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A mitochondrion is an organelle found in the cells of most eukaryotes.
Mitochondria are sometimes described as "cellular power plants" because their primary purpose is to manufacture adenosine
triphosphate (ATP), which is used as a source of energy.
The number of mitochondria found in different types of cells varies widely. At one end of the spectrum, the Trypanosome protozoan has one large
mitochondrion; by contrast, human liver cells
normally have between one and two thousand each. Mitochondria can occupy up to 25% of cell cytosol.
Structure
Mitochondria are composed of two membranes the inner of which has folds called cristae, which give a much increased
surface area on which chemical reactions can occur.
- The outer membrane encloses the entire organelle and contains channels made
of protein complexes called porins through which molecules and ions can move in and out of the mitochondrion. It is composed of 50% lipids and 50% proteins. Large
molecules are excluded from traversing this membrane.
- The inner membrane, folded into cristae, encloses the matrix (the internal fluid of the mitochondrion). It contains
several protein complexes, and is 20% lipids and 80% protein. Stalked particles are found on the cristae: these
are the ATP synthase enzyme molecules, which produce ATP.
- The intermembrane space between the two membranes contains enzymes that use ATP to phosphorylate other nucleotides and that catalyze
other reactions.
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Cross-section of a mitochondrion, showing:
- inner membrane
- outer membrane
- crista
- matrix
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"Mitochondrion" literally means 'thread granule', which is what they look like under a light microscope: tiny rod-like structures present in the cytoplasm of
all cells. The matrix contains soluble enzymes that catalyze the oxidation of pyruvate and other small organic
molecules. Parts of the Krebs cycle occur within mitochondria.
The matrix also contains several copies of the mitochondrial DNA
(usually 5-10 circular DNA molecules per mitochondrion), as well as special mitochondrial ribosomes, tRNAs, and proteins needed for DNA replication.
When the cell divides, mitochondria replicate by fission. They also
replicate if the long-term energy demands of a cell increase. For example, fat storage cells,
which require little energy, have very few mitochondria, but energy-demanding muscle
cells tend to have many. Mitochondria are generally theorised to be highly adapted symbiotic bacteria, probably belonging to the alpha-proteo bacteria (with the closest known candidate being
Rickettsia, the causative agent of typhus), and are believed to have been
incorporated only once (compare chloroplast).
Proteins
The mitochondrial proteins are found on the outer membrane, the inner membrane, or the intermembrane space.
Outer membrane proteins
Stop-transfer sequences anchor proteins to the outer membrane. Matrix-targeting sequences target the protein for the mitochondrial matrix.
Energy conversion
Mitochondria convert the potential energy of food molecules into ATP. The production of ATP is achieved by the Krebs cycle
(see citric acid cycle), electron transport and oxidative phosphorylation. Without oxygen, these
processes cannot occur.
The energy from food molecules (e.g.,
glucose) is used to produce NADH and FADH2 molecules, via glycolysis and the Krebs cycle. This energy is transferred to oxygen
(O2) in several steps. 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). An active transport system (energy requiring) pumps the protons against their physical tendency (in the
"wrong" direction) from the matrix into the intermembrane space.
As the proton concentration increases in the intermembrane space, a strong diffusion gradient is built up. The only
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 (Pi). This
process is called chemiosmosis
and is an example of facilitated diffusion. 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.
See also: chemiosmotic hypothesis, electrochemical potential, glycolysis
Other functions
Mitochondria have several important functions besides the production of ATP. This variety of functions corresponds to the
variety of mitochondrial diseases.
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. These enzymes are not made in the
mitochondria of cardiac cells.
Mitochondria also play a role in the following:
Use in population genetic studies
Because eggs destroy the mitochondria of the sperm
that 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.
Recent studies have, however, cast doubt on this hypothesis. The paper ' Kraytsberg et al., Recombination of Human
Mitochondrial DNA, Science 2004 304: 981' shows that mitochondrial recombination is possible in humans.
The endosymbiotic hypothesis
Mitochondria are unusual among organelles in that they contain ribosomes and
their own genetic material. Mitochondrial DNA is circular and employs characteristic variants
of the standard eukaryotic genetic code.
These and similar pieces of evidence motivate the endosymbiotic hypothesis — that mitochondria originated as prokaryotic
endosymbionts. Essentially this widely accepted hypothesis postulates that
the ancestors of modern mitochondria were independent bacteria that colonized the interior of the ancient precursor of all eukaryotic life.
See also
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