the matrix
specific transport proteins that regulate the passage of metabolites into and out of the matrix.

It contains more than 100 different polypeptides, and has a very high protein-to-phospholipid ratio (more than 3:1 by weight, which is about 1 protein for 15 phospholipids). Additionally, the inner membrane is rich in an unusual phospholipid, cardiolipin, which is usually characteristic of bacterial plasma membranes. Unlike the outer membrane, the inner membrane does not contain porins, and is highly-impermeable; almost all ions and molecules require special membrane transporters to enter or exit the matrix.

Mitochondria structure :
1) Inner membrane
2) Outer membrane
3) Crista
4) Matrix

The mitochondrial matrix

The matrix is the space enclosed by the inner membrane. The matrix contains a highly concentrated mixture of hundreds of enzymes, in addition to the special mitochondrial ribosomes, tRNA, and several copies of the mitochondrial DNA genome. Of the enzymes, the major functions include oxidation of pyruvate and fatty acids, and the citric acid cycle. [Alberts, 1994]

Thus, mitochondria possess their own genetic material, and the machinery to manufacture their own RNAs and proteins. (See: protein synthesis). This nonchromosomal DNA encodes a small number of mitochondrial peptides (13 in humans) that are integrated into the inner mitochondrial membrane, along with polypeptides encoded by genes that reside in the host cell's nucleus.

Mitochondrial functions

Although the primary function of mitochondria is to convert organic materials into cellular energy in the form of ATP, mitochondria play an important role in many metabolic tasks, such as: The inner mitochondrial membrane is folded into numerous cristae (see diagram above), which expand the surface area of the inner mitochondrial membrane, enhancing its ability to generate ATP. In typical liver mitochondria, for example, the surface area, including cristae, is about five times that of the outer membrane. Mitochondria of cells which have greater demand for ATP, such as muscle cells, contain even more cristae than typical liver mitochondria.

Apoptosis
Glutamate-mediated excitotoxic neuronal injury
Cellular proliferation
Regulation of the cellular redox state
Heme synthesis
Steroid synthesis
Heat production (enabling the organism to stay warm).

Some mitochondrial functions are performed only in specific types of cells. For example, mitochondria in liver cells contain enzymes that allow them to detoxify ammonia, a waste product of protein metabolism. A mutation in the genes regulating any of these functions can result in a variety of mitochondrial diseases.

Energy conversion

As stated above, the primary function of the mitochondria is the production of ATP. This is done by metabolizing the major products of glycolysis: pyruvate and NADH (glycolysis is performed outside the mitochondria, in the host cell's cytosol). This metabolism can be performed in two very different ways, depending on the type of cell and the presence or absence of oxygen.

Pyruvate: the citric acid cycle

Main article: citric acid cycle

Each pyruvate molecule produced by glycolysis is actively transported across the inner mitochondrial membrane, and into the matrix where it is combined with coenzyme A to form acetyl CoA. Once formed, acetyl CoA is fed into the citric acid cycle , also known as the tricarboxylic acid (TCA) cycle or Krebs cycle. This process creates 3 molecules of NADH and 1 molecule of FADH₂, which go on to participate in the electron transport chain.

With the exception of succinate dehydrogenase, which is bound to the inner mitochondrial membrane, all of the enzymes of the citric acid cycle are dissolved in the mitochondrial matrix.

NADH and FADH₂: the electron transport chain

This energy from NADH and FADH₂ is transferred to oxygen (O₂) in several steps via the electron transfer chain. The protein complexes in the inner membrane (NADH dehydrogenase, cytochrome c reductase, cytochrome c oxidase) that perform the transfer use the released energy to pump protons (H⁺) against a gradient (the concentration of protons in the intermembrane space is higher than that in the matrix).

As the proton concentration increases in the intermembrane space, a strong concentration gradient is built up. The main exit for these protons is through the ATP synthase complex. By transporting protons from the intermembrane space back into the matrix, the ATP synthase complex can make ATP from ADP and inorganic phosphate (P_i). This process is called chemiosmosis and is an example of facilitated diffusion. Peter Mitchell was awarded the 1978 Nobel Prize in Chemistry for his work on chemiosmosis. Later, part of the 1997 Nobel Prize in Chemistry was awarded to Paul D. Boyer and John E. Walker for their clarification of the working mechanism of ATP synthase.

