Everything about Mitochondrion totally explained
In
cell biology, a
mitochondrion (plural
mitochondria) is a membrane-enclosed
organelle found in most
eukaryotic cells. These organelles range from 1–10 micrometers (
μm) in size. Mitochondria are sometimes described as "cellular power plants" because they generate most of the cell's supply of
adenosine triphosphate (ATP), used as a source of
chemical energy. In addition to supplying cellular energy, mitochondria are involved in a range of other processes, such as
signaling,
cellular differentiation,
cell death, as well as the control of the
cell cycle and
cell growth. Mitochondria have been implicated in several human diseases, including
mental disorders, and may play a role in the
aging process. The word mitochondrion comes from the
Greek μίτος or
mitos, thread +
χονδρίον or
khondrion, granule. Their ancestry isn't fully understood, but, according to the
endosymbiotic theory, mitochondria are descended from ancient
bacteria, which were engulfed by the ancestors of eukaryotic cells more than a billion years ago.
Several characteristics make mitochondria unique. The number of mitochondria in a cell varies widely by
organism and
tissue type. Many cells have only a single mitochondrion, whereas others can contain several thousand mitochondria. The organelle is composed of compartments that carry out specialized functions. These compartments or regions include the
outer membrane, the
intermembrane space, the
inner membrane, and the
cristae and
matrix. In humans, mitochondria contain about 615 distinct types of
proteins, depending on the
tissue of origin. Although most of a cell's DNA is contained in the
cell nucleus, the mitochondrion has its own independent
genome. Further, its DNA shows substantial similarity to
bacterial
genomes.
Structure
phospholipid bilayers and
proteins. Disruption of the outer membrane permits proteins in the intermembrane space to leak into the cytosol, leading to certain cell death.
Intermembrane space
The
intermembrane space is the space between the outer membrane and the inner membrane. Because the outer membrane is freely permeable to small molecules, the concentrations of small molecules such as ions and sugars in the intermembrane space is the same as the
cytosol. Cardiolipin contains four fatty acids rather than two and may help to make the inner membrane impermeable. In typical
liver mitochondria, for example, the surface area, including cristae, is about five times that of the outer membrane. Mitochondria of cells that have greater demand for ATP, such as muscle cells, contain more cristae than typical liver mitochondria. The 13 mitochondrial
peptides in humans are integrated into the inner mitochondrial membrane, along with
proteins encoded by
genes that reside in the host cell's
nucleus.
Organization and distribution
Mitochondria are found in nearly all
eukaryotes. They vary in number and location according to cell type. Substantial numbers of mitochondria are in the liver, with about 1000–2000 mitochondria per cell making up 1/5th of the cell volume. Recent evidence suggests
vimentin, one of the components of the cytoskeleton, is critical to the association with the cytoskeleton.
Function
The most prominent roles of the mitochondrion are its production of
ATP and regulation of cellular
metabolism.
Pyruvate: the citric acid cycle
Each pyruvate molecule produced by glycolysis is
actively transported across the inner mitochondrial membrane, and into the matrix where it's
oxidized and combined with
coenzyme A to form CO
2,
acetyl-CoA, and
NADH. The citric acid cycle oxidizes the acetyl-CoA to carbon dioxide, and, in the process, produces reduced cofactors (three molecules of
NADH and one molecule of
FADH2) that are a source of electrons for the
electron transport chain, and a molecule of
GTP (that is readily converted to an ATP).
As the proton concentration increases in the intermembrane space, a strong
electrochemical gradient is established across the inner membrane. The protons can return to the matrix through the
ATP synthase complex, and their potential energy is used to synthesize
ATP from ADP and inorganic phosphate (P
i). who was awarded the 1978
Nobel Prize in Chemistry for his work. 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.
Heat production
Under certain conditions, protons can re-enter the mitochondrial matrix without contributing to ATP synthesis. This process is known as
proton leak or
mitochondrial uncoupling and is due to the
facilitated diffusion of protons into the matrix. The process results in the unharnessed potential energy of the proton electrochemical gradient being released as heat. Thermogenin is a 33k
Da protein first discovered in 1973. Thermogenin is primarily found in
brown adipose tissue, or brown fat, and is responsible for non-shivering thermogenesis. Brown adipose tissue is found in mammals, and is at its highest levels in early life and in hibernating animals. In humans, brown adipose tissue is present at birth and decreases with age. In fact, their ability to rapidly take in calcium for later release makes them very good "cytosolic buffers" for calcium. The calcium is taken up into the
matrix by a calcium
uniporter on the
inner mitochondrial membrane. It is primarily driven by the mitochondrial
membrane potential.
Glutamate-mediated excitotoxic neuronal injury
Cellular proliferation regulation
Regulation of cellular metabolism (see also: porphyrin)
Steroid synthesis.
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 mitochondrial diseases.
Origin
Mitochondria have many features in common with prokaryotes. As a result, they're believed to be originally derived from endosymbiotic prokaryotes.
Mitochondria contain DNA which is formed only during the division of other mitochondria within the cell. This DNA contains genes for ribosomes, and the twenty-one tRNA's necessary for the translation of messenger RNAs into protein. The DNA is often circular, as is most bacterial DNA, and employs a variant genetic code similar to that of Proteobacteria. This suggests that their ancestor, the so-called proto-mitochondrion, was a member of the Proteobacteria. In particular, the proto-mitochondrion was probably related to the rickettsia. However, the exact relationship of the ancestor of mitochondria to the alpha-proteobacteria remains controversial.
The ribosomes coded for by the mitochondrial DNA are similar to those from bacteria in size and structure. They closely resemble the bacterial 70S ribosome and not the 80S cytoplasmic ribosomes which are coded for by nuclear DNA.
The endosymbiotic relationship of mitochondria with their host cells was popularized by Lynn Margulis. The endosymbiotic hypothesis suggests that mitochondria descended from bacteria that somehow survived endocytosis by another cell, and became incorporated into the cytoplasm. The ability of these bacteria to conduct respiration in host cells that had relied on glycolysis and fermentation would have provided a considerable evolutionary advantage. In a similar manner, host cells with symbiotic bacteria capable of photosynthesis would also have had an advantage. The incorporation of symbiotes would have increased the number of environments in which the cells could survive. This symbiotic relationship probably developed 1.7-2 billion years ago.
A few groups of unicellular eukaryotes lack mitochondria: the microsporidians, metamonads, and archamoebae. These groups appear as the most primitive eukaryotes on phylogenetic trees constructed using rRNA information, suggesting that they appeared before the origin of mitochondria. However, this is now known to be an artifact of long-branch attraction – they're derived groups and retain genes or organelles derived from mitochondria (for example, mitosomes and hydrogenosomes). It encodes 37 genes: 13 for subunits of respiratory complexes I, III, IV, and V, 22 for mitochondrial tRNA, and 2 for rRNA.
As in prokaryotes, there's 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. Not all proteins necessary for mitochondrial function are encoded by the mitochondrial genome; most are coded by genes in the cell nucleus and the corresponding proteins imported into the mitochondrion. The exact number of genes encoded by the nucleus and the mitochondrial genome differs between species. In general, mitochondrial genomes are circular, although exceptions have been reported. Also, in general, mitochondrial DNA lacks introns, as is the case in the human mitochondrial genome; such as that of yeast and protists, including Dictyostelium discoideum.
While slight variations on the standard code had been predicted earlier, none was discovered until 1979, when researchers studying human mitochondrial genes determined that they used an alternative code. Many slight variants have been discovered since, including various alternative mitochondrial codes. Further, the AUA, AUC, and AUU codons are all allowable start codons.
Further Information
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