In cell biology, 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 thousand and two thousand each. Mitochondria can occupy up to 25% of cell cytosol.
Cross-section of a mitochondrion, showing: (1) inner membrane, (2), outer membrane, (3) cristae, (4) matrix
Mitochondria have two functionally distinct membrane systems separated by a space: the outer membrane, which surrounds the whole organelle; and the inner membrane, which is thrown into folds or shelves that project inward. These inward folds are called cristae. The number and shape of cristae in mitochondria differ, depending on the tissue and organism in which they are found, and serve to increase the surface area of the membrane.
- 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 about 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 about 20% lipid 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.
Mitochondria are visible as thread-like structures in the light microscope. Mitochondria are selectively stained with a fluorescent dye. Nucleus and cell membrane are not visible.
"Mitochondrion" literally means 'thread granule', which is what they look like under a light microscope: tiny rod-like structures present in the cytoplasm of most eukaryotic 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).
The mitochondrial proteins are found on the outer membrane, the inner membrane, or the intermembrane space. Stop-transfer sequences anchor proteins to the outer membrane. Matrix-targeting sequences target the protein for the mitochondrial matrix.
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 involving 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). 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. 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.
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:
- 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)
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. Kraytsberg et al. showed that mitochondrial recombination is possible in humans (Science 304:981, May 2004, pubmed #15143273 (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15143273)).
The endosymbiotic theory
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 theory — 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.
This article contains material from the Science Primer (http://www.ncbi.nlm.nih.gov/About/Primer) published by the NCBI, which, as a US government publication, is in the public domain  (http://www.ncbi.nlm.nih.gov/About/disclaimer.html).