Under certain conditions, protons may be allowed to re-enter the mitochondial matrix without contributing to ATP synthesis. This process, known as proton leak or mitochondrial uncoupling, results in the unharnessed energy being released as heat. This mechanism for the metabolic generation of heat is employed primarily in specialized tissues, such as the "brown fat" of newborn or hibernating mammals.

Reproduction and gene inheritance

Mitochondria replicate their DNA and divide mainly in response to the energy needs of the cell; in other words their growth and division is not linked to the cell cycle. When the energy needs of a cell are high, mitochondria grow and divide. When the energy use is low, mitochondria are destroyed or become inactive. At cell division, mitochondria are distributed to the daughter cells more or less randomly during the division of the cytoplasm. Mitochondria divide by binary fission similar to bacterial cell division. Unlike bacteria, however, mitochondria can also fuse with other mitochondria. Sometimes new mitochondria are synthesized in centers that are rich in proteins and polyribosomes needed for their synthesis.

Mitochondrial genes are not inherited by the same mechanism as nuclear genes. At fertilization of an egg by a sperm, the egg nucleus and sperm nucleus each contribute equally to the genetic makeup of the zygote nucleus. However, all of the mitochondria, and therefore all the mitochondrial genes, are contributed to the zygote by the egg. At fertilization of an egg, a single sperm enters the egg along with the mitochondria that it uses to provide the energy needed for its swimming behavior. However, the mitochondria provided by the sperm are targeted for destruction very soon after entry into the egg. The egg itself contains relatively few mitochondria, but it is these mitochondria that survive and divide to populate the cells of the adult organism. This type of inheritance is called maternal inheritance and is common to the mitochondria of all animals. Because mitochondria are inherited from the mother only, the sequence of mitochondrial DNA is sometimes used to trace the lineage of families.

Use in population genetic studies

Main article: mitochondrial genetics

Because Ovum destroys the mitochondria of the sperm that fertilize them, the mitochondrial DNA of an individual derives almost exclusively from the mother. That is to say 99.9% comes from the mother. Individuals inherit the other kinds of genes and DNA from both parents jointly. Because of the unique matrilineal transmission of mitochondrial DNA, scientists in population genetics and evolutionary biology often use data from mitochondrial DNA sequences to draw conclusions.

Origin

As mitochondria contain ribosomes and DNA, and are only formed by the division of other mitochondria, it is generally accepted that they were originally derived from endosymbiotic prokaryotes. Studies of mitochondrial DNA, which is circular and employs a variant genetic code, show their ancestor was a member of the Proteobacteria [Futuyma 2005]. In particular, the pre-mitochondrion was probably related to the rickettsias. The endosymbiotic hypothesis suggests that mitochondria descended from specialized bacteria (probably purple nonsulfur bacteria) that somehow survived endocytosis by another species of prokaryote or some other cell type, and became incorporated into the cytoplasm. The ability of symbiont bacteria to conduct cellular respiration in host cells that had relied on glycolysis and fermentation would have provided a considerable evolutionary advantage. Similarly, host cells with symbiotic bacteria capable of photosynthesis would also have an advantage. In both cases, the number of environments in which the cells could survive would have been greatly expanded.

This happened at least 2000 million years ago and mitochondria still show some signs of their ancient origin. Mitochondrial ribosomes are the 70S (bacterial) type, in contrast to the 80S ribosomes found elsewhere in the cell. As in prokaryotes, there is a very high proportion of coding DNA, and an absence of repeats. Mitochondrial genes are transcribed as multigenic transcripts which are cleaved and polyadenylated to yield mature mRNAs. Unlike their nuclear cousins, mitochondrial genes are small, generally lacking introns, and the chromosomes are circular, conforming to the bacterial pattern.

A few groups of unicellular eukaryotes lack mitochondria: the symbiotic microsporidians, metamonads, and entamoebids, and the free-living pelobionts. On rRNA trees these groups appeared as the most primitive eukaryotes, suggesting they appeared before the origin of mitochondrion, but this is now known to be an artifact of long branch attraction - they are apparently derived groups and retain genes or organelles derived from mitochondria. Thus it appears that there are no primitively amitochondriate eukaryotes, and so the origin of mitochondria may have played a critical part in the development of eukaryotic cells.

References
ISBN links support NWE through referral fees

Futuyma, Douglas J. (2005). On Darwin's Shoulders. Natural History 114 (9): 64–68.
Scheffler, I.E. (2001). A century of mitochondrial research: achievements and perspectives. Mitochondrion 1 (1): 3–31.
Alberts, Bruce and et.al. (1994). Molecular Biology of the Cell, Third Edition, New York: Garland Publishing Inc..

External links

Credits

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History of "Mitochondrion"

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@@ Line 1: / Line 1: @@
 {{Contracted}}{{Status}}
-{{primarysources}}
+{{NCBI-scienceprimer}}
-[[image:Mitochondria_in_cell.jpg|thumb|300px|'''Mitochondria''' are visible as thread-like structures in the light microscope. Mitochondria are selectively stained with a fluorescent dye.  The nucleus and cell membrane are not visible. ]]
-In [[cell biology]], a '''mitochondrion''' (from [[Greek language|Greek]] ''mitos'' thread + ''khondrion'' granule) is an [[organelle]] found in most [[eukaryote|eukaryotic]] [[cell (biology)|cells]], including those of [[plant]]s, [[animal]]s, [[fungus|fungi]], and [[protist]]s. A few cells, such as the [[trypanosome]] [[protozoa]]n, have a single large mitochondrion, but usually a cell has hundreds or thousands of mitochondria. The exact number of mitochondria depends on the cell's level of metabolic activity: more activity means more mitochondria. Mitochondria can occupy up to 25% of the cell's [[cytosol]].
+{{Commons|Mitochondrion}}
+[[Category:Cellular respiration]]
+[[Category:Organelles]]
-Mitochondria are sometimes described as "[[cell (biology)|cellular]] [[power plant]]s", because their primary function is to convert organic materials into [[energy]] in the form of [[adenosine triphosphate|ATP]] via the process of [[oxidative phosphorylation]].
+In [[cell biology]], a '''mitochondrion''' (plural '''mitochondria''') (from [[Greek language|Greek]] ''mitos'' thread + ''khondrion'' granule) is an [[organelle]] found in most [[eukaryote|eukaryotic]] [[cell (biology)|cells]]. Mitochondria are sometimes described as "[[cell (biology)|cellular]] [[power plant]]s," because their primary function is to convert organic materials into energy in the form of [[adenosine triphosphate|ATP]] via the process of [[oxidative phosphorylation]]. Usually a cell has hundreds or thousands of mitochondria, which can occupy up to 25% of the cell's [[cytoplasm]]. Mitochondria have their own [[DNA]], and, according to the generally accepted [[Endosymbiotic theory]], they were originally derived from external organisms.
 ==Mitochondrion structure==
-The outer and inner membranes of the mitochondria are composed of [[phospholipid bilayer]]s studded with [[protein]]s, much like a typical [[cell membrane]]. The two membranes, however, have very different properties. The outer mitochondrial membrane, which encloses the entire organelle, is composed of about 50% [[phospholipid]]s by weight and contains a variety of [[enzyme]]s involved in such diverse activities such as the [[oxidation]] of [[epinephrine]] (adrenaline), the [[degradation]] of [[tryptophan]], and the elongation of [[fatty acid]]s. The outer mitochondrial membrane contains numerous [[integral protein]]s called ''[[porin]]s'', which contain a relatively large internal channel (about 2-3 [[Nanometre|nm]]) that allow [[ion]]s and small [[molecule]]s to move in and out of the mitochondrion. Large molecules, however, cannot traverse the outer membrane without being [[active transport|actively transported]]. Mostly every cell contains mitochondrion but each of them have different amounts.
+A mitochondrion contains outer and inner membranes composed of [[phospholipid bilayer]]s studded with [[protein]]s, much like a typical [[cell membrane]]. The two membranes, however, have very different properties.
-The inner mitochondrial membrane, in contrast, contains more than 100 different [[polypeptide]]s, and has a very high protein to phospholipid ratio (more than 3:1 by weight, which is about 1 protein for 15 phospholipids). Additionally, the inner membrane is rich in an unusual phospholipid, [[cardiolipin]], which is usually characteristic of [[bacteria]]l [[plasma membrane]]s. The inner membrane does not contain porins, however, and is highly impermeable; almost all ions and molecules require special membrane transporters to enter or exit the matrix. The inner membrane also has foldings called the [[crista|cristae]] which allow the inner membrane to have a greater surface area for the [[electron transport chain]]s.
+The outer mitochondrial membrane, which encloses the entire organelle, contains numerous [[integral protein]]s called ''[[porin]]s'', which contain a relatively large internal channel (about 2-3 [[Nanometre|nm]]) that is permeable to all molecules of 5000 [[Atomic mass unit|dalton]]s or less [Alberts, 1994]. Larger molecules can only tranverse the outer membrane by [[active transport]]. The outer mitochondrial membrane is composed of about 50% [[phospholipid]]s by weight and contains a variety of [[enzyme]]s involved in such diverse activities as the the elongation of [[fatty acid]]s, [[oxidation]] of [[epinephrine]] (adrenaline), and the [[degradation]] of [[tryptophan]].
+The inner membrane contains proteins with three types of functions [Alberts, 1994]:
+# those that carry out the oxidation reactions of the respiratory chain
+# [[ATP synthase]], which makes [[Adenosine triphosphate|ATP]] in the matrix
+# specific transport proteins that regulate the passage of [[metabolite]]s into and out of the matrix.
+It contains more than 100 different [[polypeptide]]s, and has a very high protein-to-phospholipid ratio (more than 3:1 by weight, which is about 1 protein for 15 phospholipids). Additionally, the inner membrane is rich in an unusual phospholipid, [[cardiolipin]], which is usually characteristic of [[bacteria]]l [[plasma membrane]]s. Unlike the outer membrane, the inner membrane does not contain porins, and is highly-impermeable; almost all ions and molecules require special membrane transporters to enter or exit the matrix.
+[[image:Mitochondrie.svg|thumb|300px|
+Mitochondria structure :
+<br/>1) [[Inner membrane]]
+<br/>2) [[Outer membrane]]
+<br/>3) [[Crista]]
+<br/>4) [[Matrix (biology)|Matrix]]
+]]
 ===The mitochondrial matrix===
-In addition to various enzymes, the mitochondrial [[matrix]] also contains [[ribosomes]] and several molecules of [[DNA]]. Thus, mitochondria possess their own genetic material, and the machinery to manufacture their own [[RNA]]s and [[protein]]s. (''See: [[protein synthesis]]''). This nonchromosomal DNA encodes a small number of mitochondrial [[peptides]] (13 in humans) that are integrated into the inner mitochondrial membrane, along with [[polypeptide]]s encoded by [[gene]]s that reside in the host cell's [[nucleus]].
+The [[Matrix (biology)| matrix]] is the space enclosed by the inner membrane. The matrix contains a highly concentrated mixture of hundreds of enzymes, in addition to the special mitochondrial [[ribosomes]], [[tRNA]], and several copies of the mitochondrial [[DNA]] [[genome]]. Of the enzymes, the major functions include oxidation of [[pyruvate]] and [[fatty acids]], and the [[citric acid cycle]]. [Alberts, 1994]
+Thus, mitochondria possess their own genetic material, and the machinery to manufacture their own [[RNA]]s and [[protein]]s. (''See: [[protein synthesis]]''). This nonchromosomal DNA encodes a small number of mitochondrial [[peptides]] (13 in humans) that are integrated into the inner mitochondrial membrane, along with [[polypeptide]]s encoded by [[gene]]s that reside in the host cell's [[cell nucleus|nucleus]].
 ==Mitochondrial functions==
 Although the primary function of mitochondria is to convert organic materials into cellular energy in the form of [[adenosine triphosphate|ATP]], mitochondria play an important role in many [[metabolism|metabolic]] tasks, such as:
+The inner mitochondrial membrane is folded into numerous [[crista|cristae]] (see diagram above), which expand the surface area of the inner mitochondrial membrane, enhancing its ability to generate ATP. In typical [[liver]] mitochondria, for example, the surface area, including cristae, is about five times that of the outer membrane. Mitochondria of cells which have greater demand for ATP, such as muscle cells, contain even more cristae than typical liver mitochondria.
 * [[Apoptosis]]
@@ Line 23: / Line 45: @@
 * [[Heme]] synthesis
 * [[Steroid]] synthesis
-* Heat production (enabling the organism to stay warm)
+* Heat production (enabling the organism to stay warm).
 Some mitochondrial functions are performed only in specific types of cells. For example, mitochondria in liver cells contain enzymes that allow them to detoxify [[ammonia]], a waste product of protein metabolism. A mutation in the genes regulating any of these functions can result in a variety of [[mitochondrial disease]]s.
@@ Line 40: / Line 62: @@
 {{main|electron transport chain}}
-This energy from NADH and FADH<sub>2</sub> is transferred to oxygen (O<sub><small>2</small></sub>) in several steps via the electron transfer chain. The [[electron_transfer_chain|protein complexes]] in the inner membrane ([[NADH dehydrogenase]], [[Coenzyme Q - cytochrome c reductase|cytochrome c reductase]], [[Cytochrome_c_oxidase|cytochrome c oxidase]]) that perform the transfer use the released energy to pump [[proton]]s (H<sup>+</sup>) against a [[gradient]] (the concentration of protons in the intermembrane space is higher than that in the matrix).
+This energy from NADH and FADH<sub>2</sub> is transferred to oxygen (O<sub><small>2</small></sub>) in several steps via the electron transfer chain. The [[electron transfer chain|protein complexes]] in the inner membrane ([[NADH dehydrogenase]], [[Coenzyme Q - cytochrome c reductase|cytochrome c reductase]], [[Cytochrome c oxidase|cytochrome c oxidase]]) that perform the transfer use the released energy to pump [[proton]]s (H<sup>+</sup>) against a [[gradient]] (the concentration of protons in the intermembrane space is higher than that in the matrix).
-As the proton concentration increases in the intermembrane space, a strong ''concentration gradient'' is built up. The main exit for these protons is through the ''[[ATP synthase]]'' complex. By transporting protons from the intermembrane space back into the matrix, the ATP synthase complex can make [[Adenosine triphosphate|ATP]] from ADP and inorganic phosphate (P<sub>i</sub>). This process is called [[chemiosmosis]] and is an example of [[facilitated diffusion]].  Peter Mitchell was awarded the 1978 [[Nobel Prize]] in Chemistry for his work on chemiosmosis. Later, part of the 1997 Nobel Prize in Chemistry was awarded to [[Paul D. Boyer]] and [[John E. Walker]] for their clarification of the working mechanism of ATP synthase.
+As the proton concentration increases in the intermembrane space, a strong ''concentration gradient'' is built up. The main exit for these protons is through the ''[[ATP synthase]]'' complex. By transporting protons from the intermembrane space back into the matrix, the ATP synthase complex can make [[Adenosine triphosphate|ATP]] from ADP and inorganic phosphate (P<sub>i</sub>). This process is called [[chemiosmosis]] and is an example of [[facilitated diffusion]].  [[Peter D. Mitchell|Peter Mitchell]] was awarded the 1978 [[Nobel Prize]] in Chemistry for his work on chemiosmosis. Later, part of the 1997 Nobel Prize in Chemistry was awarded to [[Paul D. Boyer]] and [[John E. Walker]] for their clarification of the working mechanism of ATP synthase.
 Under certain conditions, protons may be allowed to re-enter the mitochondial matrix without contributing to ATP synthesis. This process, known as ''proton leak'' or ''mitochondrial uncoupling'', results in the unharnessed energy being released as heat. This mechanism for the metabolic generation of heat is employed primarily in specialized tissues, such as the "[[brown fat]]" of newborn or [[hibernation|hibernating]] mammals.
+==Reproduction and gene inheritance==
+Mitochondria replicate their DNA and divide mainly in response to the energy needs of the cell; in other words their growth and division is not linked to the [[cell cycle]]. When the energy needs of a cell are high, mitochondria grow and divide. When the energy use is low, mitochondria are destroyed or become inactive. At cell division, mitochondria are distributed to the daughter cells more or less randomly during the division of the [[cytoplasm]]. Mitochondria divide by binary fission similar to bacterial cell division. Unlike bacteria, however, mitochondria can also fuse with other mitochondria. Sometimes new mitochondria are synthesized in centers that are rich in [[proteins]] and [[polyribosomes]] needed for their synthesis.
+Mitochondrial genes are not inherited by the same mechanism as nuclear genes. At fertilization of an egg by a sperm, the egg nucleus and sperm nucleus each contribute equally to the genetic makeup of the [[zygote]] [[nucleus]]. However, all of the mitochondria, and therefore all the mitochondrial genes, are contributed to the zygote by the egg. At fertilization of an egg, a single sperm enters the egg along with the mitochondria that it uses to provide the energy needed for its swimming behavior. However, the mitochondria provided by the sperm are targeted for destruction very soon after entry into the egg. The egg itself contains relatively few mitochondria, but it is these mitochondria that survive and divide to populate the cells of the adult organism. This type of inheritance is called [[maternal inheritance]] and is common to the mitochondria of all animals. Because mitochondria are inherited from the mother only, the sequence of [[mitochondrial DNA]] is sometimes used to trace the lineage of families.
 ==Use in population genetic studies==
 ''Main article: [[mitochondrial genetics]]''
-Because [[Ovum|egg]]s destroy the mitochondria of the [[sperm]] that [[fertilization|fertilize]] them, the [[mitochondrial DNA]] of an individual derives exclusively from the mother.  Individuals inherit the other kinds of genes and DNA from both parents jointly. Because of the unique matrilineal transmission of mitochondrial DNA, scientists in [[population genetics]] and [[evolutionary biology]] often use data from mitochondrial DNA sequences to draw conclusions about [[genealogy]] and [[evolution]]. ''See: [[mitochondrial Eve]], [[genetic genealogy]], [[genealogical DNA test]].''
+Because [[Ovum]] destroys the mitochondria of the [[sperm]] that [[fertilization|fertilize]] them, the [[mitochondrial DNA]] of an individual derives almost exclusively from the mother. That is to say 99.9% comes from the mother. Individuals inherit the other kinds of genes and DNA from both parents jointly. Because of the unique matrilineal transmission of mitochondrial DNA, scientists in [[population genetics]] and [[evolutionary biology]] often use data from mitochondrial DNA sequences to draw conclusions.
-Recent studies have, however, cast doubt on this hypothesis.  Kraytsberg et al. showed that mitochondrial recombination is possible in humans (''[[Science (journal)|Science]]'' '''304''':981, May 2004, [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15143273  pubmed #15143273]).
 ==Origin==
+As mitochondria contain ribosomes and DNA, and are only formed by the division of other mitochondria, it is generally accepted that they were originally derived from [[endosymbiosis|endosymbiotic]] prokaryotes. Studies of mitochondrial DNA, which is circular and employs a variant [[genetic code]], show their ancestor was a member of the [[Proteobacteria]] [Futuyma 2005].  In particular, the pre-mitochondrion was probably related to the [[Rickettsiales|rickettsia]]s. The endosymbiotic hypothesis suggests that mitochondria descended from specialized bacteria (probably purple nonsulfur bacteria) that somehow survived [[endocytosis]] by another species of prokaryote or some other cell type, and became incorporated into the [[cytoplasm]]. The ability of symbiont bacteria to conduct cellular respiration in host cells that had relied on [[glycolysis]] and fermentation would have provided a considerable evolutionary advantage. Similarly, host cells with symbiotic bacteria capable of [[photosynthesis]] would also have an advantage. In both cases, the number of environments in which the cells could survive would have been greatly expanded.
-As mitochondria contain ribosomes and DNA, and are only formed by the division of other mitochondria, it is generally accepted that they were originally derived from [[endosymbiosis|endosymbiotic]] prokaryotes. Studies of mitochondrial DNA, which is circular and employs a variant [[genetic code]], show their ancestor was a member of the [[Proteobacteria]] [Futuyma 2005].  In particular, the pre-mitochondrion was probably related to the [[Rickettsiales|rickettsia]]s.
+This happened at least 2000 million years ago and mitochondria still show some signs of their ancient origin. Mitochondrial [[ribosomes]] are the 70S (bacterial) type, in contrast to the 80S ribosomes found elsewhere in the cell. As in prokaryotes, there is a very high proportion of coding DNA, and an absence of repeats. Mitochondrial genes are transcribed as multigenic transcripts which are cleaved and [[Polyadenylation|polyadenylated]] to yield mature mRNAs. Unlike their nuclear cousins, mitochondrial genes are small, generally lacking [[introns]], and the chromosomes are circular, conforming to the bacterial pattern.
 A few groups of unicellular eukaryotes lack mitochondria: the symbiotic [[microsporidia]]ns, [[metamonad]]s, and [[entamoebid]]s, and the free-living [[pelobiont]]s.  On [[rRNA]] trees these groups appeared as the most primitive eukaryotes, suggesting they appeared before the origin of mitochondrion, but this is now known to be an artifact of [[long branch attraction]] - they are apparently derived groups and retain genes or organelles derived from mitochondria.  Thus it appears that there are no primitively amitochondriate eukaryotes, and so the origin of mitochondria may have played a critical part in the development of eukaryotic cells.
-==Reference==
+==References ==
-* {{Journal reference | Author=Douglas J. Futuyma | Title=On Darwin's Shoulders | Journal=Natural History | Volume=114 | Issue=9 | Year=2005 | Pages=64&ndash;68}}
+* {{cite journal | first=Douglas J. |last=Futuyma | title=On Darwin's Shoulders | journal=Natural History | volume=114 | issue=9 | year=2005 | pages=64–68}}
+* {{cite journal | last=Scheffler|first= I.E. | title=A century of mitochondrial research: achievements and perspectives | journal=Mitochondrion | volume=1 | issue=1 | year=2001 | pages=3–31}}
+* {{cite book|first=Bruce |last=Alberts|coauthors= ''et.al.''|year=1994|title= Molecular Biology of the Cell|edition= Third Edition|publisher= Garland Publishing Inc.|location= New York}}
 ==External links==
+* [http://www.wadsworth.org/databank/electron/cryomito_dis2.html Video Clip of Rat-liver Mitochondrion from Cryo-electron Tomography]
 * [http://www.sci.sdsu.edu/TFrey/MitoMovie.htm Mitochondrion Reconstructed by Electron Tomography]
 * [http://www.the-elso-gazette.org/magazines/issue11/mreviews/mreviews1.asp Review of evidence addressing whether mitochondria form cellular networks or exist as discrete organelles]
+* [http://www.cytochemistry.net/Cell-biology/mitoch1.htm Mitochondria: Architecture dictates function]
+* [http://www.zytologie-online.net/mitochondrium.php Mitochondrion with Cell Biology]
+* [http://www.uni-mainz.de/FB/Medizin/Anatomie/workshop/EM/EMMitoE.html Mitochondra Atlas]
+* [http://bama.ua.edu/~hsmithso/class/bsc_495/mito-plastids/mito_web.html Other links]
 ==See also==
@@ Line 72: / Line 105: @@
 * [[Endosymbiotic theory]]
 * [[Glycolysis]]
-* [[Lynn Margulis]]
 * [[Mitochondrial disease]]
 * [[Mitochondrial DNA]]
 * [[Mitochondrial genetics]]
+* [[Mitochondrial permeability transition pore]]
 * [[Submitochondrial particle]]
-===Mitochondrial structure===
-* [[Outer membrane]]
-* [[Inner membrane]]
-* [[Intermembrane space]]
-* [[Crista]]
-* [[Matrix (biology)|Matrix]]
-===Fiction===
-* The [[midi-chlorians|midi-clorians]] of the [[Star Wars]] universe are fictional life-forms inside cells that provide [[the Force]]. George Lucas took inspiration from the [[endosymbiotic theory]].
-* ''[[A Wind in the Door]]'' posits fictional "farandolae" which are to mitochondria what mitochondria are to cells.
-* In the acclaimed novel and video game ''[[Parasite Eve]]'', mitochondria are shown to be their own independent organisms, using animals and plants as a form of "transportation," causing a major biological disaster when they decide to set themselves free.
-{{organelles}}
@@ Line 96: / Line 116: @@
-{{credit|34414755}}
+{{credit|45384311}}
 [[Category:Life sciences]]